In our last installment of Tools of the Trade, we had just finished doing the inspection of the surface mount part of the PCB. Next in the process is the through hole components. Depending on the PCB, the order may change slightly, but generally it makes more sense to get all the SMT work done before moving to the through hole work.
Through hole used to be the standard, but as the need for size reduction and automation increased, SMT gained favor. However, there are still a lot of reasons to use through hole components, so they aren’t going away entirely (at least not any time soon). One of the biggest advantages of THT is mechanical strength, which makes it better suited for connectors than SMT. If you’ve ever popped a microusb connector off a PCB by breathing on it heavily, you’ll understand. So, how do we most efficiently get through hole components on a PCB, and how do the big boys do it?
This is how most people start, and is still surprisingly common even in mass production. One of the tools involved is the helping hands, which can hold your PCB while you solder. These can get cumbersome quickly, though, so the next step up is dedicated PCB holding tools, which have rails and slotted rubber pieces that hold the edges of the PCB.
One problem with both of these, though, is that you generally have to put the component in on one side and solder it on the other side, so components have a tendency to fall out when you flip the PCB over. There are lots of ways around this, from bending the leads as you insert (or opposite corners on DIP packages), to soldering briefly on the top side to tack it on, then finishing on the other side.
Others have gone with more creative solutions, including the bed of nails approach, or foam that holds the parts in place while the PCB is flipped over and soldered by hand.
Sometimes you need to stand off a component from the board, and it has to be the same height off the board. In this case, one option is to use a standoff around one or multiple leads. Another is to design a jig that acts as the negative of the PCB. It is machined or 3D printed, and then the jig is placed over the PCB. When the pair is flipped over the components drop down to the correct height (or stay where they are), and can be soldered by hand.
Surprisingly, there are tools that automate the placement of through hole components. Or maybe not surprisingly, if you consider LED panels. These outdoor signs use through hole LEDs, and some quick math leads one to the conclusion that automatic placement of LEDs is a necessity.
Some LEDs come on strips of tape and a machine cuts the leads at the tape and places the LED where it needs to go. Lots of LEDs come in bags, though, and are placed in a hopper, which spins and spits out a line of LEDs, all oriented in the right direction. In either case, the LED makes it to a gantry which cuts the leads and places the LED in the holes. The same is done for resistors or other axial lead components.
Imagine a coffee-mug sized container with a heating element around it. Then put a bar of solder in it. No flux, just a block of solder. Heat it up until it melts, and you have a solder pot. This is good for dipping the edge of a PCB in (not all the way, just the bottom) to get the pins of a connector to solder. You need to coat the PCB with flux first, because there isn’t any in the solder pot. This method is great for certain volumes and sizes, where you may only have one or two connectors that need to be attached to an otherwise completely SMT PCB, but if you want to do a bunch of connections on a larger PCB, you’ll need to use a wave soldering machine.
As a side note, solder pots are also great tools for recovering components from PCBs, since they can heat up the whole component quickly from the underside, making it easy to remove, clean, and with minimal danger of overheating the component.
We’ve covered dangerous homemade solder pots before, but you should probably just buy a used one if you want.
The wave soldering machine takes molten solder and pushes it up through a long channel with a pump so that there is a long thin fountain of solder coming up a few millimeters. Then the PCB is dragged along some sort of conveyor so that the fountain just touches the bottom of the flux coated PCB, and the wave instantly attaches to the through-hole pads and pins and makes the connection so beautifully. This is an easily automated process, provided the bottom is clear of components. If there are components on the bottom, they will need to be masked first.
A bastard child between the wave solderer and a solder pot is the dip solderer. You put your PCB on a metal frame, and the frame dips down onto a molten sheet of solder until just the bottom is touching the solder, and then it is raised up. It’s a pretty simple mechanism that doesn’t take up a lot of space, but the boards still need to be coated underneath with flux.
There’s still a place for through hole, and even in mass production it’s very common. While the world is moving towards surface mount, there are still perfectly legitimate reasons (connectors and high power applications especially) for using through hole. It’s particularly easy to do by hand, and as production scales there are some tools that make it easier. We haven’t seen a homemade wave soldering tool yet; solder pots are about the extent of DIY tools for mass production of through hole parts, but they’re perfectly capable of handling pretty large volumes.