Is Alloy Mixing During Rework Acceptable?

Phil
Welcome to the Board Talk. This is Phil Zarrow and Jim Hall, the Assembly Brothers, also known as consultants with ITM Consulting.

We're here to attempt to answer your questions on SMT process and all the related ancillaries. What's our question today?

Jim
"We're an in-house repair facility for boards from various vendors. A board may bear a RoHS label, but the lead-free solder alloy is not known. What is the best rework procedure? Clean off the original solder residue first, or just use any lead-free alloy and not worry about the resulting mix?" This question comes from J.M.

Phil
Well, let's see. Where to start? Obviously, the best practice in the ideal situation would be to clean off as much of the old stuff as possible, and that should be pretty straightforward. There are materials, tools, available for that, soldering irons, solder wick, all kinds of other fun stuff out there. Beyond that ...

Jim
Taking the solder off and avoiding significant mixing takes time. A number of papers and technical tests have been published about mixing during rework, particularly for the manufacturers of the modified tin copper alloys. They're different from the SAC alloys, the concern of mixing is raised, and there's some good test data available.

When the military aerospace program, the JCAA/JGPP program was done - its most published author is Dave Hillman - and in setting up their test evaluation, where they were looking at - what's the military going to do in a changing lead-free world - they anticipated that, in repair, alloys are gonna get mixed, and they have included in their test matrix, where they did vibration, shock, thermal cycling through several different cycles - in a number of those tests they have included repaired boards that have mixed alloys in them.

So you might want to check that out. I'm sure there are others available. What you really want to be careful about is that you don't get lead mixed in with lead-free solders. So if you have a RoHS label, make sure that it really is lead free.

Phil
And don't forget what happens if your lead-free alloy happens to contain bismuth.

Jim
Yes, that's all the more reason for not allowing any lead to get anywhere near a lead-free assembly. Is it worthwhile to go through the trouble to wick off and move all of the existing original alloy and then replace it with a single alloy?

Phil
You're talking from a joint-strength and integrity standpoint.

Jim
What's the quality of the product? What is the expected life? Is it a stereo system that's going to sit on somebody's shelf, or is it an industrial controller that's going to see a lot of vibration, shock and temperature cycling? So that can guide you in terms of making an economic decision whether you want to go to best practice and take off all of the existing alloy.

Phil
I think we answered that question. Beyond that, this is Phil Zarrow and Jim Hall, and, whatever you do ...

Jim
Don't solder like my brother.

Phil
And don't solder like my brother, and don't remove solder like my brother.

Let's start with the premise that the assembly must meet the print requirements. The solder alloy to be used is listed on the drawing, including any exceptions (for example, Note 1. "Use Sn63Pb37 solder, except BGA U1 which may have SAC305 solder balls").

In order to rework the assembly back into compliance with the print requirement, you should always be able to get a copy of the original assembly print from the customer to determine the solder alloy requirements and exceptions, and rework accordingly. If the assembly print no longer exists, then an appropriate waiver from the customer is needed before mixing alloys or removing old solder and reworking with different alloys.

You can't just fly by the seat of your pants with issues such as solder alloys. These are critical items and must be kept controlled.

There is another situation: The product is lead-free soldered due to availability of BGA with lead-free balls. Now, we have few parts to rework, the product does not need RoHS label but was soldered with SAC305.

Now, we need to touch up the joint using regular 63/37 Tin-Lead solder alloy. The reworked parts are discrete and ICs. (if BGA reworked, it is with SAC305 only)

Do you see any problem with mixing of alloys when RoHS label is not required by the customer?