Is There a Thermal Cycle Limit?

Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting. Process Troubleshooting, Failure Analysis, Process Audits, Process Set-up CEM Selection/Qualification, SMT Training/Seminars, Legal Disputes

Phil Zarrow 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 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.

Transcript

Phil
Welcome to Board Talk. This is Phil Zarrow and Jim Hall, the Assembly Brothers also known as ITM Consulting. We're here to answer your process questions with regard to SMT materials, components, equipment, and materials. We have a theory question today.

Jim
The question is from P.W. and the wording is, "How many thermal cycles can we expose a Class 3 printed wiring assembly to before it no longer maintains a Class 3 rating?" They're talking about IPC Class 3 as spelled out in the holy Bible of our industry, IPC 610D.

This is the highest rating for critical electronics. The question continues, "In general, how many cycles should we allow at a given location on a circuit board, such as a BGA site, before we should consider the assembly compromised and thus be unreliable at any class rating?"

Now these are really good questions.

First off, we're assuming when we're talking about thermal cycles, soldering cycles, be it reflow or wave or hand soldering.

Phil
Let's say if we were putting down an SOIC, that's one thermal cycle. And let's say we have to repair it, that's another thermal cycle to remove it and another thermal cycle to replace it.

If it's on the A side of the board, it would have gone through one thermal cycle to place it on the board, and a second thermal cycle while the other side of the board gets reflowed.

Jim
So the answer is that any given site can potentially see a lot of thermal cycles and what happens? Well, one thing that can happen, particularly if you're dealing with anything but an ENIG surface, the copper tin inner metallic will continue to grow and get thicker every time you heat that joint.

We all know that the copper tin inner metallic is strong but brittle, so you want a thin layer, not a thick layer. So as you continue to heat this through repair and multiple reflow and wave soldering cycles, you stand a chance of making that inner metallic too thick and reducing the reliability of the solder joint.

This is greatly reduced if you're using an ENIG finish, electroless nickel with immersion gold, because you're not soldering to copper, you're soldering to nickel and the nickel tin inner metallic doesn't tend to form nearly as quickly.

The other issue is the circuit board. What about the materials underneath the pads? What about the adhesive material that's bonding the copper pads down to the PCB and what about the PCB materials, the laminate materials, and the vias in the same area?

Phil
The thermal cycling will affect the Z axis, as well as the XY axis.

Jim
And of course the key given here is a BGA site because when you get into BGA repair, you have one or two reflow cycles up front. Then you've got to take the BGA off and then you've got to get in and scrub the excess solder off and then you've got to put stuff down and then you have to reflow it again.

So there's a lot of concern about damage to the BGA pads in the BGA areas because when removing them, you tend to have to heat them quite intensely to make sure you melt all the joints before you pull the BGA off. The BGA repair sequence raises the bar, raises the concern.

As Phil said, unfortunately, we don't have any numbers. IPC doesn't seem to publish it but people have come up with their own numbers, although I can't recall off the top of my head the methodology they went through.

You can compromise a circuit board, a local area of a circuit board, by applying too many heat cycles and the rework cycles are particularly a concern, especially with area rays where you've got to get in and really heat them to get the parts off and then scrub them to get the excess solder off the pads before you do the repair.

P.W., you're on the right track. I'm sorry we don't have any hard numbers for you but I would say think about it and certainly think about how many BGA repairs you want to allow on a given site.

If you're talking about BGA repair, think about minimizing the intensity and duration of every one of those heating cycles because none of that heat is doing you any good. That's the bottom line with any of these materials. Extra heat cycles do you no good.

Phil
That about attempts to answer that question and we thank you for tuning in to Board Talk. And on that note, I will say a fond farewell on behalf of Jim Hall and myself, Phil Zarrow, and whatever you do, wherever you're doing it ...

Jim
Don't solder like my brother.

Phil
Please don't solder like my brother.

Comments