Solder Mask Around Vias in a BGA Fanout

[aharman]

Hi,

I hope I can explain this question clearly. Do you recommend having the solder mask or not around the vias that fanout from the BGA pads if you are soldering down the part? This is a fairly small BGA so the vias are very close to the BGA pads. We just don't want to short anything when soldering which is why I ask. Normally in the past we have done via in pad for all our BGA's but I wanted to try the BGA fanout for this particular project. Thank you for your time and any advice would be appreciated!

[suleymancskn]

Hi, Aharman.
There should be a solder mask between the pads. Because after the two pads are soldered, there may be a short circuit.

I think the solder mask expansion value for VIA should be> 0. Because via top silkscreen can be closed. It becomes difficult to reach Via. I even encountered VIA related bugs during testing.

I hope that will be useful.

[aharman]

Hi Suleymancskn.

Thank you for the response! I figured that would be the case. I am just not sure what the proper solder mask expansion value for this should be since they are so close. I will mess around with it.

[robertferanec]

I always mask VIAs.

PS: I believe, if VIAs under BGA are not masked, you may have problems with assembly ... there may be short circuits between VIAs & Pads & BGA balls

[qdrives]

What about the chemical residue in the via? Often they advise to leave the via itself open (ie 50um annular ring). But yes, I had shorts between vias that were not tented in the past (wave soldering).

[aharman]

Thank you Robert. Is it ok for the BGA pads and the vias to have solder masks that are overlapping or do I need to reduce the solder mask expansion until both are not physically touching each other?

[robertferanec]

Often they advise to leave the via itself open

- I asked about this and even if you do not open mask in via, there still will have a hole (mask will flow inside). I think, we talked about it in this video: https://youtu.be/FM3pRM0CxGw

[robertferanec]

Is it ok for the BGA pads and the vias to have solder masks that are overlapping or do I need to reduce the solder mask expansion until both are not physically touching each other?

- when you say solder masks overlaping, you mean the opening, right? I explained, it is recommended to mask VIAs under BGA. If you really would like to use unmasked VIAs under BGA, there has to be mask between pad and VIA otherwise the solder (tin) can flow inside of the VIA (simply to say, the ball of the BGA can flow inside of the VIA) and you will lose the connection between BGA and PCB.