When thinking of soldering, the first image that comes to mind is the soldering iron and solder in the form of a small spool of a wire that has a rosin core which works as a fluxing agent. Soldering irons are pretty simple, consisting of a base that holds the wand and a small sponge. The wand has a tip that can be replaced with different sizes and shapes. Most soldering irons have a knob that allows the user to adjust the temperature as well as a readout of the temperature setting. Some economy irons have a preset temperature and a fixed tip.
The spools of solder come in different diameters and include the tin/lead formula that was the standard everywhere until the Reduction of Hazardous Substances (RoHS) directive went into effect twenty years ago. The home brewer of electronics still has access to the Sn63 eutectic solder while the industrial users lean into SAC305 which is Sn96 with impurities of silver and copper. The noble goal is to avoid putting more lead into landfills. The end result is that our PCB dielectric materials now have more options for resilience to the higher temperature soldering that comes with the “greener” solder.
Let’s unpack these two typical examples of solder wire. The Sn63 is 63% tin with lead making up the remainder. At some point, product labeling has changed from “Sn63” to “Sn63 - Pb37” to highlight the fact that this solder uses lead. There are several other formulas out there with their own trace metals so designating just the primary metal isn’t enough anymore. Aside from different diameters, it also comes with or without flux. The one with water soluble flux is known as no-clean solder.
Figure 1. Image Credit: Weller - A different type of soldering iron can use blow air on the leads which is useful when the component leads are on the small side.
Truth is that you can use either one and don’t really have to clean off the flux if that’s your thing. In the factory, we were told to clean it off to facilitate inspection of the solder joints. The rosin, essentially tree sap, is inert at normal temperatures but becomes a strong cleaning solvent when the heat is on. We used to routinely remove the flux from the boards after soldering by dunking the boards in a vat of bubbling freon. Isopropyl alcohol and a stiff brush will do the trick without destroying the atmosphere. Times change for the better and today’s no-clean solder is water soluble.
We got the lead out as well, thus the 96% tin in SAC305. It would be 100% tin except that would give rise to tin whiskers. There are other formulas available but this ratio hits a sweet spot where we can call the solder eutectic. This means that the solder has a lower melting point than either of the metals would in their pure state. It also means that there is a very small thermal window between the melting point and the solidification point. We don’t want the solder existing between those two states for long. While Sn63 is the eutectic ratio of tin and lead, the process happens at a much lower temperature than with Sn96.
Figure 2. Image Credit: Grainger - Soldering has been around for quite a while. Minor injuries have always been a part of it. Always wear personal protective equipment over your eyes and don’t lean in too much.
To hand-solder a wire or lead, you melt a little dab of solder onto the preheated tip of the iron and press it up to the wire. It only takes a moment for the heat of the tip to raise the temperature of the wire to the point where the little blob of solder suddenly jumps over and “wets” to the other metal surface and is said to be in flux. You can probably think of other situations where things were in flux. We get this fun little word from the latin fluxus, meaning to flow. Accordingly, the solder will continue to react by flowing into the hole in the board or the strands of the wire. At this precise moment, a little more solder can be fed into the spot where the iron’s tip touches the surface to be soldered.
Avoiding Solder Defects
As soon as there is enough solder to form a good connection, the soldering iron should be removed. Leaving it on for too long can roast the insulation of the wire or, in the case of a PCB, a few issues can arise. Lifted pads, solder balls and measles (local delamination) are three of the common defects when the soldering process runs too hot or spends too much time in the liquid state. Cutting the process short can lead to other defects such as non-wetting (cold solder) or disturbed solder. Solder peaks and bridges can occur if there is too much solder or the tip isn’t moving with purpose.
These and many other defects are enumerated in IPC-A-610. Meanwhile J-STD-001 is more of a practical how-to guide rather than laying out the inspection criteria as in the first document. These docs are helpful in pinpointing the root cause of the defects. Every one of the defects mentioned above leaves a tell-tale signature on the otherwise shiny and smooth connection.
Knowing the difference between a good quality solder joint and one that would be rejected is the first step to learning how…
Knowing the difference between a good quality solder joint and one that would be rejected is the first step to learning how to perform this task. It takes a good eye and a steady hand even with through-hole components. That said, most people could learn how to solder and desolder in a day given enough material and equipment.
Wave Solder and Drag Soldering To Automate Through-Hole PCB Assembly
Mass production of PCB assemblies goes well beyond hand soldering operations. A wave soldering machine has a “wave pool from hell” that sends a pulse of melted solder across the bottom of a waiting batch of PCBs that dream of becoming PCBAs. There are table top models but higher throughput calls for units the size of a clown car.
Meanwhile, drag soldering starts with a quiet pool of molten metal and has a conveyor belt that tracks across the top of the pool of solder for long enough to perform the same operation. Both of these types of equipment are huge investments in cost, upkeep and floorspace, not to mention heavy bars of solder.
Getting down into the surface mount devices (SMD) technicians typically do soldering under a microscope. Installing or reworking large packages are best handled with special equipment that blows extremely hot air into a rectangular nozzle that fits around the part. This is partially why certain types of components want a fairly large component clearance - known as a courtyard. This stuff is generally not for the hobbyist, particularly when it comes to the vanishingly small passive components and the ball grid array packages.
Figure 3. Image credit: Metcal - A hot air desoldering machine with a board preheater and a nozzle that can use adapters to rework different size integrated circuit packages.
While those items have become industry staples, there are still plenty of makers out there using the old-school through-hole components. With more and more components available in SMD packages, it has become possible to use only SMD parts for the entire build. That has become an expectation with a good number of contract manufacturers who want to get the soldering done in a single batch process.
That process is known as reflow. The solder bars and wires are out of the picture; replaced by paste. The paste is deposited on the PCB with a stainless steel stencil that has apertures that line up with all of the component pads. Then, while still at room temperature, the components are placed on the board by a robot known as a pick and place machine. Next comes some time in an oven. The oven preheats the boards until the paste is about to do what solder does best. A spike in the temperature sends all of the solderable lands into reflow followed by a cooldown solidification phase.
The preheat-reflow-cooldown cycle has to be dialed in for the component mix. Some parts will readily reflow while others may need a bit more time to reach the optimal temperature owing to their intrinsic thermal loads and, of course, the way you designed the layout. Most data sheets include a recommended temperature profile indicating the rise, dwell and post reflow ramp down.
If you’ve submitted enough boards to the assembly house, you’ve probably heard about treating each pad of the part in a similar way. If you flood copper over one side of the cap, you should do the same to the other pin. Otherwise, use a thermal relief for all of the pins.
Being consistent gives the assembers a better chance of bonding the parts to the board without the most infamous soldering defect of all, the tombstone. That’s when the experience of one pad is so different to the other that the part actually lifts up and away from one of the pads - like a drawbridge while in the reflow stage.