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Low Melt Solder Chemistry for use in Rework Process

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#47303

Low Melt Solder Chemistry for use in Rework Process | 7 February, 2007

After some precursory experimentation, I am planing on employing a form of low melting temp solder for use in reworking some very dense, copper core PCB's, both SMT and THT, which were originally mounted with 63/37 solder.

The main problem with these assemblies is that they are designed to be efficient heat-sinks, and as such, do not lend themselves well to typical rework processes or even high-wattage soldering equipment. Further, there are some delicate components nearby (LED's) which can not tolerate heat above 120C for any extended amount of time and the thermagon board material does not share the typical pad adhesion strength o an FR-4 PCB.

The experimentation mentioned was completed with Zephyrtronics "Low Melt" solder. The alloy was used in conjunction with a preheater and temps aroun 100C to cause cometalization to occur between the low melt solder and the 63/37 in order to remove components at lower temperatures than normally possible, hence minimizing the risk of lifting lands and damaging barrels. I have a sample of "Chip Quik" coming in soon as well, but I would like suggestions on similar products to try from those who may be employing something similar or who may be up against similar challenges. The proprietary solutions I mentioned which are marketed as such seem extremely expensive, though we are willing to pay the cost if necessary to save our product.

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#47324

Low Melt Solder Chemistry for use in Rework Process | 7 February, 2007

Bismuth is a low-melting metal with poor mechanical properties; if you melt the bismuth wire into a standard tin-lead solder joint, then the bismuth alloys with the solder, and forms a material that will crumble if you scrape it with your fingernail. The idea is that if you mix bismuth into a solder joint that you want to remove, then the joint will become so mechanically weak that you can pull the component off with your fingers, and the joint will break, with no damage to the component or the board. At that point you can clean the lands with flux and copper braid. [Electronic Parts and Construction; Jonathan Westhues; jwesthues at cq.cx]

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RDR

#47325

Low Melt Solder Chemistry for use in Rework Process | 7 February, 2007

Be sure that this component temp during operation does not exceed the low temp of the solder. (it has happened in similar situation)

Very good idea though

Russ

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#47332

Low Melt Solder Chemistry for use in Rework Process | 7 February, 2007

Thanks Russ and Dave.

Russ-

I was planning on thoroughly cleaning out the remainder of the low temp solder and going back to 63/37 for re-installation of a new device (our company does not ever re-use a device). Given these circumstances, would you still be concerned about the quality of the reworked joint?

Dave-

Should I try to find a 100% Bizmuth solder wire, or is there a specific formulation you might recommend in going for this fracturing method you outlined?

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RDR

#47333

Low Melt Solder Chemistry for use in Rework Process | 7 February, 2007

I would not be concerned about the reliability when doing this, I re-read your post and am familiar with this type of process.

Russ

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#47335

Low Melt Solder Chemistry for use in Rework Process | 7 February, 2007

If you can't find bismuth wire, use 42Sn58Bi.

For more, look here: http://www.mrs.org/s_mrs/bin.asp?CID=2274&DID=48689&DOC=FILE.PDF

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greg york

#47336

Low Melt Solder Chemistry for use in Rework Process | 7 February, 2007

or low melt Sn/Pb/Bi alloy, readily available and fairly cheap.If your UK based I will send you a sample if it will help you prove it cheers greg

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