Fixing untented vias under BGA's

[elmohandis]

Hi Guys,

I recently spun a board with 3 BGA's (1.0mm pitch 676-pin FPGA + 2x DDR3 SDRAM 0.8mm pitch).

The board just came in and I noticed that I forgot to tent the vias under the BGA's (oops!).

I am considering the following fix before sending it off for assembly:

1. Order a stencil with cutouts in place of the vias to be tented.
2. Apply curable solder mask to the vias only using the stencil.
3. Cure with UV.

Can someone share some advice? Is there a better way?

Thanks!

[elmohandis]

Considering somethig like this to use as solder mask:

https://buerklin.com/default.asp?event=ShowArtikel%2811L470%29&context=subset:0;sel:SE;search:solder%20mask;se_name:vt;patchid:;tags:;pagecount:100&l=e&jump=ArtNr_11L470&ch=26481

[marshallh]

Try that method if you want, better is probably just to assemble it anyway. Just be more careful with the paste.

[John_ITIC]

I recently did some BGA site repair and came across this product: http://www.soldertools.net/bga-stencils-stencilquik/

Essentially, the stencil has adhesive on the bottom such that it acts as solder mask as well as stencil for solder paste application. It is left in place when reflowing the BGA, which also prevents solder ball collapse and shorts.

I also purchased a PCB rework kit, which contained high-temperature epoxy that can be used for touching up solder mask in case of damage. However, it was very hard to do properly since my pitch was 0.8 mm. The Stencilquik approach would likely be easier for you.
http://www.circuitmedic.com/products/201-2100.shtml
/John.