Material Witness: Low-Flow Prepregs–Defining the Process


Reading time ( words)

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.

Guiles Fig3.JPG 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 10oF/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


About Arlon Arlon Electronic Materials Division, now part of Rogers Corporation (www.rogerscorp.com), is a major manufacturer of specialty high performance laminate and prepreg materials for use in a wide variety of printed circuit board (PCB) applications and in several distinctive markets. http://www.arlon-med.com

Share

Print


Suggested Items

AltiumLive Munich: Day 1 Keynotes

01/28/2019 | Pete Starkey, I-Connect007
The weather forecast was wrong! Despite my apprehension and winter clothes, there was very little snow at the Hilton Munich Airport. It could have been any season of the year inside the splendid convention facility, which was also the venue for the second European AltiumLive design summit. AltiumLive brought together a family of over 220 electronics engineers and designers eager to learn from top industry experts and applications specialists who were equally eager to share their knowledge and experience freely.

The Quest for Perfect Design Data Packages

01/18/2019 | Barry Matties, I-Connect007
There’s an ongoing problem in the PCB industry: fabrication shops are receiving incomplete or inadequate design data packages, leaving manufacturers scrambling to fill in the blanks. For a quick-turn prototype shop like Washington-based Prototron, with over 5,000 customers and up to 60% of orders coming from new customers each month, that can add up to a lot of wasted time and effort just in the quoting stage. Dave Ryder, Prototron president, and Mark Thompson, engineering support, delve into this continuing issue and more.

A Fractal Conversation with Jim Howard and Greg Lucas

01/15/2019 | Barry Matties and Andy Shaughnessy, I-Connect007
Veteran PCB technologists Jim Howard and Greg Lucas have made an interesting discovery: Certain shapes of copper planes make a PCB run more efficiently than other shapes, a process they dubbed fractal design. It doesn’t appear to cost a penny more, and testing suggests that fractal design techniques could eliminate edge noise. Barry Matties and Andy Shaughnessy asked Jim and Greg to discuss the fractal design process, and the advantages of using this technique.



Copyright © 2019 I-Connect007. All rights reserved.