Types of Solder Delivery Material and Solder Composition

Welcome back to WISDOM WEDNESDAY! In this post we’ll start by looking at the different types of solder delivery materials and then we’ll take a closer look at the constituents of the two most common solder compositions before briefly touching on the flux contained in those solders. Wisdom Wednesday is brought to you by MICRON CORPORATION, where electronic assemblies are built with Micron precision.

Three Types of Solder

There are three major physical structures for delivering solder to the electronics manufacturing industry. These are bar solder, wire solder, and solder paste. They are used in different parts of production processes. Bar solder is used in wave machines and smaller solder pots. Wave machines are mostly used for soldering through-hole parts (THT), though they can certainly be used for surface mount parts (SMT) as well, usually with the help of some sort of epoxy or glue to hold those SMT parts in place. Wire solder is used for hand soldering and for feeding into automated soldering equipment. Solder paste is a more developed technology that is used to print, place, and reflow PCB assemblies. Mostly used for SMT components, it is also used for a technique known as Pin-in-Paste for the soldering of THT components in a reflow oven. It is estimated that at least 90% of components present in watches, cameras, memory cards, smart cards, thermal print heads, laser-jet printers, televisions, calculators, PCs, laptops, notebooks, and handheld devices are SMT components (“The Mechanics of Solder Alloys: Interconnects,” Darrel Frear, Harold Morgan, Steven Burchett, John Lau: Van Nostrand Reinhold, 1994).

Solder Composition

There are many, and I mean many, different solder compositions for numerous purposes. And several different elements make up those solders. The most popular among them are Tin (Sn), Lead (Pb), Silver (Ag), Copper (Cu), Nickel (Ni) and Bismuth (Bi). There are two solder compositions that we consider in this post. Those two are the Sn/Pb solder and SAC305 solders. These two types, like many others, can be deployed in bar, wire, and paste forms.

Tin-lead solder remains in use in some military, aerospace, and other high-reliability applications. Alloy selection depends on the product’s qualification requirements and service conditions. AIM Solder’s Military and Aerospace Applications section includes both tin-lead and lead-free materials. A component’s temperature rating alone does not determine which solder alloy should be used.

SAC305

Almost all solder involves at least two elements. Though some high temperature solders do not. But most solders are alloys, that is to say, they are a combination of two or more metallic elements. SAC305 has three elements, and that gets more complicated in terms of understanding the phase systems involved. Let’s break down the alphanumeric acronym for this solder. S stands for Tin (Sn), A stands for Silver (Ag), and C stands for Copper (Cu). The numeric characters that follow represent the percentages of the last two (A and C) with the first (S) comprising the rest. Ag (Silver) is 3 percent of the composition of SAC305. Cu (Copper) is 0.5 percent. The rest is Tin (Sn). This is a system more complicated than the Sn/Pb system, and perhaps the best suited, almost certainly the most used, to replace solder containing Lead.

Fluxes and No Clean Solder

Solder paste is made up of metal alloys and a variety of liquids and suspended solids that we’ll collectively call flux. Flux is useful for several aspects of the soldering process including preparation of the surface, activation of the solder, and wetting of the surface. These are accomplished with what are known as vehicles, activators, and additives. Flux is a useful, and often necessary, component for soldering. And the residues it leaves behind were usually cleaned from the final PCB assembly, often with chloro-fluoro-carbons or CFCs. When those cleaning agents were banned in the 1990s a type of flux that is referred to as “No Clean” became widely adopted. Since then No Clean has become a major category of fluxes in the industry. And then came the cleaning of the “No Clean.”

That brings us to the end of this Wisdom Wednesday. We look forward to sharing much more about solder, flux, and cleaning PCB assemblies. You can always share information with us at MICRON CORPORATION by contacting info@microncorp.com.

Continue the discussion in Solder Paste Flux, or explore Intermetallics and Why They Matter.