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

Stitching Capacitor: Crosstalk Mitigation for Return Path Discontinuity

06/13/2019 | Chang Fei Yee, Keysight Technologies
When the return path is broken due to the switching of reference planes with different potential, e.g., from ground to power or vice versa after layer transition on PCB, the return current might detour and propagate on a longer path, which causes a rise in loop inductance. This might lead to the sharing of a common return path by different signals that pose a high risk of interference among the signals due to higher mutual inductance. This interference results in signal crosstalk. To mitigate the crosstalk due to return path discontinuity (RPD), stitching capacitors are mounted on the PCB to serve as a bridge between the two reference planes of interest on different PCB layers.

Technically Appropriate Material Choices are Key to Design Success

05/16/2019 | Nolan Johnson, I-Connect007
Materials are no longer a passive part of the design; they play an active role in the manufacturability, reliability, and speed of a PCB. I-Connect007’s Nolan Johnson and Mike Creeden, founder of San Diego PCB Design, discuss several key characteristics that designers should consider in their material selection process.

EM Modeling: The Impact of Copper Ground Pour on Loss and Impedance

05/02/2019 | Chang Fei Yee, Keysight Technologies
This article briefly introduces the general purposes of copper ground pour on printed circuit boards. Subsequently, the impact of copper ground pour on PCB channel loss in terms of insertion loss and impedance in terms of time domain reflectometry (TDR) is studied with electromagnetic modeling using Mentor HyperLynx.



Copyright © 2019 I-Connect007. All rights reserved.