Like many other industries, the PCB design and manufacturing industry is one of continual changes. Many years back, PCB designers were laying out thru-hole circuit boards at four times the size on a light table using black tape and sticky dots. Eventually, CAD systems were introduced, which really simplified this work until small fine-pitch surface mount technology parts changed the game once again. Could you imagine trying to lay out today’s fine-pitch BGA parts on a light table with tape and dots?
As PCB layout technology evolved with parts and pin pitches getting smaller and smaller, the manufacturing of circuit boards had to change as well. The fabrication processes that used to work well for wide, hand-taped traces had to be refined to handle the smaller trace, via, and spacing required for BGAs and other high-density, fine-pitch parts. The assembly processes had to keep up with this growth as well to place and solder these parts. Assembling these boards has become a real challenge. Fortunately, there is yet another leap forward in the industry that has proven to be a great help: the selective soldering process.
What Is the Selective Soldering Process?
A selective soldering system is a machine that will solder thru-hole leads on a board or a panel. It uses a solder reservoir and pump system to deliver molten solder up through a nozzle to the leads coming through the bottom of the circuit board. This solder mechanism is built on an assembly that moves in multiple directions, and the movement is under the control of a program developed by the operators for the board or panel being assembled.
In addition to the movement, the programming also gives the operator precise control of the length of time that the solder is applied as well as the temperature of the solder to ensure the best solder joints. The programming can be developed on a computer with the selective solder system software in it based on the same Gerber data that is used for fabricating the board.
Selective solder fills the gap between the automated wave solder process and manual hand-soldering. Wave solder is the first choice for thru-hole component soldering, as it is quick and takes very little preparation or programming. Wave soldering does have its limitations, however, with boards that have dense parts placement on the back of the board. Some boards require a custom pallet to mask off surface mount components on the bottom side, allowing the solder wave to contact only the thru-hole pins.
In cases where the surface mount parts are too close and a masking pallet can’t be used, the only other option would be to manually solder the thru-hole leads. Manual hand-soldering is a time-consuming process that could introduce human error. With the selective soldering process, however, the potential problems with manual soldering can be eliminated.
What Are the Benefits of Selective Soldering?
One of the disadvantages of manual hand-soldering is that it takes time, and it requires that the circuit board undergo a great deal of heating to achieve a good solder joint. Applying that much heat can eventually cause thermal problems for the board, the components, and other solder joints that are already in place.
The selective soldering system, however, gives the operator the flexibility to manipulate the variables of soldering to achieve good solder joints in less time and with less overall heat. During the selective soldering process, the operator:
- Programs exactly where the nozzle of molten solder is supposed to go for applying the solder.
- Specifies how fast the nozzle should move to provide adequate heating time for the solder to fill the thru-hole.
- Programs how much solder is to be used and at what temperature it should be at.
This much control gives the operator much better precision in soldering without needing a steady hand to hold a soldering iron. What used to take a team of technicians hand-soldering thru-hole leads that weren’t able to go through the wave can now be done much faster using the selective solder system.
How Selective Soldering Can Help Your CM to Assemble PCBs
Consider for a moment a backplane with connectors on both sides. The tall profile of the connectors makes creating a custom pallet for the wave solder very difficult. It used to be that the only thing that would work on these boards was the tip of a handheld soldering iron. Now, instead of relying on manually hand-soldering the hundreds of pins of each connector on these backplanes, the automated tip of the selective solder nozzle can do the job in far less time with much better results.
Your contract manufacturer will usually default to wave soldering for speed of assembly. If there is only a very small amount of additional soldering required after wave soldering, manual hand-soldering can still be done in less time than running boards through selective solder. But if your board needs more than a little touch-up soldering, the selective soldering system is a much better choice for speed and accuracy while at the same time reducing the thermal abuse of the board.
