Material Witness: Low-Flow Prepregs–Defining the Process

Estimated reading time: 5 minutes

What this looks like is shown in Figure 3.  The sample involves three pieces of prepreg into which are punched two 1” diameter holes, as shown.  After test, the resin has flowed into the circles (irregularly as shown in the middle diagram) and the average reduction in diameter of the circle as measured along several diameters is defined as the “flow.  A typical low-flow product may flow into the holes in a range of 0.030” to as much as 0.150” depending on the grade and type. Measuring this manually has proved to have a great deal of inherent variability (as much as +/- 30% of nominal!), so use of a computerized automated measurement system as is indicated by the test coupon on the far right has been developed in which 500 to 1000 individual measurements are taken around the “diameter” of the flow bead and a statistical “best fit” circle is defined to determine the flow.

 Although we have gotten something of a handle on the measurement method, the test itself remains somewhat variable, and correlation between test presses and between test facilities remains problematic.  To be practical as a “real time” manufacturing test, the test procedure needs to be able to be completed in a relatively few minutes.  The quality of die punched holes in the prepreg is critical, since any damage to prepreg edges will result in irregular flow.  The IPC method also results in unrealistically high heat-up rates (several hundred degrees F per minute!) and not unexpectedly, irregular flow. 

Users who employ test procedures based on normal PWB manufacturing processes with heat-up rates around 10^oF/minute get better results, but the testing takes as long as a normal press cycle, far too long for a real-time prepreg manufacturing test.  So what happens?  We test using the IPC procedure.  Many of our customers test in a realistic process simulation.  And there is (surprise, surprise!) often poor correlation and the potential for issues in terms of how and whether specs have been met.

One of the unintended consequences of test methods that relate only marginally to in-use parameters is that individual products (rather than generic slash sheet designations) become locked into processes because engineers and shop floor people become familiar with their use and make the necessary adjustments in pressure and temperature, prepreg cut-backs, etc. so that they will work with a variety of designs.  They come to have the belief that the product itself is infinitely process-flexible, and so anything new seems never to work quite like “Product X.”  Different products, even if they are “the same” according to IPC testing (remember, this test uses a heat-up rate of several hundred degrees F/minute 200 psi on a 5.5 x 7 inch test specimen), do not necessarily work the same way in a real PWB process and the only way to really get the best out of any low flow product is to work with it in your own process until you are sufficiently familiar with it to make it jump through hoops.

I’m sure there are a few “miracle” prepregs out there that have inherent organic bio-feedback loops that adapt flow and viscosity to the specific design being manufactured, but for the most part we in the business have to be constrained by the laws of chemistry and physics, the limitations of human-designed processes, and the constraints of standard testing.  Doing “the best we can with what we’ve got” is not a cheap excuse to avoid getting better; over the years we’ve improved materials and methods, and so have the guys producing PWBs.  Working together we can evolve newer and better materials, provided we are willing to tune our processes to get the best out of them.

A topic for the future:  How low-flow materials work in-process and what kinds of modifications of flow and viscosity have been made to open the process window with a minimum of pain.

Chet Guiles is a consultant for Arlon Electronic Materials. 

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