Electronics Assembly Knowledge, Vision & Wisdom
Tin-lead Reflowed at Lead-free Temps
Tin-lead Reflowed at Lead-free Temps
We reflowed a batch of tin-lead circuit boards with a lead-free profile at higher temperatures than specified. Would you scrap these board assemblies? The Assembly Brothers, Phil Zarrow and Jim Hall, discuss this topic.
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.
Transcript

Phil
Welcome to Board Talk. This is Phil Zarrow and Jim Hall, the Assembly Brothers, also members of the ITM Consulting elite team. We're here to attempt to solve various process questions and problems. And what's today's dilemma, Jim?

Jim
It's about using a lead-free profile on a tin-lead assembly.

The question, "We recently reflowed a batch of tin-lead soldered circuit board assemblies with a lead-free profile. This was a mistake.

The lead-free profile temperatures were significantly higher than specified for these tin-lead assemblies. The assemblies appear fine, but we're concerned about reliability. Would you scrap these board assemblies? What additional testing can we do to confirm the long-term reliability of these assemblies?"

Phil
I'll start off by saying before you get into the testing, you might have to do a bit of research.

We're assuming that the joints look okay from visual inspection and there is no conspicuous blistering of boards, or parts, or strange chemicals oozing around.

So starting at that point, visually, at least, things look good. The concerns with the metal, Jim would be?

Jim
If you reflowed tin-lead solder it should be able to go to that higher temperature if they soldered well on the way up. My concern would be time above liquidous. You have to take that profile, and recalculate the time above liquidous for 183 C, instead of 217 C.

The higher temps might have exceeded your specifications, and then you would be concerned about inner metallics forming on your joints, unless you have a nickel-gold board, in which case, that would not be a problem.

The real concern that we have are components and the printed circuit board. For components, you should be able to go back to your spec sheets, and find out what the peak temperature rating is, and what the moisture sensitivity rating was.

If you had a part that was only rated as a tin-lead part, at a level three, and it was exposed for six days, so you're near the seven day limit for tin-lead, then you may very well have exceeded the moisture sensitivity limit by going to the higher temperature.

The only way to test that is a scanning electron microscopy. That is really the only testing that I know, short of accelerated life-testing them, and testing them to failure, and see how they correlate.

That's a really long, expensive, and rigorous process. So should you throw them away? It depends on the reliability requirements, but I think you can get most of the way by looking at the material specs on your components, and your circuit board.

What was the T-Sub-G of the laminate used? And then, as Phil said, visual inspection, looking for blistering on the boards, bad solder joints, warping, cracking of your components due to moisture sensitivity. But regarding testing, there's not a lot of non-destructive testing you can do.

Phil
Right, okay. This has been Phil Zarrow and Jim Hall, we are the Assembly Brothers, pick and place.

Jim
Whatever you do, don't solder like my brother.

Phil
And don't solder like my brother.

Jim
And keep those kids away from the flux pot.

Reader Comment

Most components these days are tinned with lead-free metal, i.e. tin. So in fact they SHOULD be taken up to 217degC minimum to get proper flow. A standard hot air reflow machine set for tin-lead typically overshoots to around 220C or higher, and this is actually GOOD! if it didn't then you would get incomplete flow and some bad joints between the tin on the components and the tin-lead solder paste.


Steve OBrien, Controlex
Reader Comment

You don't mention the peak temperature of the lead-free reflow but it is one of the main concerns for assessing the risk to the ICs.

You may have lucked out in that many of the material sets for leaded ICs may be the same ones used for lead-free ICs but you'd need to check with IC suppliers on this. The JEDEC standard for leaded ICs is 235C peak for smaller ICs and 220C peak for larger ones.

J-STD-020D lead-free profiles has different categories with it for peak reflow ratings that is based on semiconductor package volume (260C smallest, 250C or 245C for largest) but in practice, board reflow temperature may be constrained by the solder paste manufacturer not by the components. I have been surprised to see 235C to 240C as peak temperatures for some board manufacturers.

Thus a 240C peak temperatures may be survivable for smaller lead-free ICs even if rated for 235C peak. However, larger IC components rated for 220C peak may need SAM assessment irrespective of floor life as the temperatures may cause internal delamination / damage.

However note that JEDEC leaded vs lead-free, has different maximum exposures times for example time at peak may be longer for lead-free so don't just look at peak temperatures.

I cannot answer for the metallurgy of the lead solder joints but you could do some cross sections of accidental reflow vs intended reflow to see if there is any noticeable differences. You should focus on any BGA solder joints.


Allan Webber, Texas Instruments
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