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dendritic growth

#31699

dendritic growth | 14 December, 2004

We have a problem with dendritic growth that is affecting a new board going into production. This is of concern because they were subjected to Damp Heat Cyclic Tests as part of the product validation testing during their development.

Fails to date

1.Fast track boards, populated and varnished. Unacceptable fail rate. Severe crystal and dendritic growth causing shorts. Oven profile checked and found to be good. Trials done at higher than recommended profile temperatures to ensure flux had been activated. Made no difference.

2.Fast track boards, cleaned then populated, and varnished. Unacceptable fail rate. Obvious crystal and some dendritic growth causing shorts.

3.Fast track boards, populated then cleaned and varnished. All passed. Little evidence of crystal growth. No evidence of dendritic growth.

Results suggest that starting off with clean boards has an effect. The boards may be dirtier than normal supply by virtue they were from a fast track supplier. However starting out with clean boards, whilst it reduces the problem, is not the solution.

Results show that cleaning the boards after population solves the problem and suggests that the solder paste and the �no clean� fluxes therein significantly contribute to the problem. However introducing cleaning in the process is not considered to be a particularly attractive solution.

Understood that lead free pastes are less prone to crystal and dendritic growth under these conditions. Also advised that non lead free components can be soldered using lead free pastes. (NB have since had conflicting information � this needs to be further checked out)

Clearly we do not want to clean after placement as this is very painful in high volume production.

Has anyone any ideas what else this might be or a better way round this problem?

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#31701

dendritic growth | 14 December, 2004

On contaminated assemblies: While you isolated part of your dendritic growth to contaminated boards, could it be that you have a similar problem with other components, rather than your flux? Is this flux new? Have you had similar problems with this flux in the past? If you soldering SMT components with this flux and you have a reasonable reflow recipe, may be the culprit is elsewhere.

On no-lead solder with leaded components: We spoke about this recently here on SMT. Search the Archives. It could boil-down to: "if it work in your application, good. If it doesn't, be wary.

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#31703

dendritic growth | 14 December, 2004

Our paste is no new we have used it before [Cobar no-clean paste]. The solder joints look like they are bursting out salty looking crystals. Maybe the board cleaning is a red herring in that if the boards are clean it stops this crystalisation traveling? Is this possible?

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#31706

dendritic growth | 14 December, 2004

Hi Paul,

One thing I can tell you for sure "lead free" is not going to solve your problem, because spectrum analysis have shown that dendrites at there final stage always consists of pure tin. The higher the tin content the higher the risk for dead shorts when conditions promoting dendrite formation are present.

I assume that the boards were fully functional when undergoing the heat cycling, what is the percentage humidity reached in the test room. Also with varnish do you mean conformal coating? If so what type are you using. Is the dendritic growth always in the same areas on the board or random, same for the crystal growth. Do you see dendritic growth without the crystals or are the crystals always present on locations with dendritic growth. How did you clean the boards with water or solvent based cleaning.

Analyzing the sequence of tests you did (and not having answers to previous questions) it looks like the paste flux is hydroscopic absorbing humidity from your test chamber, but on the other hand if the boards are conformal coated humidity shut not get to the flux. If you could give some more detail it would be easier to get too the root cause.

Best Regards, Patrick

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#31707

dendritic growth | 14 December, 2004

Yep fully functional and failed during the test. 25 to 55C in 3 hour hold for 9 hours then 55C to 25C in 3 hours hold for 9 hours then repeat. Humidity at 95 percent!

Boards only part conformally coated. Growth with and without the coating. seems to be on every solder joint. large white crystals and then dendtritic growth. sometimes white crystals look like balls embedded in the solder joint? Solvent based cleaner.

Still stumpted!

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#31708

dendritic growth | 14 December, 2004

Yep that is a tough environment and no-clean will not work. The only two solutions you have is cleaning with a better cleanable paste (No-clean doesn't clean very well) Or you can go to no-residue and skip the cleaning.

If you want to further discuss contact me at 214-350-5565

Patrick

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#31710

dendritic growth | 14 December, 2004

You may also want to contact Terry Munson at Foresite - he is very knowledgeable about the root causes of dendritic growth - you can reach him at 765-457-8095 - he can more than likely provide you with some good advice. Sara

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