-
- News
- Books
Featured Books
- design007 Magazine
Latest Issues
Current IssueLearning to Speak ‘Fab’
Our expert contributors clear up many of the miscommunication problems between PCB designers and their fab and assembly stakeholders. As you will see, a little extra planning early in the design cycle can go a long way toward maintaining open lines of communication with the fab and assembly folks.
Training New Designers
Where will we find the next generation of PCB designers and design engineers? Once we locate them, how will we train and educate them? What will PCB designers of the future need to master to deal with tomorrow’s technology?
The Designer of the Future
Our expert contributors peer into their crystal balls and offer their thoughts on the designers and design engineers of tomorrow, and what their jobs will look like.
- Articles
- Columns
Search Console
- Links
- Media kit
||| MENU - design007 Magazine
Material Witness: Low-Flow Prepregs–Defining the Process
March 19, 2015 | Chet GuilesEstimated 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 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.
Page 2 of 2Suggested Items
Real Time with... IPC APEX EXPO 2025: Tariffs and Supply Chains in U.S. Electronics Manufacturing
04/01/2025 | Real Time with...IPC APEX EXPOChris Mitchell, VP of Global Government Relations for IPC, discusses IPC's concerns about tariffs on copper and their impact on U.S. electronics manufacturing. He emphasizes the complexity of supply chains and the need for policymakers to understand their effects.
The Chemical Connection: Surface Finishes for PCBs
03/31/2025 | Don Ball -- Column: The Chemical ConnectionWriting about surface finishes brings a feeling of nostalgia. You see, one of my first jobs in the industry was providing technical support for surface cleaning processes and finishes to enhance dry film adhesion to copper surfaces. I’d like to take this opportunity to revisit the basics, indulge in my nostalgia, and perhaps provide some insight into why we do things the way we do them in the here and now.
NUS Physicists Discover a Copper-free High-temperature Superconducting Oxide
03/28/2025 | PRNewswireProfessor Ariando and Dr Stephen Lin Er Chow from the National University of Singapore (NUS) Department of Physics have designed and synthesised a groundbreaking new material—a copper-free superconducting oxide—capable of superconducting at approximately 40 Kelvin (K), or about minus 233 degrees Celsius (deg C), under ambient pressure.
AT&S Sets New Standards in the Recycling of Copper and Chemicals
03/25/2025 | AT&SAT&S has been working for years to reduce the ecological footprint of its production sites worldwide with a comprehensive sustainability strategy and considerable investments.
Empowering the Future of Advanced Computing and Connectivity: DuPont Unveils Innovative Advanced Circuit Materials in Shanghai
03/24/2025 | DuPontDuPont will showcase how we are shaping the next generation of electronics at the International Electronic Circuits (Shanghai) Exhibition 2025.