Material Witness: Low-Flow Prepregs–Defining the Process


Reading time ( words)

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.)

 

guiles new Fig1.JPG

Guiles Fig2.JPG

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 5oC/minute, and a standard epoxy low-flow product—47N—with  a minimum viscosity about 8000 poise at heat-up rate of 5oC/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. 

Share

Print


Suggested Items

New Book From Isola Highlights Importance of Material Selection

01/14/2022 | I-Connect007 Editorial Team
In "The Printed Circuit Designer’s Guide to... High Performance Materials," the latest release from I-007eBooks, readers will learn how to overcome challenges associated with choosing the right material for their specific application. Author Michael Gay of Isola provides a clearer picture of what to know when determining which material is the most desirable for which products. PCB materials and DFM expert Mark Thompson says, “I love this book, particularly the sections on the effects of the glass weave, the history of laminate, and the difference between Dk and effective Dk."

Living in a Material World: High-Speed Design Strategies

01/13/2022 | I-Connect007 Editorial Team
Any discussion about high-speed PCB design techniques would be incomplete without considering the properties and requirements of the materials. Your material selection drives much of your design strategy when you’re operating at 28 gigabits per second or faster. We recently spoke with high-speed design expert Lee Ritchey of Speeding Edge, and electronic materials veteran Tarun Amla of Avishtech and Thintronics, about the relationship between advanced PCB materials and high-speed design techniques. They discuss the challenges facing designers and engineers working with materials at speeds that were considered unreachable not long ago, and what designers need to know about material selection as board speeds continue rising toward the stratosphere.

RealTime with... American Standard Circuits: Thermal Management

12/16/2021 | Pete Starkey, I-Connect007
In the third of a series of three RealTime with... interviews, I-Connect007 managing editor Nolan Johnson received knowledgeable and informative answers from Anaya Vardya, John Bushie, and Dave Lackey of American Standard Circuits to his questions on the topic of thermal management. Anaya Vardya began by clarifying the terminology, describing thermal conductivity as a material property defining how quickly heat was transmitted through a piece of that material, whereas thermal management was about analysing the entire system, trying to understand how much heat was being generated, and using appropriate techniques to dissipate that heat as efficiently as possible.



Copyright © 2022 I-Connect007. All rights reserved.