Have you ever had a project around the house where you needed to remove an old screw and discovered that it had a square head on it instead of a standard slotted or Phillips head? Those square-headed screws are called “Robertson,” in case you didn’t know. (And, in the interest of full disclosure, I didn’t know it either—I had to look it up.) Anyway, I ran into one of these screws the other day while doing some simple home repairs. Since I didn’t have a Robertson driver at hand, I tried to remove it with a regular Phillips driver instead and ended up stripping the screw head. You know what happened next: pliers, saws, frustration, some regrettable language, and way too much time used up by this one annoying problem.
The best way to deal with problems like these, whether it is a simple home repair or building a circuit board, is to use the correct tool for the job. There are many times during PCB assembly when the more traditional tools or methods—such as wave soldering—are not always the best choice for one reason or the other. The assembly technicians could spend a lot of time trying to make these traditional processes work. Or, they could use a tool that is specifically designed for the problem at hand.
The selective soldering system gives PCB assemblers the right tool they need to resolve some of the problems that wave soldering isn’t able to. Let’s take a look here at why selective soldering vs. wave soldering can help out the assembly of your printed circuit board.
The Basics of Selective Soldering vs. Wave Soldering
Wave soldering is the standard process that has been in use for many years to solder parts onto a circuit board. Although initially created for boards with all thru-hole parts, it will solder some surface-mount technology (SMT) parts as well. The circuit board rides a conveyor belt through a molten wave of solder that wicks up through the holes and around the pins to create the solder joints.
Wave soldering is the preferred method for production soldering because it can be quickly set up, and it will solder a lot of boards in very little time. Wave soldering does have some limitations, however, and not all boards can be run through this process. If a board cannot be wave soldered, then it must either soldered by hand or run through the selective soldering process.
Selective soldering is an automated system that pumps molten solder up from a reservoir through a nozzle to coat the leads extending out the bottom of a circuit board. The system will move in multiple directions to apply solder on every lead the operators program it for. In addition, the selective soldering system can be programmed for the length of time and the temperature of the solder that is being applied to get the best results. The selective solder system is an excellent tool that bridges the gap between wave soldering and hand soldering, and in some cases, it can be the most effective soldering choice.
When Selective Soldering Is the Most Effective
There are some conditions in which wave solder cannot be used, and hand soldering is not effective. In these cases, the only choice left is using the selective solder system. Some of these conditions include:
- Tall component height: The height of the solder wave has limitations, and some components are tall enough that they will block the wave from soldering the board.
- Tight component spacing: When thru-hole components are placed too close to SMT components, there might not be enough room to fit a protective fixture around the SMT parts that will allow effective wave soldering.
- Uneven heating: Thick boards, or boards with thick copper layers for power and ground planes, can cause a problem for hand soldering. It is difficult to get a single soldering iron to heat all of the thermally connected metal in the board sufficiently enough for the solder to flow through the holes to form a good solder joint.
- Dense concentration of thru-hole pins: When large connectors are being used with hundreds of pins, it can be difficult for a soldering iron to get into each pin to solder them all effectively.
In all of these situations, selective soldering can provide the solution. In addition to its ability to be programmed to handle unique pin configurations, selective solder also has other benefits as well. The selective solder system will utilize a far-side infrared preheater to balance uneven heating of thick copper boards. It can also use a wider nozzle that can solder two or three rows of connector pins in a single pass, considerably cutting the amount of time needed for soldering. This ability to quickly solder hard-to-reach areas that were traditionally only able to be hand-soldered is where selective solder really shines.
Another benefit that a contract manufacturer that offers selective soldering is in its consistency. Hand soldering can understandably produce variations in its results, but the selective soldering system will repeatedly give you the same consistent solder joint each and every time.
How Your CM Can Use Selective Soldering to Best Help You
Usually, the circuit board that you have built by a CM will have a part configuration that can be wave soldered. There are those conditions, though, that can make wave soldering difficult or even expensive, depending on the amount of fixturing that is required. Additionally, there may be scheduling requirements that won’t allow time to create the fixturing needed for wave soldering. In cases like these, it may make more sense to go straight to using selective solder to give you the results you need for your build.
At VSE, our manufacturing engineers have extensive experience, and they know which soldering processes will make the most sense for your PCBA. Our technicians have years of experience running boards just like yours through all of our different assembly processes, and you can be assured that your board will be run through the soldering process that will best fit your needs.
