Electronics Assembly Knowledge, Vision & Wisdom
Acceptable Rate for Head in Pillow?
Acceptable Rate for Head in Pillow?
We have rejects in 2-3 per 15,000 PCBs that look like Head in Pillow. Should we tinker with the reflow process, or is this reject rate acceptable?
Board Talk

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Board Talk is presented by ITM Consulting

Phil Zarrow
Phil Zarrow, ITM Consulting
With over 35 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis. He has vast experience in SMT equipment, materials and processes.

Jim Hall
Jim Hall, ITM Consulting
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.

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And welcome to Board Talk with Jim Hall and Phil Zarrow, the Assembly Brothers, here to talk about assembly processes - equipment, materials, and other things.

And speaking of other things, today I see we have a solder defect question.

Question is, "We have rejects that look like Head in Pillow. We see rejects in 2-3 per 15,000 PCBs. Should we tinker with the reflow process, or would you consider this reject rate acceptable?"


Head and Pillow is a real issue. First up I want to say before you think about tinkering with anything, are you using an up-to-date solder paste, because solder paste manufacturers have done significant work to try to minimize this with newer formulations.

As to what is an acceptable rate, it's very hard to say with the numbers quoted, 2 or 3 rejects per 15,000 printed circuit boards.

So, that would be about 1 in 5,000 circuit boards or about how many parts per 5,000. That's 3,000 parts per million. That's not too bad.

That's a three sigma process.

But that's not really the way we would define defects, because if we're talking head and pillows occurring on BGA parts, all of which have multiple balls, any one of which could have the defect.

So, the proper way to do this would be to define it not per number of PCBs, but for parts per million opportunities, so you would have to count all of the BGA balls on those 15,000 PCBs.

Since that's probably greater than 100, that would drop that down to about 30 parts per million, and that is certainly a very low defect level to try to go optimize just in general optimization strategies.

So, congratulations, A.B. The answer is probably not to tinker with the reflow process.

Movement towards continuous improvement is good and trying to fix the defect rate would be good, but it may be things beyond your immediate control.

If you really want to address a defect such as head and pillow, which is going to become more common, because there's going to be more and more BGA parts, thinner BGA parts, which are going to warp more - seriously look at getting an up-to-date solder paste.

That's true for all of the newer types of defects, such as graping, and so forth. Solder paste technology is evolving daily or at least monthly, and you may be making your life a lot harder than you have to by not evaluating a modern, up-to-date paste that could significantly reduce your defects such as head and pillow right out of the box.

Well, I think we answered that question hopefully to AB's satisfaction.
Reader Comment

From my point of view, the warpage of BGA during soldering is the most important source of Head in Pillow! That means the component supplier is responsible for such a problem.

The question is this rate acceptable or not should also to be discussed under the view: Do we find all the Head in Pillow Defects? I think, not at all. A little bit more printed solder paste can be help to minimize this problem, but this is not the root cause.

Heinz Wohlrabe, TU Dresden, Germany
Reader Comment

One consideration not addressed is the detection of the defect. The HiP defect is very difficult to detect since you need to have an open or significant change in joint geometry for x-ray to detect. The marginal conditions that escape inspection can lead to latent field failures which in some product markets can be catastrophic.

Kirk Van Dreel, Plexus Corp., USA
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