Solder balls on pads - lead free

Hi,

maybe solderability of PCB...

reply »

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.

reply »

msg - delete because picture posted not viewed.

reply »

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

reply »

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...

reply »