Clever Use of Different Solder Melting Points to Solve a PCB Problem

Quote of the Day

It may be an unwise man that doesn't learn from his own mistakes, but it's an absolute idiot that doesn't learn from other people's [mistakes].

— Frasier Crane


Figure 1: Common 63/37 Lead Solder (Melting Point 183 °C ).

Figure 1: Common 63Sn/37Pb Solder (Melting Point 183 °C ).

Just before lunch, some of the hardware folks here were having a discussion about a Printed Circuit Board (PCB) issue that had come up and how we had solved this issue in the past. During this discussion, one of the hardware managers reminded me of how he cleverly resolved a part problem we had a few years ago by using solders with different melting points. At the time, I thought his solution was brilliant and it saved us a bunch of time and money. His solution is worth documenting here.

Figure 2: No Lead Solder, 99.5/0.5 Sn/Cu (Melting Point 227°C).

Figure 2: No Lead Solder, 99.5/0.5 Sn/Cu (Melting Point 227°C).

The problem we had a few years ago involved a complex circuit card with a layout that assumed a specific memory part. After we had fabricated the PCB but before we had soldered parts on it, a problem was discovered with a component and we needed to use an alternative device. Unfortunately, all potential substitutes had a different pin configuration than the part that the PCB was designed for.  The substitute parts had the same pins, but in different positions. We did not have time to design and fabricate a new version of the complex PCB.  The path to a clever solution began with asking the right question.

The question was, "What if we made a small, quick-turn, PCB that would connect the alternative part's pins to the pin positions of the faulty part?" This approach would work, but how would we align and solder a pair of PCBs together at the same time?

His clever answer was not to solder them together at the same time. We could solder the substitute part to the small PCB using high-temperature solder (Figure 2: no lead, melting point 227 °C) and then solder the bottom of the small board to the large board using low-temperature solder (Figure 1: leaded, melting point 183 °C).

Over a period of a few days, we designed the small PCB, fabricated it, and soldered our alternative part onto its top surface. We then used low-temperature solder to attach the small board in place of the faulty part. The high-temperature solder did not reflow when the small PCB was placed on the large PCB and everything worked perfectly.

Figure 3 illustrates the small circuit board and different solders that we used to solve our part substitution problem.

Figure 2: Solving An Interconnect Problem Using Different Solders.

Figure 3: Solving An Interconnect Problem Using Different Solders.

Figure 4 shows the actual, small PCB that we built to solve our problem. Sorry for the poor photography – all I have is my phone.

Figure 4: Small PCB to Fix Interconnect Issue.

Figure 4: Small PCB to Fix Interconnect Issue.

 
This entry was posted in Electronics. Bookmark the permalink.

Leave a Reply

Your email address will not be published. Required fields are marked *