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 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.
|Figure 1: Four rows of pins make it difficult to access inside pins.|
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.
Like most hand soldering, it used to be easier. Now heavy heat draws and microscopic sizes challenge technician's dexterity, vision and choice of materials.
Figure 1 displays a particularly daunting application that showed up at our facility. Not only were the pins gold, the board stacked with heavy ground planes, and the pins were four rows deep.
|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:
- Acquiring access to the inner rows of pins while delivering sufficient heat and not contaminating adjacent pins with solder.
- Wetting to the gold surface of the pins.
- Wetting to the walls of the plated through holes.
- Maintaining the topside solder fillet below the specified height.
To meet these targets a variety of techniques were applied.
|Figure 3: Solder donuts in position for soldering.|
The key to ensuring the holes properly wet with solder was to accomplish this during the gold pin connector removal step - waiting until the connector re-installation step was too late and would result in a variety of solder flow issues. Utilizing a mini-wave system, the connectors were removed from the board. Plated through holes were desoldered 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 lack of wetting in the plated through holes. After solder removal and hole tinning, the holes were carefully inspected with a microscope and back-light.
Once the connector locations were certified "placement ready" a combination of hand and convection soldering were used to access the pins. Control of solder volume was essential to ensuring proper fill and preventing improper wicking up the pins. This was accomplished at challenging locations by adding pre-manufactured "solder donuts" (see Figure 3).
|Figure 4: Completed rework.|
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 spend and the results were pretty darn good.
Several members of the Circuit Technology Center team contributed to this feature story.