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Poor Solderability on TSOPs with Alloy 42 Leadframes

#35077

Poor Solderability on TSOPs with Alloy 42 Leadframes | 22 June, 2005

Our manufacturing site has been struggling with soldering Alloy 42 TSOPs. Currently, we have to hand solder in one of our applications. Here are the particulars.

Component: 54 leads, Sn plated, 400microinches +/-200.

Process: Eutectic 63/37 Sn/Pb solder, mildly activated flux. 220C peak temp, over 183C for approximately 90 seconds.

PCB: HASL. Standard laminate and Tg. CTE of PCB is approximately 16ppm/C in both X and Y directions.

Activities we are working on to eliminate this issue:

Changing to higher Tg laminate.

Wetting balance testing to determine if we should change flux.

Analysis of module to determine if we can run to peak of 225C.

We have done many tests, analyses, etc. and are really frustrated with this problem. Does anyone have a suggestion on how to alleviate this problem? We cannot change to a Cu leadframe part because it is not offered.

Thanks.

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#35081

Poor Solderability on TSOPs with Alloy 42 Leadframes | 22 June, 2005

Not sure what a higher Tg laminate will accomplish. Can't hurt though.

Remember, when soldering to Alloy 42, the surface is 42% nickel. So, you need to raise the reflow temperature some 15 to 20*C over what is needed for copper. Also, sometimes Alloy 42 is not solderable at all. So, you are simply encasing the gullwing feet in solder without any metallurgical bond.

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KEN

#35104

Poor Solderability on TSOPs with Alloy 42 Leadframes | 23 June, 2005

....how does the part perform in a wetting test. Is it solderable on a bench or dip-plating in a pot? If you add flux and heat can you fix your problem assemblies? If yes, plating finish or substrate is not your problem.

What flux are you using?

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Paul

#35205

Poor Solderability on TSOPs with Alloy 42 Leadframes | 28 June, 2005

I'm with DaveF on this one. I think you'll be trying solve this one for a long time. Alloy42 just doesn't like to be soldered.

What exactly is the issue? Lack of a toe fillets at on the cropped lead? There is no spec for a toe fillet other than to say there should be some wetting evident (check IPC standards). It's very subjective. If you have good heel and side fillets then you have a good joint. A toe fillet is merely a cosmetic indicator. It doesn't add any strength to your solder joint.

We have tried everything to solve this and the only solution was to plate the toe after cropping which is obviously not a practical solution. We then went down the road of trying to find out what the consequences of having no toe fillet were and discovered through pull testing that a toe fillet added very little to joint strength.

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GS

#35233

Poor Solderability on TSOPs with Alloy 42 Leadframes | 29 June, 2005

Several years ago, we had almost same pbm, best results have been obtained by using water soluble paste (those time the old AM1208). Any way, to get a good toe meniscus is not so easy.

Talking about CTE, keep in consideration the internal die of TSOP (Type I ?) has pretty large area and the CTE is very low 4-5ppm/C* and this some time could generate local CTE mismatch with PCB'laminate. But this should not effect the solder joint (!?).

Regards

GS

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Rob

#35283

Poor Solderability on TSOPs with Alloy 42 Leadframes | 4 July, 2005

Hi,

I don't know if it will help but we talked about soldering Nickel on this thread:

http://www.smtnet.com//forums/index.cfm?fuseaction=view_thread&CFApp=1&Thread_ID=8051&#Message31803

where we used Cobar solder paste for soldering all types of nickel strips to PCBs with great success (well, they passed all sorts of pull tests).

Cheers,

Rob.

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