Manually tenting vias for an existing PCB

[gperoni]

Hello guys,

I had this prototyping board made where I incorrectly specified the soldermask clearance and as a consequence I'm having an extremely hard time soldering a 0.4mm pitch BGA (I went the cheapo way and did a 0.4mm BGA with standard PCB techniques). Here is a picture:



Is there any way I can apply soldermask to the exposed vias copper, tenting the vias? I figured purchasing some chemical is a better option than re-spinning the board, but I will be going that way as a fallback.

Thank you!

[RoGeorge]

I've seen soldermask for sale (on Aliexpress, for manual made PCBs). Never tested it, but it might work.

[stmdude]

I've seen soldermask for sale (on Aliexpress, for manual made PCBs). Never tested it, but it might work.

Yep, it's a thing. I use it every now and then when repairing lifted/corroded tracks. You can apply it with a small paintbrush, or from the tip of a toothpick.

[rx8pilot]

Look forward to seeing the solution for this. I have been in the same situation and never really found a workable solution.

Good luck and share the final outcome if possible.

[langwadt]

something like this? https://youtu.be/PRO03Jv3qRo

[JustMeHere]

I wonder how well fingernail polish would work

[rx8pilot]

My guess is that nail polish will be a problem when exposed to the reflow thermal cycle. Potentially bubbling up and having hard to predict chemical reactions.
Just a guess though....

[mairo]

I have done something similar with thin kapton tape, but I only had to do it on two boards. The difficult part for me was to 'master' how to cut the small peaces of tape without touching the adhesive. It work for me that time, but was a lot of work. I did respin the board later on.
My solution might be a bit to 'high' for you application tho. 

[janekm]

Since presumably that's 0.8mm between the rows/columns (one row missing from 0.4mm pitch) I can't imagine you'd succeed in filling in the soldermask though it could make for a fun experiment.

However it looks like you have left off the pads for some of the solder balls in the BGA? If that's the case your chances for success in soldering it are slim as the non-soldered balls will leave too much space for the soldered ones leading to broken connections.

[nanofrog]

I've seen soldermask for sale (on Aliexpress, for manual made PCBs). Never tested it, but it might work.

Yep, it's a thing. I use it every now and then when repairing lifted/corroded tracks. You can apply it with a small paintbrush, or from the tip of a toothpick.

If I understand you correctly, you're referring to UV curable solder mask. Great stuff for that purpose ^, but I'm not so sure for the OP's situation.

Dry film solder mask would be a better solution IMHO, but I'm not sure if there's sufficient PCB surface area for it to sufficiently adhere.

[asmi]

Well since you went cheapo - presumably a respin will be cheap too

[gperoni]

I actually ordered a respin last Sunday after seeing the first comments as I couldn't find anything suitable on Farnell. I'm also fairly certain manually applying it would have made it difficult for the pads to touch the solder balls... And yes, the respin was cheap!

However it looks like you have left off the pads for some of the solder balls in the BGA? If that's the case your chances for success in soldering it are slim as the non-soldered balls will leave too much space for the soldered ones leading to broken connections.

No, that's just how the chip is designed. 0.4mm pitch with missing columns. Go figure, it's ancient technology.

Also damn, the factory400 guy answered to my thread! I'm a fan!

[rx8pilot]

Also damn, the factory400 guy answered to my thread! I'm a fan!

Ha! Awesome!

I am dying to release some videos....I have shot 8 videos that need to be edited but I am swamped. Totally over my limit, working 7 days/week.