Circuit Technology Center
SEARCH  

Soldering Long Gold Plated Pins

Soldering to gold has never been a flawless exercise. Concerns about the quality and quantity of gold finishes buzz around the soldering process like annoying horseflies, drawing blood whenever your guard is dropped.

Soldering Long Gold Plated Pins
Figure 1: Four rows of pins make it difficult to access inside pins.
Soldering gold connecter pins into through holes can shackle us with further constraints regarding the allowable height of solder fillets up the length of gold pins. Beyond initial manufacture, the rework of these same through-hole gold connectors, that is, removing and replacing them by hand, is difficult, very difficult.

It's difficult to get solder to flow down holes to sufficiently wet gold pins while keeping the solder from running upward into a pin's restricted zone. The longer a hot soldering iron tip dwells to drop molten solder down the plated hole, the more likely it is that this same solder will wick up the connector pin.

Figure 1 displays a particularly daunting application that showed up at our facility. Not only were the pins gold, but the board was stacked with heavy ground planes, and the pins were four rows deep.

Soldering Long Gold Plated Pins
Figure 2: Removing pin connectors from board.
Using the usual packet of rework tricks, expert technicians could find ways to solder 50 to 75 percent of the pins, but there would still be a few locations stubbornly resisting every effort to drop solder down holes without the molten metal likewise creeping too far up the pin.

In order to succeed, we had to try something different.

There were four key performance targets:
  1. Acquiring access to the inner rows of pins while delivering sufficient heat and not contaminating adjacent pins with solder.
  2. Wetting to the gold surface of the pins.
  3. Wetting to the walls of the plated through holes.
  4. Maintaining the topside solder fillet below the specified height.
Soldering Long Gold Plated Pins
Figure 3: Solder donuts in position for soldering.
To meet these targets, a variety of techniques were applied.

Using a mini-wave system, the connectors were removed from the board. Then, the plated-through holes were cleared with a vacuum desoldering tool.

A good deal of attention was focused on this process inasmuch as one of the main reasons for this rework was the lack of wetting in the plated through holes. After solder removal, the plated-holes were carefully inspected with a microscope and back-light.

Soldering Long Gold Plated Pins
Figure 4: Completed rework.
Once the connector locations were fully inspected, a combination of hand and convection soldering was used to access the pins. Control of solder volume was essential to ensuring proper fill and preventing unacceptable solder wicking up the pins. This was accomplished at challenging locations by adding pre-manufactured "solder donuts" (see Figure 3).

All-in-all a pretty time-consuming process, but sometimes that's what it takes to be successful in rework. As you can see from Figure 4, the time was well spent, and the results were excellent.

Several members of the Circuit Technology Center team contributed to this feature story.