Material Witness
by: Chet Guiles
August 23, 2010
“Panta Rei" – “Everything Flows” — Part 1
The Greek metaphysician Heraclitus, writing in the 5th
century BC said, "Panta Rei" – “Everything Flows.”
I’d like to spend a little time visiting the science of
rheology (how and why things flow) and in particular
how understanding the rheology of the prepregs you
use, during the time they melt and flow, can help you
as a process engineer to manage and control your
lamination process.
it. All other things being roughly equal, the amount of
flow is proportional to the amount of pressure per unit
area (P/A) multiplied by a constant (determined by the
geometry of the system) and integrated over time as a
function of viscosity.
Anyone who has ever scratched his or her head
studying the flash or edge trim of a laminate cycle that
has not gone as expected knows empirically about
some of the effects of flow, or not flow, on a laminated
MLB.
Too much flow means boards that are
“footballed” – too little flow means boards with dry
edges and maybe inadequate fill around circuit traces
or in low pressure areas. Application of the science of
rheology, and an understanding of how and why your
materials behave the way they do, can reduce the
dependency on prayer, magic and guesswork in
designing press cycles.
F = c*(P/A) ∫ (1/v) dt
t is time (viscosity changes with time as the resin
starts to cure)
But let’s start at the beginning. What is “rheology” and
why does “everything flow”? We are interested in
viscosity (the resistance of a material to flow) and in
flow itself: how much, how quickly, and what
factors affect it. Viscosity in very simple and
nonmathematical terms, is a measurement of the
resistance of a fluid to flow when a force is applied to
You can see that the ratio of the pressure to the area is
critical. As an example of what that might mean to you
as a PWB processor, consider the old standard IPC
test for flow, where a stack of 4”x4” samples is
pressed at 200 psi and the flow measured. The P/A
ratio is 200/16 or 12.5. How does that relate to your
laminating an 18”x24” panel at 350 psi? Not very well.
A “resin flow parameter”
following formula:
(1)
has been defined by the
Where:
F is the resin flow parameter
c is a constant based on system geometry
P/A is the pressure divided by the area of the board
v is the viscosity
www.arlon-med.com
The P/A ratio there is 0.81! The scaled flow test (IPC
TM-650, 2.4.38A) which uses 31 psi on a 5.5”x7”
sample area is a closer approximation of the kind of
flow expected in a normal lamination process. The P/A
ratio there is 31/38.5 or 0.81!
An unexpected benefit of this information is that
given a known resin system, it is possible to adjust
laminating pressure depending on the area of the
board being fabricated to keep the P/A ratio constant.
It goes like this:
P = k * SQRT(A)
Where:
P = pressure in psi
k is a constant based on the resin system (usually
determined empirically)
A = the area in square inches of the laminated panel
If you want to laminate an 18” x 24” panel, you
can guesstimate the proper pressure as
13.2 * SQRT(18 x 24) or 18.7 * 20.8 = 390 psi.
Next column we will talk about the effect of your
process on the actual viscosity of the prepregs you
use, and how to make simple adjustments of process
based on two easy to adjust parameters: pressure and
heat-up rate.
Thanks to Paul Kyle, formerly of Arlon, for much of the
theoretical and “spade work” on rheology that helped
in our development of practical rheological tools. Paul
has moved on, but his work on rheology brought us –
and me in particular – out of the “stone age” of simple
P/A calculations, to a place where we can use
some sophisticated rheological tools to evaluate and
characterize newer resins.
(1)
Bartlett, Bloechele and Mazieka, “The Use of Scaled Flow Testing
for B-Stage Prepreg”, IPC Technical Paper TP-281, IPC Annual
Meeting, April 1979
Let’s say you are using a particular polyimide prepreg
and you laminate a 12” x 18” panel at 275 psi with
good results. So working the math:
275 = k * SQRT(12 * 18)
and so k = 275/14.7 and k = 18.7
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