Best PCB pad finish for repeated solder jobs?

[ryaamaak]

I'm designing a PCB with large exposed metallic pads for an experiment I'm running. The idea is I'll be able to solder directly and repeatedly onto the pads, so ideally the pads should be able to withstand multiple removals of solder.

Any thoughts on whether ENIG or bare copper would be better for this? Is there another finish I'm missing?

 

[CWatters]

Cant really help but... Perhaps use PTH to "stake" the copper pads to the board? Solder resist also helps keep small pads attached.

 

[berkeman]

Can you say more about what you are intending to solder onto the pads? Is it for SMT devices, or plated-through-holes (PTH) or some combination? How big will the pads be, and how big are the components.

+1 on @CWatters comment about stitching the pads through to pads on the other side of the PCB -- that's very important to help avoid delamination. Also, are you using a Metcal soldering iron (RF heated tip)? They have superior temperature control.

Here are some of the notes from the Fab Drawing for a board that I recently worked on. You could increase the thickness of the outer layer copper features if that would help to make the pads more robust...

 

[Rive]

For such kind of mechanical loads it's common to use solder pins or rivets.

 

[ryaamaak]

Can you say more about what you are intending to solder onto the pads? Is it for SMT devices, or plated-through-holes (PTH) or some combination? How big will the pads be, and how big are the components.

Sorry for the delayed reply. I'm running an experiment involving repeated low current measurements. Right now I'm using gold wire a couple mils thick, and soldering it to a protoboard with wires attached to the instrumentation. It's messy, the connections are noisy, and there are more solder joints than really necessary. In the past, I designed a PCB with exposed copper areas for cryostat measurements--the wires were attached to the exposed copper using cold solder or silver paint rather than heat. In this experiment though, I will need to detach and reattach the wires to whatever PCB I design repeatedly, probably using hot solder, so I'm worried about the finish on the exposed pads degrading.

The pads will prob be in the 5mmx10mm range. The board doesn't need to be grounded or anything because I'm just using it to simplify wiring. No components, just gold wire and probably some PTH connectors.

 

[Tom.G]

The problem that arises with repeated soldering to a PCB pad is the bond between the Copper and the board is degraded by the heat. This is especially true when hand soldering with a soldering iron. Since you are in a cyrostat that is temperature cycled, rivets or anything staked thru th board is pretty much ruled out. One possible approach would be to bond a pigtail lead to the pad and then do your connection changes by soldering to the pigtail. Another idea would be to use temperature controlled hot air soldering directly to the pads, but it's probably cheaper/easier/safer to use the pigtails if you can.

Please keep us updated as to what works best.

Cheers,
Tom

 

[ryaamaak]

The problem that arises with repeated soldering to a PCB pad is the bond between the Copper and the board is degraded by the heat. This is especially true when hand soldering with a soldering iron. Since you are in a cyrostat that is temperature cycled, rivets or anything staked thru th board is pretty much ruled out. One possible approach would be to bond a pigtail lead to the pad and then do your connection changes by soldering to the pigtail. Another idea would be to use temperature controlled hot air soldering directly to the pads, but it's probably cheaper/easier/safer to use the pigtails if you can.

Please keep us updated as to what works best.

Cheers,
Tom

Thanks for the input! I should have clarified that this board will not be going in a cryostat like the previous board, but instead will only be used at room temperature and at standard air pressure (no vacuum). That being said, I'm still concerned about the thermal cycling with hot solder eventually scorching the board. I'm also worrying that the process of removing solder will damage the board. I'm not worried about anything staked through the board, but I'll consider the pigtail leads as a way to control damage.