Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

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SMT electronics assembly manufacturing forum.


Printing problem, water soluble paste with gold plating PCB..

Frank

#7439

Printing problem, water soluble paste with gold plating PCB.. | 28 July, 2001

Hi guy

Does anyone have experiance with printing problem between solder paste water soluble 62/36/2 and gold plating PCB? I do have problem about insufficient solder after printing although I used the same printing parameter as when I print with another solder paste or PCB. The paste look properly roll but it seem like paste can not hold on gold surface. So i have to decrease saperation speed to very slow. Do I have to change solder paste or not? If you know what the problem Pls, let me know

Thank.

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#7443

Printing problem, water soluble paste with gold plating PCB.. | 29 July, 2001

You have provided very little information.

Two similar possibilites come to mind: poor pad solderability protection and old boards.

POOR SOLDERABILITY PROTECTION: Maybe the nickel plating under the gold is oxidized, contaminated, or otherwise very difficult to solder.

What are the specified and actual thickness of the gold, nickel, etc plating on your pads? Typical ENIG thickness specification: 3-8 uinches of gold and 150-250 uinches of nickel.

If the gold is too thin, the nickel can oxidize. If the gold is too thick, the solder connection can become brittle.

OLD BOARDS: If the solderability protection specification and supplier performance is accpeptable, boards that have exceed their shelf life produce similar results.

If either of these apply, you need a paste with more agressive flux. Talk to your supplier for suggestions.

Please provide information about your process, boards, and solder paste.

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Frank

#7450

Printing problem, water soluble paste with gold plating PCB.. | 29 July, 2001

Hi Dave

Thanks for your information. The problem that I found is paste not stick well on pads ( solder paste printing process). When the table move down (PCB saperate from stencil) some of paste stick on aperture instead pads. Solder paste : Delta 738S2 water soluble. PCB : FR4 1.6 mm thickness. Surface protection : Soft gold 0.2 um Solder mask : Photo-definable liquid film 15-30um I'm in doubt becuase with the same printing parameter this paste can be easily print on another PCB surface.

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mugen

#7451

Printing problem, water soluble paste with gold plating PCB.. | 30 July, 2001

Best thing to do, is consult your solder paste supplier, bring him/her in, and ask for their humble professional opinion? If they refuse, strongly consider switching to a supplier, with responsive "humble professional opinions" to share, one that springs to mind, is "OMG microbond" paste suppliers, they have excellant ranges of customized paste, at normal market $$$ ranges....

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Brian W.

#7481

Printing problem, water soluble paste with gold plating PCB.. | 30 July, 2001

You did not provide the stencil thickness or your aspect ratio. If your aspect ratio (Aperture width/stencil thickness) is less than 1.5, the paste will not release correctly from the aperture, leaving paste in the aperture. For example, if your paste aperture is 12 mils wide, and your stencil is 8 mils thick: 12/8=1.5.

Brian W.

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#7486

Printing problem, water soluble paste with gold plating PCB.. | 30 July, 2001

"Poor release" (paste hanging-up in the stencil holes) could have three main (or combination of) causes:

1 Equipment: Stencil is too thick. Specifically, a 6 thou stencil should be OK, providing you don�t violate the area and aspect ratios in the fine SMTnet Archives too wildly.

2 Process: Snap-off distance is too small.

3 Material: * Solder paste viscosity is too high * Solder paste is too tacky * Solder paste particle size is too large

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Frank

#7491

Printing problem, water soluble paste with gold plating PCB.. | 31 July, 2001

Sorry I forgot to provide enough information.

Stencil thickness is 6 mil Aperture width is 10 mil Aspect ratio (aperture area / wall area) = 0.72 Solder paste mesh size, type 3 (-325 +500)

With this aspect ratio we can printing without any problem when I use another paste with another PCB. Thanks for your information. I may try to use the new paste.

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Hussman

#7500

Printing problem, water soluble paste with gold plating PCB.. | 31 July, 2001

How fast is your print head speed? I've found that I had to decrease my print head speed to get the paste to stick to gold pads. Not exactly sure why - but the ol' instincts worked well.

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Mark

#7521

Printing problem, water soluble paste with gold plating PCB.. | 1 August, 2001

please look at trying lazer cut stencils. They have trapozoidal shaped apertures and this could assist with the paste releasing onto y the PCB

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