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Air inclusion in solder paste

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Air inclusion in solder paste | 26 March, 2006


We are running automatic stencil printers, open squeegees, stainless stencil, stainless blades, print speed 25-30mm/sec.

We have a frustrating consistency issue printing .37x.3mm apertures on a 45pin LGA (11 pads each side, 1 large ground in middle)

We get what appears to be random insufficient paste issues that affect only a couple of pads at a time, yet in most cases the very next print is fine.

So to my question: Is it common for solder paste to maintain air bubbles? Can this cause the faults we are seeing? Can this be a factor of the solder paste?

When mixing paste we are careful not to fold in air when exercising before putting on the stencil. What I am wondering is if the printing process itself is bringing air into the paste. At the start and end of the print stroke the blades lift, generating a slight peak. As the next blade comes in contact with the paste from the opposite side the paste collapses in on itself (like a wave breaking) This looks like it could aerate the paste. As the paste drops into apertures is there a moment when air could get under the wave front?

If you let the paste rest after a few print strokes you can see small bubbles slowly being released.

When printing over a 6.5mm dia test aperture small bubbles can be seen in the resulting deposit. Wouldn't this be able to cause faults in the fine pitch?


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Air inclusion in solder paste | 27 March, 2006

It seems highly unlikely that you could actually trap enough air in your paste for it to be releasing air after you've worked it a few times unless it's not rolling properly to begin with.

I suppose maybe a contaminant could be present that could be causing some outgassing but hopefully you're not "refreshing" your paste with an unknown solvent...right?

Is your paste handling procedure up to snuff as far as expiration dates, refrigeration, etc.?

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Air inclusion in solder paste | 27 March, 2006

Blasted back button. :/

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Air inclusion in solder paste | 27 March, 2006

Hi Steve Thanks for your reply. Your comments make it sound like it may be the nature of this paste. Any one else agree?

Paste control is fairly good. Paste is put in fridge straight after it arrives at inwards goods, (4deg). Removed from fridge 24hrs before production use. Paste on the stencil is topped up as required with fresh material, and replaced every 4 to 6 hours. Certainly no solvent addition.

This particular batch of paste was manufactured 12/05, and has an expiration date of 06/30/06. So is not an age issue. It is something you have to look for, and I have seen this a few times so am confident it is not a batch related issue.

The outgassing is not huge amounts. In the bead at rest for example it is just small bubbles resulting in pinholes.

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Air inclusion in solder paste | 27 March, 2006

You mention "large ground plane pads in the center". Could it be that these large apertures are allowing the blades to "dig" which somehow pulls paste from adjacent apertures as the blades recover?

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Air inclusion in solder paste | 27 March, 2006

Thanks for the suggestion. The large ground pad appears as a single large pad on the component and PCB but we print a matrix of 28 small dots of paste to provide the solder for the joint.

The inconsistent volume transfer can occur on any of the four sides.

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Air inclusion in solder paste | 27 March, 2006

If you believe: * There is air in your paste AND * You are not whipping the air into the paste ...

You should: * Return the paste to your supplier * Get it replaced * Begin qualifying a replacement supplier.

One thing before you go racing off to lynch your supplier, tell us about the area and aspect ratios for the apertures where you see this issue.

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Air inclusion in solder paste | 27 March, 2006

Thanks Dave

I'm not convinced the air is necessarily in the paste from manufacture - suspicious that it may be folding in during the change in direction of the print cycle - or as the paste travels across apertures. It is not necessarily in the first few prints that it is seen. It is there after a number of print strokes.

So you are indicating you do not see pin hole bubbles in your solder paste deposits? Time to try out some other pastes then.

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Air inclusion in solder paste | 27 March, 2006


The air 'contained' in the aperture would prefer to escape between the board and the bottom of the stencil rather than through your paste while you're printing over it.

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