Question: I am performing some modifications to a Raspberry Pi circuit board for a university research group and I was asked to substitute a USB cable for the stock, on-board USB port. I’m having difficulties soldering the braided shield to the board. It appears that the braided shield may be made of aluminum. The braid simply will not accept the solder. No wetting, whatsoever, even after cleaning and fluxing. Are there any special techniques for soldering aluminum wire?
Answer: Soldering to aluminum is very difficult and quite different than soldering to copper or solder coated surfaces. The issue with aluminum is the oxide on the aluminum itself and it needs to be removed so the solder can make a electromechanical bond to the shield itself.
You need an aggressive flux, a high activity flux with activators, such as an ORH1 flux. This flux is very active and cleaning is a must to removed the residual fluxes left behind after the solder joint is created, and in your case, after the shield is soldered. I would recommend an ultrasonic cleaner to do this to verify the flux is removed from beneath the shield. You can use existing tin/lead solders and lead-free solders to do this soldering. If the product is new, or was built after 2008, then the lead-free solder is a must to do this work.
I would contact the solder manufactures to find out exactly which alloy is best to use for this application and which flux they would recommend, so you can achieve a good solder joint.