Supporting Material
Student Guide
Block 3
Risk Management
Part 1 Natural constraint
1/ Book keeping:
- Let’s say Philip’s annual income is twenty pounds.
-
Philip’s annual expenditure is nineteen pounds and ninety six pennies.
Result = Happiness
- Let’s say David’s annual income is twenty pounds.
- David’s annual expenditure is twenty pounds.
- Result = Misery
- The income & expenditure totals can generally each be resolved into several
smaller transactions but the overall statement must still account for
everything.
- A balance sheet shows dispassionately what is going on.
- Philip’s Sheet indicates a happy balance sheet, while David’s sheet indicates
a miserable balance sheet.
- In spite of free will, almost everything we do is constrained by something.
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- The boundaries to choice are set by the society we live through customs &
laws in addition to financial costs.
- To an engineer, very few things can be admitted to be impossible, example:
lack of adequate fund for a scheme yet it won’t be impossible.
- There are occasions when some things are really impossible, it contradicts
one or more of the fundamental laws of nature. For example: you cannot
make gold from lead.
2- The energy balance – getting it right and what
happens when you don’t:
- My personal computer (PC) sits on my desk humming away to itself. All the
time heat is generated by the computer’s internal components.
- Main source of heat is the transformer.
- Transformer reduces main electricity from 230 Volt sacs. To that required.
- The CPU, disks & all innards that are involved in the activity of the
computer also generate heat.
- What happens to all this heat & what are the consequences of simply
ignoring it?
Power & energy:
- Power is the rate at which energy passes from one form to another. (From
electricity to heat).
- Energy is measured in Joules; power is the number of Joules per second.
- Joules/ second is known as Watt.
- How much heat is generated within a computer? Refer to table 4.3
(Electrical power consumption of PC components).
- Power is consumed as electronic switches open & close million times per
second.
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The first part of balance sheet:
- Say that the thermal power generated equals to the electrical power
consumed.
- In saying this, any energy that ends up as light or sound is neglected.
Balance in terms of power:
- 70 Watts of electrical power is continuously converted to 70 Watts of
thermal power, which is 70 Joules per second.
- Also a balance in terms of energy can be made.
The principle of conservation of energy:- Energy cannot be created or destroyed it can be transformed to different
forms.
- Therefore 5000 Joules of chemical energy from fuel can be converted by a
power station turbine into 4500 Joules of thermal energy & 500 Joules of
mechanical energy.
- Refer to table 4.4 (Balancing energy transactions for a system).
- Table 4.5 (Power consumption of the body & some comparisons).
- It is realized that the incandescent light bulbs rated 100 Watts produces
barely 6 Watts of light, the rest being heat.
- In terms of its use, such a light bulb has “ efficiency” of only 6%.
Efficiency:
- It is the amount of useful output divided by the amount of input, both
measured in the same units.
- Therefore
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Efficiency = Useful output
Input
- Example:
A light bulb produces 6 watt of light while being supplied with 100 watts of
electrical power has an efficiency of:6W = useful output = Efficiency
100 W
input
- Maximum possible efficiency is one, when output and input are equal.
- Efficiency can be expressed in percentage.
- Therefore
efficiency in% = useful output x 100%
Input
- Example:
Determine the % efficiency of a light bulb that produces 3 Watts of
useful light while consuming 60 Watts of electrical power:
Efficiency in% = 3 watts x 100% = 5%
60 watts
Converting Energy:
- Power is a measure of the ability to do work.
- From table 4.5 it can be seen that a horse has ten times more capacity to
supply energy than does a human.
- Take a look at the energy and power in relation to human body from table
4.5.
- As mentioned earlier :
-
energy(Joules) = Power(watts) x time( seconds)
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- Dividing both sides of the equation by time.
-
The result will be : Power = energy/ time
- So power is the rate at which energy is supplied or consumed.
-
Work done = force x distance
- Energy to heat conversion occurs in the computer, while energy of earth
surface is being converted to gravitational potential energy.
- For example a car rolling down the hill is converting gravitational potential
energy into kinetic energy
(Energy associated with motion).
- According to the Equation above , force is multiplied by distance h that the
object rises or falls.
- That is to say for an object on earth, the force required lifting it or the force
pulling it downwards is just equal to weight.
- Gravitational potential energy = Mass x acceleration due to gravity x H.
- Calculate the power expended while climbing the stairs. I climbed the stairs
a couple of times, once quickly which took four seconds and once at normal
rate which took 8 seconds. The power would be 2400/4 = 600 watts in the
first case. Table 4.6 shows the values of power output for a typical adult at
three ages.
- Going upstairs converts’ nutritional energy into potential energy. Running
along the flat converts nutritional energy into kinetic energy.
- Calculation of kinetic energy is = 1 MV
2
- V stands for velocity.
- Refer to SAQ 4.1
2.1 Balancing heating with increased temperature:- From table 4.3, the thermal power generated inside a PC is about 100 Watts.
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- The PC would be warm if 100 Watts light bulb switched inside the case.
- How does the temperature rise within the enclosed space of a PC calculated
as a consequence of its power consumption?
- This is achieved by specific heat capacity.
- Therefore specific heat capacity is the relation between added heat(Delta Q)
& increased temperature (Delta T) for a mass (M) of a substance with a
specific heat capacity of (C) is:
- Delta Q = M C Delta T
- The heat capacity of a substance tells you how much energy is required to
raise its temperature by a given amount.
- SI (units), It’s expected that to be one degree Celsius.
- Refer to table 4.7 to see (Specific heat capacity for some materials.
- Given the same amount of energy input, the temperature of aluminium will
raise almost four times more than the temperature of concrete will rise.
(Refer to table 4.7).
- The formula linking energy, temperature, specific heat capacity & mass is =
Delta Q = MC Delta T
- Delta Q = amount of heat required, M = Specific heat capacity, C =
temperature change.
- Refer to SAQ 4.2 and SAQ 4.3
- By rearranging the Formula you should be able to see that the expression :
-
Delta T = Delta Q
MC
The equation expresses the temperature rise that a given amount of heat energy
will produce in a given amount of a given substance.
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Transferring Heat:- Heat can be transferred in three ways:1/ By radiation
2/ By convection.
3/ By conduction.
- Conduction involves transferring heat through a material.
- Therefore if you hold a metal knife in a gas flame, the end you hold will
become hot as heat is conducted along the knife.
- Convection involves heat transfer by setting up a flow of air.
- Radiation is the transmission of heat without contact, either directly or
through a convection medium.
- Therefore the sun reaches us by radiation.
- Refer to table 4.1 ( temperature change of a PC consuming 100 Watts)
2.2 Balancing heat generation with cooling:- If the PC generates 100 joules of thermal energy per second, this is also the
rate at which we will have to remove the heat to maintain a steady
temperature.
- The plan is to let the thermal energy flow into air inside the box & to replace
the air fast enough to carry out 100 Joules each second. (Refer to figure 4.2)
- Let's say 2o degrees C above room temperature.
- The next step is to work out the mass of air that is heated by 20 degrees C by
100 Joules of thermal energy.
-
Delta Q = MC DELTA T again.
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- By dividing both sides by C Delta T we can rearrange the equation :-
M = Delta Q
C Delta T
- From table 4.7 the specific heat capacity of air is about 1000 JKG C
- M = 100 J………………..
1000 JkgcC x 20 C
= 0.005 Kg
- Refer to SAQ 4.4 & 4.5
2.3 Ignore it at your peril:- Thermal management is important in engineering design.
- Inside a PC it is not just the solder joints that need to be kept cool.
- PVC plastics insulation or cables degrade on heating beyond 100 degrees C.
- If not isolated, cable looses structural integrity & in some cases catches fire.
- Electronic components too are susceptible to excess heat.
- Consequences of inadequate cooling are serious.
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Balancing Mass
- Mass is another quantity which book keeping can be applied to, in order to
check what's going on in a process or reaction.
- Total mass of stuff coming out of any process has to be the same as the total
mass going in, with the exception of nuclear reactions.
- An emergency generation is powered by natural gas.
- Consumes a certain amount of gas an hour when in operation.
- Refer to figure 4.3.
- Enough gas must be burnt to provide enough heat for generating that
required amount of electric power.
- The gas is mixed with air for combustion.
3.1 A chemical balance – atomic scales:-Natural gas is predominantly a substance called methane.
-The simplest of the Family is known as hydrocarbons.
-It's chemical formula is CH4.
CH4 + 2O = co2 + 2H2o this is a balanced reaction equation.
- Hydrocarbons are chemical compounds that contain atoms of carbon and
hydrogen only.
- A whole family of hydrocarbons exists, combining more and more carbon
and hydrogen atoms.
- Examples :- C3H8 , C2 H6 which is known as Ethane.
- Refer to SAQ 4.6 and SAQ 4.7
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3 :The role of engineering in identifying
managing risks
Aims:
¾ To introduce the concept of risk.
¾ To illustrate the need for identifying priorities in risk assessment.
¾ To review some major incidents and identify issues from which preventive
measures can be developed/
The UK's engineering council stresses the need for proper balance between:
(1) Efficiency.
(2) Public safety.
(3) Needs of the environment when carrying out engineering activities.
¾ The code of professional practice points out that if engineers address
risk thoroughly they may help to encourage greater awareness of risk
in other engineers.
¾ Their ability to design, develop and construct means they sometimes
have to risk the consequences of their engineering being misapplied or
misused by others.
The key element of the council’s code of professional practice on RUK issues are
listed on table 3.1 (page 9).
Many accidents are inevitability linked by the public to engineering activity,
Where incidents have since been analysed thoroughly and their causes
established, table 3-2 (contributory careers).
Some incidents were the result of compartment or structural failure but often
there was some human error involved as well because it is often difficult to
separate engineering issues from human factors so called hard and soft elements.
Like wise, a perception of the relative significance of many of these issues is a
personal one.
Table 3-2 shows some engineering disasters and contributory causes.
Exercise 3.1
Perception of Risk
Activity:- Table 3-3 perception of risk and safety test of some of your
perceptions against some commonly held views.
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Perception of safety is often linked with perception of likelihood of an
accident happening.
What is an accident?
There are many ways in which we can define accidents but the key characteristics
of any event that could be described as an accident seems to be:
¾ The degree of expectedness- the less we expect the event, the more we regard it
as an accident.
¾ The avoid ability – the less likely we can avoid the event, the more it is
accidental.
¾ The lack of deliberateness: the less some one is actually involved in causing an
event to occur the more we view it as an accident.
Table 3.1
Point Codes
1.Professional Responsibility
2.Law
3.Conduct
4.Approach
5.Judgement
6.
7.
8.
9.
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Risk Issues
Table 3.2 Some engineering disasters 1986 – 1989
Incident
Date
Outcome
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Contributory
Causes
Definition:An accident is a non-deliberate, unplanned event which may produce
undesirable effects and is preceded by unsafe, avoidable act(s) and/or
condition (s).
The definition probably matches the common perception of the word quite well,
nevertheless it has some limitations.
Non-deliberate acts include trips and falls as well as earthquakes.
Good engineering design may help to prevent former but can only help to
minimize the effects of the latter.
The “Unplanned” element of the definition can be taken to imply that accidents are
inevitable and uncontrollable, this is not necessarily true and much engineering
practice aiming to prevent such events.
The phrase “undesirable effect” raises another problem. Value judgement
determines whether something is desirable or undesirable and whether a change
is required or not.
-
The inconsistency has an impact on the perception of an accident.
Examples:
Someone swallow a pesticide incorrectly stored in a lemonade bottle.
Eating canned meat infected with toxins.
The above examples have undesirable effects, but not all accidents are
undesirable because sometimes the outcome maybe beneficial.
There is no doubt that engineers learn from accidents and disasters have lead to
changes in design or legislation that improve product or customer safety and that
engineering aims to designing and manufacturing artefacts to reduce the
likelihood of and minimize to consequences of engineering failure.
In this Context engineering failure could include both failures of components
and failures of system.
RISK MANAGEMENT:
In the engineering council policy risk management is the process of evaluating
alternative actions and selecting the most appropriate.
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¾
¾
¾
¾
¾
Risk management is done on daily basis whether consciously or subconsciously.
It is practice with process, methods and tools for managing risks in a project.
It provide a disciplined environment for proactive decision making to:
Asses continuously what could go wrong i.e. identify risks?
Determine which risks are important to deal with.
Implement strategies to deal with those risks.
Risk management is a decision – making process that involves the consideration
of political, social, economic and engineering information together with risk
related information.
Exercise 3-3
What is risk?
Three definitions are derived from the guidelines on risk issues from the
engineering council. These are:
1- Risk is the chance of an adverse event.
2- Risk is the likelihood of a hazard being realized.
3- Risk is the combination of the probability or frequency of occurrence of a
defined hazard and the magnitude of the consequence of the occurrence.
Therefore it is a measure of the likelihood of a specific undesired event and
unwanted consequences or loss.
All these three definitions deal with the likelihood of an event rather than the
definition of the event itself.
Events range from disasters to relatively minor inconvenience.
Sometimes even a relatively minor event could have enormous consequences e.g.
fatal car accident though unlikely to receive national attention. Major events or
incidents may involve much loss of life and therefore receive the greatest publicity
and tend to raise general concerns about the risks of engineering failure.
Perceptions of risks are not based solely on quantitative measures but include
subjective value judgement.
The word risk& hazard are used interchangeably but hazard is a potential source of
harm while risk is a combination of the chance of exposure to that harm and the
consequences of the exposure.
READINGS
Fear of flying (Chapter 3, page 13).
Risks of flying (Chapter 3, page 14).
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People can be very emotional or irrational about certain risks as illustrated in
“Fear of flying” even though the actual risks from air craft flight are very low
indeed as noted in “Risk of flying”.
SAQ 3.1
ANALYSING INCIDENTS:
From previous discussions it is clear that whether a particular event is an incident,
or a disaster is a subjective judgement. This part will focus on events that are or
might be connected to engineering failure.
Incidents of various scales that are linked to some engineering endeavour will be
examined.
In this context, an accident will be defined as follows:
“An accident is an undesired event which results in physical harm and/or
property damage. It usually results from a contact with a source of energy
above the threshold limit of the body or structure.”
Exercise 3-4
Different types of energy:Energy comes in many forms and from many sources:
Kinetic Energy (due to motion).
Chemical Energy (From foods).
Gravitational Potential Energy.
Heat Energy.
Sources:
Burning Gas.
Electricity.
Electromagnetic Radiation.
Exercise 3-5
Whilst the causes identified in exercise 3.5 may lead to an accident, they are
not clearly the root cause.
Incidents are divided into two categories. These are:
Unsafe conditions.
Unsafe practices.
Whilst causes may lead to accident they are not clearly its roots causes could be
due to failures of management controls relating to the following:Human factor (Lack of knowledge, skills or training)
System factors (Inadequacies to work standards design)
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Engineering causes (component failures).
¾ It is essential to address possible failures at any level to prevent the causes and
direct and correct them before they can lead to incidents. As mentioned earlier
on incident may or may not result in loss or effects maybe considered to be
negligible depending on the level of energy transfer.
¾ These incidents sometimes called (Near Misses) can be very important ,by
identifying the cause of such minor incidents corrective measures may be put in
place before serious accidents occur.
¾ A great care should be taken in identifying causes of incidents. Causes are
many varied and often interlinked. Identification of roots causes maybe
different depending on ones own perspective or opinion.
¾ Therefore it is very important to look beyond the obvious cause and to identify
more fundamental ways of eliminating the hazard and reducing the risks by
looking at multiple causes and their interactions.
Refer to figure 3-3 page 7.
Assessing Risk
To approach “estimation of risk” in a more scientific and calculated manner it is
worth studying the statistical aspects of risk or it is probability. Professional risk
analysts often quote various figures showing the risks associated with everyday
activities in comparison with risks in which they have an interest.
Table 3-5 page19
Probability: is a way of expressing risk in mathematical terms.
It is always a number between 0 and 1 and is therefore expressed as a function or
decimal. An impossible event that is certain to happen has a value of 1 e.g. the risk
of dying is 1.o for everyone.
Other outcomes may be less certain and scientist lives with uncertainty by
measuring probability. Probability refer to the proportion of cases in which an
event may occur and therefore it is different from statistics which through the same
language deals with what did happen but probability deals with what may happen
but it is often that deal with what may happen or to put it in another way
probability deals with what may happen ,but often our best guess at what may
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happen is that it will mirror what has happened in the past which can be described
by statistics.
No matter how risks are defined or quantified they are usually expressed as a
probability of adverse effects associated with particular activity.
Table 3-4
The de minimis concept:
This assumes that there is a level of risk that is so low that it can be reasonably
ignored.
6 Risk perception:
Research consistently shows that people’s perception of risk is a function not
just of the possible harm but also of the attributes of the hazard and the benefits
associated with the thing in questions. Unfamiliar things excite more fear than
things that are familiar ones.
The term risk and safety are consistent with the common acceptance that
something is safe if the risks associated with it are low and vice versa.
ALARP (AS LOW AS REASONABLY PRACTCABLE)
The approach was first developed in the area of environmental engineering.
The ALARP concept was often referred to as BATNEGC “being an acronym for'
Best Available Techniques Not Entailing Excessive Cost”
The term originated from the European Union directive but in the UK it’s use came
from Integrated Pollution Control legislation set out in the environmental
protection Act of 1990.
¾ A low level of risk maybe reliable and generally acceptable.
¾ Over a certain high level, risks are not tolerable and legislation may take an
active part in their regulation.
¾ Between the two ends of the spectrum, risks may be tolerable provided that it
can be shown that the financial costs and efforts required reducing the risks
further out of proportion to the benefits gained.
¾ The ALARP concept implies that ultimately there is a trade-off between the
cost of risk reduction and the benefits obtained.
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¾ The scientific community has a very important role to play in measuring risks
and presenting information as clear as possible with the appropriate cautions
about uncertainty.
¾ It is then a responsibility of society as a whole to determine what is tolerable
and acceptable based on social, political, cultural and even economic
considerations.
¾ Clearly there are areas where the risk is so high as to be un acceptable and
others where it is so low to be negligible.
¾ Legislations, attitudes and hence behavioural changes are important channels
for reducing the risk.
¾ Many hazards cannot be abolished in the sense that they are completely
eliminated.
¾ Therefore reducing risk is often a question of reducing exposure.
Table 3-9
Appreciating very low risk:
Risk are typically measured using factors of 10-6 (one in a million)
or less.
On the other hand the consequences of some engineering failures
such as nuclear plant if that occur are significant e.g. Bhopal chemical
plant and the Chernobyl.
This small combination of small scale probabilities and large possible
outcomes are known to cause problems with perception of risks
involved and a large amount of research has been performed to try to
measure these perceptions. Several studies have sought to measure
people’s perception of the estimated risks.
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Part 5
INTRODUCTION
Four systems of "rules" in engineering environment have been
located at:(1) Standards for methods & methodologies.
(2) Patents for invention protection.
(3) Rise assessment for product safety.
(4) Natural rules that constrain what engineers can do.
- Engineering is not just a technical subject.
- Skills & understand underpin the fundamentals of engineering/
-
Engineering development is strongly influenced by external factors such
as economics.
- It's a big problem that electronics is expected to become the largest
industrial section in the world.
- The problem centers on a key feature of engineering decision making.
- Electronic equipment is getting smaller yet more is expected in terms of
performance.
- Notice in Fig 5.1 (design evolution of mobile phones) from bigger to
smaller phones with substantial improvement in performance.
-
In electronic, goods is a so called "Printed circuit board" (PCB), it is a plat
form for a large number of electronic components.
- The board material is usually fiber reinforced polymer.
-
Copper small conducting pads are printed on to the board as the base & to
conduct electricity between them.
-
The weak point of the PCB is the joint between the components & the
board or between the components & a track on the board.
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-
In this case the joint is in the form of a soldered interconnection.
The soldered joint serves several purposes:1/ it glue the component on the board.
2/ provides an electrical conduction path between the component & the tracks
on the board.
3/ it conducts heat away from the components
(prevents over heating).
4/ it help to give strength to the structure.
- As miniaturization of electronic goods has continued, the packing density
has increased, the dimensions of some individual components have grown
whilst others have become smaller & size of soldered joints has taller.
Factors of Safety:- It is very poor engineering to design too close to the limit of performance
of a material.
- Example:Let's say the factor of safety is F
Mass of fully loaded lift is X
Cable can support a mass of 10X
So the equation is:F= 10x / x = 10
- Thicker cable would be heavier & more costly, than a thinned cable
stronger cable material would be more expensive, so while there would be
price to pay if F is increased yet the essence of engineering design is to
optimize.
Adverse health effects of lead:- Lead compounds have been identified by us environmental protection
agency as one of the top 20 chemicals that pose hazard to human life &
environment.
- Lead accumulates in body over time.
- It binds to proteins & inhibits many normal functions of human body.
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- It may result in adverse health risks including nervous & reproductive
system disorders, reduced hemoglobin in blood.
- Lead level even below established official threshold 50mg/dl of blood
could be hazardous to children.
- In the U k lead has been banned from use in domestic paint for many years
and vehicles.
- Sophisticated design approaches & as power generation applications is
now being employed to guarantee the structural integrity of PCB systems.
- Electronics designers & manufacturers are facing another challenge in the
solder field.
- Virtually all current solder materials contain element lead. (Adverse health
effects of lead).
- There is the occupational hazard to workers.
- Most electrical & electronic goods are currently disposed in land fill sites
at the end of their working lives.
- With time, possibly aided acid rain May lead to lead dissolving in
containers ground water.
- It is considered to be uneconomic to separate the PCBS component &
recycle them.
- The producer will always be responsible for products throughout their life
& including recycling, this is known as manufacturer responsibility.
Legislating for lead:- In 1991 the US congress proposed that lead should be banned in some
materials & limited to less than 0.1% in others.
- US interest in the development of suitable lead free solder alloys waned.
While Europe & Japan maintained their efforts to eliminate lead.
- An article no. 4 states:(Member states shall ensure that the use of lead is phased out by 1st of
January 2004).
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- Electronics producers in Japan have also responded to guidelines from
their government & the electronic producers to reduce the use of lead.
- Although much of legislations is yet to be enacted , the development of
suitable lead free solders is now big business.
- Table 5.1 shows us lead usage in 1995.
- At the present there is no suitable alternative to lead containing batteries
for cars, so this product is currently exempt from all suggested legislation.
Green alternatives to lead free solder
- Green alternatives to lead free soldering recovery, recycling and reuse are
more effective in preserving the environment.
- The American Electronics Association has responded to the proposal of
lead ban, saying that the ban on lead is not adequately justified and needs
further review.
- In Japan, lead free product is proclaimed to be environmentally superior.
- Will the elimination of lead in solder reduce the amount of lead mined or
lessen the pollution associated with extraction and purification of this
resource? The answer is no.
- Less than 1% of the lead being mined is used for electronic solder.
- In concern for the environment and our children's health and welfare, we
are not asking the right question. We should be asking how we could set up
electronic recovery and recycling centers.
- We need to shift our scientific and engineering efforts away from leadfree solder and focus on end of life recovery and re use of the precious
resources in our electronic product.
- Our natural resources are dwindling and we have a responsibility to
preserve them for future generations.
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- Is lead-free solder good for the environment? I don’t think so! Are end of
life recovery, recycling and re use good? Yes! This is where we should be
putting our efforts.
- Refer to exercise 5.1 and SAQ 5.1.
What is soldering?
- It is the joining of two surfaces using a low melting point alloy (or metal)
as a glue
- Solder is heated to liquid state, placed onto joint & then allowed to cool
until it has solidified.
- The solder melts but the materials being joined do not.
- Soldering has existed since around 3000BC for many uses.
- Mostly used solders are 60% tin & 40% lead or 63% tin & 37% lead.
- Soldering applications includes plumbing, electrical & electronics
manufacturing.
- Solders provide electrical, thermal & mechanical continuity.
-
The performance & quality of the solder are crucial to the integrity of a
solder joint.
- Solder provides mechanical connection between the silicon chip & its
bonding pad.
- Within the packaging of typical "Microchip", the bonding of the chip to a
substrate & its encapsulation is referred to "Level" packaging.
- This takes place long before the chip is mounted on a PCB.
- Wire bonding is a traditional way through which electrical connection is
provided to a silicon chip.
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- In figure 5.5, silicon chip is turned upside down, the approach used is flip
chip configuration.
- The next level of electronic assembly, termed "Level 2 packaging" is
where the component is mounted on a PCB. Soldering is the primary
means of interconnection in level 2 packaging.
- All microelectronic devices are attached on PCBs using solder using one of
two principle methods:1) Pin through hole (PTH) 5.6
2) Surface mount technology 5.7
- Assembly & soldering PCBs can involve purely surface mount
components or mixed technology where SMT & PTH are used.
- Soldering of surface mounted devices, commonly called REFLOW,
soldering is done by application of solder paste on mating surface.
- Solder paste is a mixture of solder powder, a flux to aid floe & prevent
oxidation of metals being joined & other additives.
- Additives are included to promote wetting & control the properties of
paste.
- Pin through hole connections are made by wave soldering, check 6.8
- PCB is transported on a molten solder bath from which the solder rises up
the protruding leads by capillary action & forms solder joint.
- Solder & heat are applied simultaneously in wave soldering where as they
are applied sequentially in re flow soldering.
- It has been estimated that there are more than 10 joints made every year.
MEASURING TEMPERATURE:- There is more than one scale that can be used to measure temperature.
- The two most common are Fahrenheit & Celsius.
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- Celsius defined the freezing point to be 0 C & the boiling point of water to
be 100 C.
-
Fahrenheit scale has 32 F & 212 F for the same point.
- There fore 1 C > 1 F.
- In engineering C is preferred.
- It is possible for temperatures to be much cooler than 0 C that is known as
the coolest possible temperature.
- It is -273.16, at this temperature mater has no thermal energy and this is
known as "ABSOLUTE ZERO".
- The scale that uses the absolute zero as a reference point is known as
Kelvin scale.
- In order to convert, Celsius to Kelvin, one has only to add 273 ( figure 5.9)
-
Refer to exercise 5.2 & SAQ 5.2.
Mechanism of Failure:- When an electronic device in operation, solder connections are subjected to
stress & strains.
- This arise from two main sources:(1) Mechanical loading.
(2) Changes in temperature.
- Materials present in a joint on a circuit board are quite different. They could
be copper, alumina, polymer other than solder.
- Different materials expand or contract to different extents on heating/ cooling
then strains can be build up with in the joint due to change of temperature.
THERMAL EXPANSION:- When a material is heated, it increases in size slightly, when cooled slight
shrinkage occurs.
- Refer to fig. 5.10 as an example.
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- The amount of expansion for a given temperature rise is material
dependent.
- Thermal expansion by property called co efficient of thermal expansion.
- Refer to Fig 5.11.
- If a bar is heated up by a temperature change of Delta T, then it will get
longer. Relative to it original length (L) there will be a strain.
- Strain is defined as Epsilon, as being equal to CTE for the material
multiplied by the temperature change.
-
E= a X Delta T
- Table 5.2 shows co efficient of thermal expansion for a range of materials.
The units of gamma are the amount of strain generated per degree Kelvin
of temperature rise.
- From the table 5.2 you will notice that the steel lid on a glass jar will
expand more than the glass of the jar by a factor of (11/9) when they are
both heated by the same amount.
- For
- Refer to exercise 5.3.
- An example of how differential thermal expansion creates stress between
the solder, the silicon chip & the substrate in the flip chip package in figure
5.12.
Shear deformation:- In addition to simple tension and compression forces, there are more
complicated ways in which objects can be stressed; one such way is by
shear forces.
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- This is illustrated in figure 5.13; one part of an object is pushed in one
direction and another part in an opposite direction.
- Associated with shear force there will be a shear stress and a shear strain.
- Refer to figure 5.14 showing a cubic object subjected to shear loading.
- Shear stress is given the symbol tau, she strain symbol is gamma and shear
modulus is given the symbol G so:
-
Tau = G x Gamma
- Many manufacturing operations are dependent on shear processes.
- The shear modulus is generally some where between one half and one third
of the young's modulus.
- Devices can experience a very large number of vibrations, with the direct
result that the solder joint is subjected to cyclic loading.
- For such applications the fatigue life of the solder joint becomes critical.
- Differential thermal expansion can result in a constant static load being
applied to the solder connection.
- This results in the solder connection being susceptible to creep.
- Creep is deformation at elevated temperatures.
The gas turbine laptop:-The operating temperatures are clearly widely different; the common
denominator in terms of material behaviors is the HONOLOGOUG
TEMPERATURE, T h, which is the ratio of the operating temperature T of a
material to it's melting point T m ( in Kelvin, K).
Th=T
Tm
- The higher the homologous temperature for a given material, the easier it is
for its atoms or molecules to move about relative to one another, thermal
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energy increases the vibrations of atoms in a solid, so materials become
softer as the temperature rises, eventually becoming liquid and then
gaseous at higher temperatures still.
- In all materials exposed to stress at high homologous temperature
eventually permanent plastic strain will occur.
- The amount of recoverable creep strain in metals is small, so creep can be
a serious problem that designers must account for in alloys that will have
to experience high homologous temperatures for long period of times.
- Many metallurgical tricks are used to design high temperature creep
resistant alloys such as those employed in gas turbine engines.
- Attempts have been made to make jet engine turbine blades from ceramic
materials which have even higher melting points than the best metal alloys
but ceramics are also intrinsically brittle and so the best modern
technology uses a combination of two technologies, nickel super alloy
blades with ceramic coatings.
- Refer to figure 5.16.
- Solder joints like gas turbine engines are subject to cyclic stresses and so
can suffer from metal fatigue.
- The plastic thermal strains created in solder joints are also much, much
greater than those in a typical gas turbine engine.
- Refer to SAQ 5
What do we want from a solder?
- The solid solder should conduct electricity and heat and posses mechanical
strength at its operating temperatures:
- It should have a low melting point both in order to conserve energy in
melting and avoid heat damage to the board material or its components:
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- Solidification of the liquid solder should be rapid to facilitate rapid
production, this contrasts with the solder requirements for other
applications, in particular the plumbing of old lead pipes, where gradual
solidification allows time for manipulation of the pieces being joined and
smoothing of the joint profile:
- The solder constituents should be readily available and economically
priced.
The melting point challenge:- Refer to table 5.3 lists the most common elements that have been
suggested for inclusion in possible solders.
- Electronic components may have to operate at temperatures up to 80 C and
in extreme situations such as under-bonnet locations in automobiles up to
130 C.
- On this basis mercury, rubidium, potassium and sodium are very reactive,
which makes them hazardous to handle and process.
- One element already a principle component of lead-based solders, tin,
appears to be suitable for use in lead- free solder alloys.
- Zinc also seems a possibility, although its melting point is somewhat
higher.
- The principle of lowering the melting point by mixing it with a second
metal is described in Alloying.
- Refer to exercise 5.4.
Alloying:- The terms describe a particular mixture of metals.
- Most pure metals have high melting points and are relatively soft.
- When we mix metals together an alloy that has a lower melting point and
is stronger is produced.
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- In order to know the melting point of a metal we alloy, we simply measure
the melting point for different alloys and then plot this information on a
graph.
- A graph of this information is known as a Phase Diagram.
- The melting point is transition from the solid phase to the liquid phase.
- In alloy phases can be a lot complicated.
- For one thing, there may be more one type of 'solid' present.
- Binary alloy system is one that consists of a mixture of just two metals.
- Refer to Figure 5.27.
- The semi solid area is often termed the mushy zone.
- Only pure metals have single, well defined melting points.
Eutectics in action:- The lead tin system is in fact an excellent example of eutectic system in
materials science.
- Refer to figure 5.18 that the eutectic composition is 63% tin 37% lead by
weight and the eutectic temperature is 183 C.
- Notice that between about 20% tin and 98% tin, any alloy composition will
begin to melt at this eutectic temperature is reached and that temperature is
composition dependent.
- There are three phases on the diagram.
- Alloy solidifies over a wide range of temperatures, in contrast for the
eutectic composition the alloy solidifies at single temperature that is the
lowest possible for the binary alloy system.
- Nearly all modern casting alloys are eutectics.
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- Refer to Figure 5.20 shows phase diagrams covering that part of the iron
carbon system that is responsible for cast irons and the Al-Si system which
is the basis of most aluminum casting alloys.
- Figure 5.12 shows components made from each of these systems.
- It is also possible to add small amounts of a third, or even fourth, element
to the binary eutectic to enhance mechanical properties even further.
- Refer to SAQ 5.4.
Cost implications:- The relative raw material costs of some of the metals that might be used in
the production of lead-free solders are shown in table 5.4 for comparison
purposes, lead is taken as the unit base.
- First glance to the column in table 5.4 would suggest that replacing lead in
solders will inevitably produce a substantial increase in cost.
- However, several factors mitigate against this problem.
- Lead is a high density material, so cost per unit mass is exaggerated.
- The right hand column of table 5.4 shows that the more appropriate
volume costs are less than the mass costs.
- The binary alloy systems of tin and copper and tin so although silver is the
most expensive material on our list, we would not have to use much of it.
Where are we now?
- Most promising replacement solder materials are the alloys, tin, silver and
copper.
- The potential replacements retain the disadvantage that their melting points
lie in the range 217 to 227 C.
- Recent research has shown that there is unlikely to be single alloy to
replace the ubiquitous eutectic 63% tin 37% lead system.
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- Solder alloys will be chosen according to their application.
- Without efforts of the environmental lobby to reduce pollution, there
would be little interest in lead- free solders.
- The replacement of virtually all lead in solders is probably now inevitable,
what is not sure is when it will happen.
- Refer to SAQ 5.5 (block revision).
- It is important to understand arguments relating to replacement of lead for
environmental reasons.
- Understand and be able to use the Kelvin temperature scale.
- Understand the concept of shear loading.
- Understand the concept of thermal expansion and be able to make simple
calculations of thermal strain.
- Be able to read melting temperatures from a phase diagram.
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