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Printed Circuit Board Assembly & PCB Design Forum

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PASTE LEACHING

Sal

#4274

PASTE LEACHING | 3 May, 2000

GUYS

SEEING A PROBLEM WITH SOLDER PASTE LEACHING. THE PCB IS HASL FINISH , AND THE DEVICE IN QUESTION IS A O.O30" PITCH RESNET. THE SIZE OF THE PADS ARE 0.020 x 0.0.047 THOU , THE APERTURE SIZES ARE 0.018 x 0.042 THOU . THE LEACHING IS OCCURING DUE TO TRACKS RUNNING OF THE PADS . DOES ANYONE HAVE ANY ADVICE OUT THERE REGARDING THE PAD AND APERTURE RELATIONSHIP TO COUNTER THIS PROBLEM ?

SAL

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#4275

Re: PASTE LEACHING | 3 May, 2000

Sal: Sorry, sometimes (more often than I�d like to admit) I�m a bit dense. But first �

Leaching. Dissolution of a metal coating into liquid solder. Leaching applies to liquid solder and is applied inaccurately to metallurgical effects, like diffusion, that occur after the solder has solidified.

Please take us through a description of the symptoms again. What�s the deal with the traces and the pads? One more time.

Hallllp Dave F

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Sal

#4276

Re: PASTE LEACHING | 4 May, 2000

THE TERMINONOLY I USED SEEMS TO BE INCORRECT , I THINK THE TERM I WAS LOOKING FOR IS WICKING . BACK TO THE PROBLEM , ISSUES WITH RESNETS, WHERE THE SOLDER PASTE SEEMS TO BE WICKING AWAY FROM THE TERMINATIONS OF THE COMPONENT TOWARDS THE VIAS WHICH ARE CONNECTED TO THE ENDS OF THE TRACK WITH THE TRACKS RUNNING OF THE PADS , LEAVING INSUFFICIENT SOLDER PASTE AND CAUSING OPEN CIRCUITS ? THE SIZE OF THE PADS AND THE APERTURES ARE AS MENTIONED ON THE PREVIOUS MAIL. WHAT CAN I DO TO MINIMISE THIS PROBLEM ?

SAL.

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Boca

#4277

Re: PASTE LEACHING | 4 May, 2000

Solder 'follows' heat, sounds like the fab is heating up faster than the component. Or the components are solderability 'chanllenged' and the paste turns liquid, and would rather spead out on the land than wet to the component termination.

Are these leaded Rnets?

Boca

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#4278

Re: PASTE LEACHING | 6 May, 2000

Sal: Now you�re talkin�, buddy!!!

Darrell is correct. All things being equal, thermal relieved vias (and through holes) should be the hottest spot on the board at reflow temperatures. Vias are gonna take on solder like drippin�s take to biscuits. And that�ll reduce the reliability of your product, and that�s in addition to the problems created by your poor solder connections.

So, the via wets with solder better than the component leads. My take on the source of the problem is one of the following: * Amount of solder paste (Well, not really, but Sal want�s to talk about this one) ;-) * Solder paste activity * Component solderability, relative to the activity of the flux of the paste you are using. * Less than optimal profile * Design of the board.

There that�s it!!! Covered all the bases. (Followed by the sound of wiping hands, patting each other on the back, and light patter of polite applause from the bank holiday crowd.)

OK ... OK, let�s go through each one. AMOUNT OF SOLDER PASTE: You probably have enough solder paste on the board, before reflow, assuming you paste is close to 50% metal, by volume and the foil is at least 6 thou. The aperture reduction of 10% from the pad size of your 30 pitch resistor networks is not necessary. On the other hand, a reduction of this amount is not uncommon and unlikely to be the cause of the insufficient solder paste and open circuit problem, you discuss. Even if there was too little paste, the component lead should still wet and complete the connection. Sal, put down from that aperture increasurometer and step away!!!! ;-) SOLDER PASTE ACTIVITY: Ahma guessin� that the solder paste is a little wimpy (prob�ly soma thet North Carolina stuff), but probably OK, because you�re suggesting that only the leads on the resistor networks are soldering poorly. Sal, you�re not holding back on us are you?? ;-) COMPONENT SOLDERABILITY: Kinda goes hip and elbow with the previous item, but if ah had to point stubby my finger at anything as being the bad boy in this, component solderability would be the one. WACKED PROFILE: If you run a long preheat and a steep ramp, you could be burnin� off your (whimpy) flux everywhere (especially your slightly corroded resnet leads) �cept the vias. Have you profiled this board to be sure your oven temperatures are good (match-up well with supplier recommendations)? DESIGN OF THE BOARD: It would be nice to have a 20 thou solder mask dam between the pad and the via, anna, may be even covering the via. It also would be nice if these vias were removed, may be 20-30 thou, from the pads.

My2� Dave F

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