Is possible to completely remove solder from a PCB pad?

[f95toli]

TL;DR Summary

Is there a method that could be used to completely remove the solder and get back to the clean, bare, pad?

We recently ordered a batch of populated PCB. Unfortunately, due to a misunderstanding around the solder mask some gold plated pads that should have been left bare were covered in solder. The PCBs are part of sample holders and the bare pads are used to wire bond (using aluminium wire) to the DUT. Because of the unwanted solder the PCBs are now essentially useless and they aren't exactly cheap.
Sometimes is possible to wire bond to solder (I've done it in the past) but for some reason it doesn't work on these PCBs (probably because of the solder used).

Is there a method that could be used to completely remove the solder and get back to the clean, bare, gold?
I am familiar with normal desoldering methods but AFAIK they all leave a thin layer of solder on the pad.

 

[berkeman]

I'm guessing that you've tried solder wick and it is still leaving a solder coating? When you hit the pad with hot air at an angle, does it look like it is trying to expose any gold?

Have you tried mechanical scraping? I'm guessing solder is a little softer than the gold, but probably not by much...

 

[Baluncore]

Maybe this process will work.
1. Remove excess solder with fine solder wick.
2. Press gold leaf onto the flat pad.
3. Re-flow with hot air.

 

[DaveE]

You could try gold plating over it after you've removed as much as possible. I've never tried it, and am pretty ignorant about this stuff, but there are gold brush plating tools that you could investigate that might leave the rest of the PCBA undisturbed.

https://www.goldplating.com/products/gold-smith-machine

 

[DaveE]

Anyway, you'll never get it all off. At the microscopic level the atoms mix (dissolve?).

You can also plate by dipping it in various solutions, which could work if the pads are near the edge.

https://en.wikipedia.org/wiki/Gold_plating

 

[Rive]

I think that gold is already mixed with the solder => unusable.
I would try to remove as much solder as possible: first with a solder wick, then with a 'fiber pen' down to bare copper.
Then, there are some PCB Gold Repair Kits...

 

[Baluncore]

PCB gold repair kits can be very expensive. They have a limited life.

A pack of 100 sheets, (90x90 mm squares), of "24k gold leaf", should last you a lifetime.
On-line, that costs less than US$10, including postage.

 

[Rive]

Sure cheaper, but I wonder how a copper-solder-gold sandwich would work as a base for wire bonding.
Experiments will be needed.

 

[Svein]

Turn the problem around - if you solder wire-wrap pins to the problematic areas, you can use wire-wrap techniques to connect - or is there something that I do not understand in the problem description?

 

[davenn]

or is there something that I do not understand in the problem description?

he wanted to clean all the solder off the gold plated pads

which would be mostly impossible, as the gold and solder would form an amalgam

 

[Svein]

he wanted to clean all the solder off the gold plated pads

which would be mostly impossible, as the gold and solder would form an amalgam

As I read the OP, he stated "the bare pads are used to wire bond (using aluminium wire) to the DUT". I replied to that part, not the derived problem of removing the solder.

 

[Rive]

"the bare pads are used to wire bond (using aluminium wire) to the DUT"

For me that sounds like they are testing some 'bare die' or alike: so not the usual modules, chips or anything 'big' (for wire wraps), but something really small and delicate.

 

[f95toli]

Firstly, sorry for not replying sooner. For some reason the "alert bell" did not show that anyone had replied to the thread(!). Thanks for all the replies.

Anyway, . I think I've also concluded that getting the solder off won't be possible, at least not without using some nasty chemicals which I doubt I would be allowed to use,
I've now been in contact with the company that made the PCBs and they've agreed to split the cost of making new ones (we are splitting the cost because I should have been more clear about what was needed, but they accepted it as a repeat order and still did something differently from last time)

Turn the problem around - if you solder wire-wrap pins to the problematic areas, you can use wire-wrap techniques to connect - or is there something that I do not understand in the problem description?

As I read the OP, he stated "the bare pads are used to wire bond (using aluminium wire) to the DUT". I replied to that part, not the derived problem of removing the solder.

No, we use wire bonding to connect to quire small microfabricated devices
https://www.fsbondtec.at/wire-bonder/?lang=en

There is no solder involved, and the pads needs to be very clean for the process to work.