Material Witness: Low-Flow Prepregs–Defining the Process

Estimated reading time: 5 minutes

One of my favorite authors once wrote about perception:  “For now we see in a mirror, dimly…” speaking in an era when mirrors were at best polished metal surfaces.  In the infancy of low-flow products, we used to speak about “no-flow” prepregs as if “no-flow” was sufficient definition of the product, but as we have recently pointed out in our discussions of rheology, “everything flows.” And as we consider these increasingly essential but sometimes hard-to-define products, much consideration will have to be given to what the products need to do, how they do it, how we test them, what the testing means, and how or if the testing relates meaningfully to how the products work in a PWB rigid-flex production environment.

Let’s try to define “low flow” in terms that will make sense to both suppliers and users of the products.  A low-flow prepreg is a prepreg that flows sufficiently to wet out and adhere to bonding surfaces and to fill inner layer copper details, but does not flow so much as to fill in cut-out areas in a heat sink or run unevenly out of the interface between rigid and flexible elements of a rigid-flex PWB.  That being said, how to define that flow quantitatively and to control it in such a way that the resulting product has wide applicability in a variety of PWB heat-sink and rigid-flex designs has been an issue with both producers and users of the products since the introduction of the concept.  (My personal involvement in low flow materials began in the Early Mesozoic Period.)

How low is low flow compared to “normal” prepregs?  Figure 1, the chart labeled as 35N Rheology, and Figure 2, 47N Rheology, are respectively a standard polyimide prepreg—35N—with a minimum viscosity about 800 poise at heat-up rate 5^oC/minute, and a standard epoxy low-flow product—47N—with  a minimum viscosity about 8000 poise at heat-up rate of 5^oC/minute. As you can see, the viscosity of the low-flow product is about an order of magnitude higher than that of the full-flow prepreg. 

Note also that as the heat-up rate is adjusted, the minimum viscosity of the low-flow product behaves similar to that of a standard-flow resin.  As the heat-up rate increases, the minimum melt viscosity is lower, and hence, the product will flow more, all other conditions being equal.  If only it were practical to use a three-temperature ramp rate test to characterize low-flow products during manufacture! 

Instead, we have an IPC test procedure (IPC TM-650 2.3.17.2) that defines low flow in terms of average reduction of the diameter of a cut-out circle when the material is tested under “standard” conditions of temperature and pressure. Page 1 of 2

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