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Solder Wetting Problems, Need Referee's

#34645

Solder Wetting Problems, Need Referee's | 31 May, 2005

We have some boards that exhibit poor solder wetting when reflowed. Nothing fancy, smallest pitch size .050.

See photo's at http://www.sipad.net/SnPbwetting.htm

Is this ; Contamination on the HASL finish? Insufficient solder,not enough to cover pad? Stencil = - 5% Worn out paste. (50% new added this morning)

We have several opinions but no definitive answers.

Please help.

mkehoe@sipad.net

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Scott B

#34649

Solder Wetting Problems, Need Referee's | 31 May, 2005

We have found that when the HASL thickness is less than 5 microns the tin in the HASL reacts with the copper leaving a SnCu surface with poor solderability and microscopic grainy Pb islands.

The IPC does not specify minimum HASL thickness but we now spec' a minimum limit with all our PCB vendors.

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#34651

Solder Wetting Problems, Need Referee's | 31 May, 2005

There are a host of possibilities. Virtually any fabrication process prior to outer layer manufacturing prior to HAL could contribute to dewetting. That goes all the way to excessive brighteners in the copper plating line. Anything that prevents the solder from coming in contact with the copper can cause dewetting or non-wetting.

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#34655

Solder Wetting Problems, Need Referee's | 31 May, 2005

Hi Matt,

Get definitive answers!

Contamination on the HASL finish?: From the pix you posted, it looks like something fishy is going on. I�d reject them back to the board house and demand an answer. Most board houses and purchasing people will give you that confused dog look and ask what�s wrong with them. So, to test your process, take a sample board and try tinning them yourself with an iron and some wire solder. If it still doesn�t wet after an iron, you really have a board problem. If it does tin, solder wick it flat to take a closer look. If it�s OK, try pasting and reflowing. If it reflows OK, you localized the problem to the board and there�s nothing wrong with your stencil or paste. Unfortunately you still have a board problem.

Insufficient solder, not enough to cover pad? Stencil = - 5%: Look at the past history of this product. Has this always been an issue? If so it could be your stencil, although at �5% I doubt it. If it hasn�t always been a problem, then it�s either your board or paste. Your stencil is a fixed tool and cannot change under normal process conditions. You machine set-up could cause problems too and may need to be looked at.

Worn out paste. (50% new added this morning). Well, you can�t �wear out� paste, but it can dry out. This would cause the paste not to roll and you�d get poor prints. Check your print! If it doesn�t look good, change the paste. Does it look OK with new paste? If not, there may be something wrong in your set-up or the way you release the board print from the stencil.

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