How to Solder - for Beginners and Experts

Copyright 2005 R.G. Keen. All rights reserved. No permission for display from web sites other than without written permission.

Soldering is a mysterious process, at some level even to metallurgists that study it for a living. We know a little bit about how it works, and a few Rules to make it work most (99.99999%) of the time.

How it works (as far as we know).
Soldering is a process where a metal is melted to the point where it is a liquid, and then the liquid wets a surface before freezing again. Liquid solder is not as aggressive a wetting agent as water is, so we have to use chemicals to help it wet the surface of the metals. Those are the fluxes we use. In addition, the solder and metals being soldered have to be hot enough, as there seems to be a minimum temperature below which the specific metals involved won't be wetted. This temperature is different for each solder.

The metallurgical literature talks about the formation of intermetallic alloy regions in the interfaces between adjoining surfaces under conditions of.... ACK!!! It's wetting, like water wets some surfaces and rolls off others (think freshly waxed car here).

Each solder??? There's more than one?
Yes. A "solder" is just a wetting-metal. Solders may be single metals (especially in high temperature jewelry soldering) or mixtures (almost all low temperature solders). Electronic solder is usually a mixture of about 60% tin and 40% lead referred to for some reason as 60-40. A mixture is used because often a mixture of metals melts at a temperature lower than either of the constituent metals. For instance, a 63-37 mixture of tin-lead melts at a lower temperature than any other recipe for tin and lead from 100% tin to 100% lead. This lowest-temperature mixture goes directly from solid to liquid and vice-versa, no slushy plastic transition temperature. This quality is called being a eutectic solder. 60-40 electronic solder is almost eutectic.

For electronics, never, never, never use anything but 60-40 electronic solder or 63-37 eutectic. Do NOT use 50-50 plumbing solder even if you can find it.

Non-eutectic solders are prone to making cold joints by being moved as they cool through the plastic region. In the plastic region, crystals of excess pure tin or lead form in the liquid melt of eutectic liquid. These crystals make the joint get grainier as it cools, and movement can make cracks along the crystals, forming a high resistance joint. This is why you should never move a solder joint while it cools.

Now for some how-to's:

Rule 1: Get the iron clean first. Before you even think about soldering an effect, get your soldering iron tip clean and tinned with a smooth, shiny layer of melted solder. This may require replacing the tip, or it may require you sanding or filing the tip down to bright shiny metal in extreme cases and then fluxing and applying solder to the cleaned tip. If the tip is not mirror-shiny with a micro-thin layer of molten solder, you cannot solder successfully.

Rule 2: Get the joint clean. To wet the metals of the base joint, the metals have to be clean. No grease, oxide, leftover chocolate, silly-putty, etc. If the metals are not clean, the solder will not wet uniformly, and this fact will be hidden under a blob of solder. Metals vary in their ability to be wetted. Frozen solder is probably the easiest to solder. Metal with a layer of pure tin or pur lead are next easiest, and this is why resistor leads are tinned with either pure tin or tin-lead solder. Eventually even solder, tin, or lead grow an oxide layer that is resistant to the wetting of solder. This is why old resistors sometimes will not solder without cleaning their leads.
Gold seems to be solderable forever, which is why semiconductors from the Second Golden Age of electronics had gold-plated leads. Copper is moderate. Bare copper PCB stock needs to be cleaned until it is shiny and salmon pink to have the best solderability.

Rule 3: Use flux. The simplest thing to do is to use rosin-flux cored solder. This is solder that has a central channel filled with rosin flux. Never, never, never, never use plumbing flux, paste flux, or any other flux, no matter how it tells you it's non-corrosive, unless it's explicitly intended for electronic use. If you do, it will eat the copper under the joint if you don't clean it better than a home constructor can clean a board.

Rule 4: Get in fast with high heat, and get out. It is easier to heat damage components with a cooler iron than a hotter one (within reason). This is because it takes longer for a cooler iron to bring a joint up to temperature than a hotter one. Heat travels in the joint at a fixed rate, so if you use a high temperature iron and deposit a lot of heat quickly, the joint reaches soldering temperature before the heat has conducted down the conductors very far.

Rule 5: Heat the joint, not the solder.  Never, never, never, melt a blob of solder on the iron and drip it on the joint. It will not stick because the joint metals are not hot enough to wet. Instead, touch the tinned iron to the joint and heat it until the solder melts when touched to a part of the joint that is not the iron. Ideally, you put the iron on the far side of the joint, touching all the bits of metal to be soldered, and touch the solder on the side away from the iron. When that far side is hot enough to flow the solder, you are certain that the bit nearer the iron are above the soldering temperature, and you (probably) get a good joint. When this happens, get that iron off the joint before you heat damage something.

Rule 6: Let it flow, let it flow, let it flow. When solder is properly melted into a clean, well fluxed joint, it suddenly flows like water into the joint and wets all the metal surfaces. The surface is mirror shiny, and the solder will not hold itself up in blobs above the joint. If the solder is a bead-like blob obscuring the inner pieces of the joint, it's likely that the joint is bad; at best, it's an unreliable joint.

Rule 7: Let it cool in place. It is by far best if the joint is mechanically arranged so it holds itself in position all by itself and none of the pieces move if you let go of them. What you want is for the joint to start cooling without any of the pieces inside the solder joint moving until the solder is completely frozen. Moving a piece as it cools makes an internally cracked joint that can oxidize along the cracks over time and make for an intractably difficult debugging problem in something that used to work before the joint oxidized.

An experienced solder-er will prepare a joint (or likely, a number of joints) for soldering by cleaning and arranging the pieces to lie still while he solders. He will then pick up the iron in his dominant hand, the solder in his non-dominant hand, clean the iron tip of any last-minute oxides by swiping it on a cleaning sponge or stainless-wool cleaner. He will then start the dance: move iron and solder to the joint; touch the joint with the iron for a tiny interval of time, then touch the solder to the far side of the joint; the flux will flow out of the solder's core, a bit of rosin smoke will appear and then the solder will flow throughout the joint; then the solder and iron come off the joint quickly and decisively.

All this is maybe two seconds. Next joint.