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Eutectic Solder On A Lead Free HASL PCB

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#59561

Eutectic Solder On A Lead Free HASL PCB | 19 August, 2009

I have a customer who approved and asked us to process PCB's built with Lead-Free HASL Finish using Eutectic Solder. We did as directed and inspected the result from a 10-PCB Run and couldn't find any problem with solderability or issues.

What Issues do I expect to see? Is there any impact on the PCB Reliability? What about the solder joint strength of the component between the Pad that has Lead Free Hassle?

Your inputs are very much welcome!

Thanks and regards,

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#59562

Eutectic Solder On A Lead Free HASL PCB | 19 August, 2009

You say, "using Eutectic Solder." * There are lots of eutectic solders. * When melted, a eutectic solder goes directly from a solid to liquid phase with no pasty phase.

Do you mean 63/37 tin lead solder?

If so, probably your assembly won't be RoHS. More directly, the reliability of your solder connections depend on the alloys of the solder and the surface HASL. Larger number of materials have increased the uncertainty. Studies by Sandia Lab have show that lead contamination of lead-free solder connections at levels below 8% show little solder joint degradation.

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#59565

Eutectic Solder On A Lead Free HASL PCB | 19 August, 2009

Thanks Sir Dave. Yeah my apologies. To clarify, we built the product PCB whcih has a Lead-Free HASL Finish using a Sn63/Pb37 Solder paste.

My major concern regarding reliability is that my profile whish is between 215'C-225'C was insufficient to reflow the Lead Free Finish on the surface of the PCB. What happens on the layer between the bonding of the Pad which is Copper then Lead-Free HASL then Tin-Lead Layer. Or what sort of Intermetallic Layer does the PCB has now? Is it reliable in both long and short term?

anymore comment from other experts will be of great Eye Opener.

Thanks and regards,

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#59581

Eutectic Solder On A Lead Free HASL PCB | 20 August, 2009

Responding to your points: * The intermettalic layers [ie, Cu6Sn5, Cu3Sn] between the solder and the copper pad is the same composition with both lead free and leaded solder. The lead in solder has no impact on the intermetallic. * As you say, increasing the amount of tin in a solder alloy requires a higher the peak temperature to properly reflow the solder connection than you would use in tin lead solder reflow. * It's tough to comment on the reliability of solder connections. Bottom-line: we all know the lead free solder connections are less reliable than tin lead solder connections. If you form a proper solder connection and don't exceed the above guideline, you'll be OK from a reliability standpoint in relatively benign end-use environments.

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#59583

Eutectic Solder On A Lead Free HASL PCB | 20 August, 2009

Hi davef,

thanks again for that enlightenment. I'm just curious about what's happening with the solder joint of the said process. Again to add to my unending queries for now:

So on top of that copper is a thin layer of lead-free solder composition that melts at 217'C and requires at least 230'C or so at certain amount of time to dwell to achieve good solder joint.

On top of this Lead Free solder is the Tin-Lead 63/37 Solder that was reflowed at 225'C Max with a Minimum of 215'C.

During the process, the 2 different solder were at liquidous when they were at 217'C. What happened with the Compounding of this 2 solder? What's the effect? Will they merge together and produced a good bond? Or Do I have a cold solder joint layer or a solder joint layer that is half processed - mainly the lead free solder.

regards,

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#59586

Eutectic Solder On A Lead Free HASL PCB | 20 August, 2009

It's a safe bet that your solder alloys will merge and form a proper solder connection at ~230*C for 15 to 20 seconds. At process conditions with less time and temperature, it's a lot of bla-bla-bla. In order to understand the quality of the solder connection at less than ideal processing, you need to cut, polish, and analyze the solder connections.

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#59625

Eutectic Solder On A Lead Free HASL PCB | 24 August, 2009

Don't make that assumption. Once we had concerns about PCB's not matching what was ordered. I said "if we got HASL instead of ENIG, how do we know we got lead-free?" I mean if one thing got mixed up in translation, maybe two did. So we put a bare PCB on a rework station to check the melting temp of the pads. It was well below 217C. Actually just above 183C. We took the boards to a nearby lab for analysis. It was pure tin on the pads. No lead at all.

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