Solder balls under components

[Neilm]

I just received a small batch of prototype boards from a PCB assembler. The PCBs were SMD populated and as there were not many through hole components I was going to fit those devices.

When inspecting the build quality, I noticed that there were small balls of solder under many of the components. It was most noticeable under a series of MELF resistors, but there were some visible at the edge of some 2512 components as well.

When I spoke to the assembler, there comment was "Oh this always happens with those devices, it's industry standard". My question is, is this true?

If it is then I am rather worried as the clearance over the MELF resistors is critical - any solder balls would reduce the clearance and I could have some serious issues.

I have checked the data sheet for the resistors and the footprint seems correct. The only thing I am not so sure of is the resistors could get quite warm so there is a reasonable amount of copper around the pads to help get rid of the heat. The 2512 component footprint is as on the datasheet.

Neil

[jeroen74]

See http://www.ami.ac.uk/courses/topics/0170_wsp/index.html for some information on solderballing. Seems to be a rather complex issue.

[free_electron]

industry standard my ass...

shouldn;t be there.. main cause for this is running to steep a rampup on the profile. the flux has no time to evaporate and 'bubbles' flinging molten solder everywhere. the flux need to evaporate completely before the solder goes liquid.

[Smokey]

I was just talking about solder balls with my assembly guy.  He was telling me about a board they ran from another company where the guys changed the layout last minute, but didn't send an updated paste stencil so they were applying paste on top of mask where there were no longer any pads.  Well that paste just balled up and rolled wherever it wanted in the oven.  Bad news.  Always check your final copper, mask, and paste layers line up.

[poptones]

I'm not gonna solder my balls under anything.

[ablacon64]

Me neither! kkkkkkk
Neilm, can you post some pics?

[Kremmen]

industry standard my ass...

shouldn;t be there.. main cause for this is running to steep a rampup on the profile. the flux has no time to evaporate and 'bubbles' flinging molten solder everywhere. the flux need to evaporate completely before the solder goes liquid.

Or running with stale paste.

[Neilm]

Neilm, can you post some pics?

No pics I'm afraid. I have been through them and removed all the balls. I did try to get some for QA as the assemblers provide some our production PCBs but they didn't come out too well.

Many thanks for the replies so far.

Neil

[Neilm]

I have finally found the problem with the solder balls. Turns out that the MELF package needs a peculiar solder mask. The one in the footprint in the library was a rectangle, the one recommended by the IPC is more of a U shape. I changed the mask and ran it on the next build of the board and it seemed to work fine. It isn't something mentioned on the datasheet.

Neil

[Smokey]

That sounds a little off to me.  Solder balls because the corners of the pads are rectangular?  All these MELF datasheets call for rectangular pads, as does the IPC_7351.pdf.

http://pcbget.ru/Files/Standarts/IPC_7351.pdf

http://www.vishay.com/docs/28713/melfprof.pdf
http://www.nxp.com/documents/data_sheet/BZV55_SER.pdf
http://www.token.com.tw/pdf/melf-resistor.pdf
http://www.comchiptech.com/cms/UserFiles/CZRL5225-G%20Thru.%20CZRL5267-G%20RevB.pdf

Are you talking about having rectangular pads but covering the corners with an elliptical mask?

I'm not saying you're wrong, just that I've never heard that before.  What IPC document were you looking at?  I'd put my money more on the assembly house realized they screwed up and fixed their process for your next batch since you brought it to their attention.  Who are you using to do your assembly?

[Neilm]

The actual pads are rectangular - looking at the solder mask it looked like a U.

I'm not sure what IPC standard it was. It did cover the solder mask rather than the land. As we own the stencil I could compare them to see that it had been modified.

Yours

Neil