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Solder balls on pads - lead free

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#67272

Solder balls on pads - lead free | 17 September, 2012

Hi guys,

We have an interesting case we haven't seen before (see the picture below)

http://postimage.org/image/lu2f4ae55/

Solder balls form on unpopulated pads after reflow.

1. Reflow profile is fine. 2. Stencil thickness / paste volume is also fine (never had this type of error before). 3. Sometimes the balls form on populated pads and even push away small components from the pads. 4. The same solder paste is used for other products without issues.

Any ideas what could cause this issue?

Thanks for your anticipated help.

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ben

#67274

Solder balls on pads - lead free | 17 September, 2012

Hi,

maybe solderability of PCB...

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#67275

Solder balls on pads - lead free | 17 September, 2012

Hi Ben,

Yes, this could be the reason but have you ever seen solderability issues for (new) boards with LF-HAL surface finish? I have seen this only if there are surface finish quality issues for bare boards (exposed copper etc.). In this case we didn't notice any deformities in surface finish.

Could it be some residue remaining on bare boards after the process?

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#67290

Solder balls on pads - lead free | 18 September, 2012

Sorry. I don't see any solder balls.

To me, it looks like the solder wetted the pad and then pulled back. It's called dewetting. It's a defect. Ask your supplier for a SEM analysis. I'm interested to know what he / she says.

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#67292

Solder balls on pads - lead free | 18 September, 2012

msg - delete because picture posted not viewed.

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#67302

Solder balls on pads - lead free | 18 September, 2012

> Sorry. I don't see any solder balls. > > To me, it > looks like the solder wetted the pad and then > pulled back. It's called dewetting. It's a > defect. Ask your supplier for a SEM analysis. I'm > interested to know what he / she says.

I see like 12 solder balls on there right off the bat.

Melissa

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#67311

Solder balls on pads - lead free | 19 September, 2012

My email mail machine couldn't send this to you ...

Hi Stivais ... I didn't read the report. It seems to require downloading of a 'reader' of some sort. No thanks. If you would, just attach the report to this email. Regardless, energy-dispersive X-ray spectroscopy (EDS or EDX) won't tell you shit. EDS uses xrays to analyze to the surface of a material.

The solder is thin at the edges of the pad because there's something metallurgically incorrect about the base metal of the pad. So, the solder can't form a good bond with the metal and moves away from that area. Unfortunately for the solder, there is no no good metal. As a result, as the solder moves away from the edges of the pad [where it first sees the heat], it ends-up in the center of the pad. [That's why you described the problem as solder balling.]

Some of the QFP pads in your picture are fascinating. The solder has retreated to the end of the pad and just kept piling-up on top of itself, trying to get away. [It went to the end, rather tan the center probably because it was cooler there, either as the result of a via or some connection to a copper plane on a inner layer of the board or something else about the design of the board.]

Usually, foreign materials embedded in the copper of the pads causes this type of dewetting problem. Often this foreign material is bits of abrasive used to polish the board.

Don't get lost in talking about internetallics. I'd guess that there is a proper intermetallic bond in the thin area, because initially the solder flowed over that portion of the pad.

On balance, I've wrecked boards to look like this. If you reflow boards ~30*C above the prescribed peak temperature, you could end-up with this type of problem. Usually when you do that, your legend turns tan. I don't see that on your board.

You're correct. This has nothing to do with your solder alloy. 95/100 it's your board fab, 5/100 it's your process controls.

Regards, Dave

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#67312

Solder balls on pads - lead free | 19 September, 2012

It's both solderpaste or PCB pad problem... 1st it's possible that your PCB exposure problem or old stocks, remedy? try to bake your PWB. 2nd, it's possible that PWB coating (wax) are more ticked so that solder paste flux are cannot penetrate to clean the surface of pad. remedy, ask solder paste vendor for technical support it's free...

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#67313

Solder balls on pads - lead free | 19 September, 2012

Melissa ... Are you calling the blobs of solder on the pads solder balls?

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