TECH solution: solder The Basics of Soldering

summAry By Chris Nash, Indium Corporation

In In this article, I will present a basic overview of soldering for those Soldering uses a filler metal and, in who are new to the world of most cases, an appropriate flux. The soldering and for those who could filler metals are typically alloys (al- though there are some pure metal sol- use a refresher. I will discuss the ders) that have liquidus temperatures below 350°C. Elemental metals com- definition of soldering, the basics monly alloyed in the filler metals or of metallurgy, how to choose the solders are tin, lead, antimony, bis- muth, indium, gold, silver, cadmium, proper alloy, the purpose of a zinc, and copper. By far, the most flux, soldering temperatures, and common solders are based on tin. Fluxes often contain rosin, acids (or- typical heating sources for soldering ganic or mineral), and/or halides, de- operations. pending on the desired flux strength. These ingredients reduce the oxides on the solder and mating pieces. hroughout history, as society has evolved, so has the need for bond- Basic Solder Metallurgy ing metals to metals. Whether the As heat is gradually applied to sol- need for bonding metals is mechani- der, the temperature rises until the Tcal, electrical, or thermal, it can be accom- alloy’s solidus point is reached. The plished by using solder. solidus point is the highest temper- ature at which an alloy is completely Metallurgical Bonding Processes solid. At temperatures just above Attachment of one metal to another can solidus, the solder is a mixture of be accomplished in three ways: welding, liquid and solid phases (analogous brazing, and soldering. The most signif- to ice mixed with water). Further icant difference between these methods temperature increases bring the is the processing temperature and melt- liquidus point, the lowest tempera- Figure 1. Cross-section of an ing temperatures of the metal. Welding ture at which the alloy is completely acceptable solder joint with surface is the highest temperature process, often molten. The solder remains in the graping. performed at temperatures above 1500°C. fully liquid or molten state at tem- While brazing is accomplished at temper- peratures above the liquidus point. Upon Soldering 101 — A Basic Overview atures above 450°C, only soldering can be removal of the heat source, the cycle is The temperature range between the soli- performed at low enough temperatures to reversed, i.e. the solder’s physical form dus and liquidus is known as the plastic take place in the presence of organic mate- changes from completely liquid to liq- zone of the solder. If the solder joint is rials, such as printed wiring boards (PWBs) uid and solid to completely solid. Graphs mechanically disturbed while the assem- and the plastics in electronic component that plot temperature vs. composition are bly is cooling through its plastic region, packages. Thus, soldering is a fundamen- known as phase diagrams and are widely the solder crystal structure can be dis- tal technology that enables electronic as- used to determine the phases and inter- rupted, resulting in a high electrical resis- sembly. Without soldering, we would not metallic compositions of solder at a given tance. Such solder joints with high elec- have an electronics industry. temperature. trical resistance are referred to as cold

www.smtonline.com ı July/August 2009 ­­ı 1 solder joints and are undesirable. To avoid Soldering Temperature this problem, it is best to select a solder that As a rule of thumb, the soldering tempera- has a narrow plastic range, typically less ture should be 30°–50°C higher than the liq- than 10°C. There are some solder alloys uidus temperature of the alloy. This ensures that have no plastic region (liquidus = soli- that enough heat energy is available to form a dus). These are known as eutectic alloys. As good metallurgical bond between solder and heat is applied to a eutectic alloy, the sol- substrate. With lower temperature solders, it der passes directly from solid to liquid in- is better to gravitate toward the 50°C end of stantaneously at the eutectic melting point this range. In a lower temperature process, of the solder. there is not much energy inherently present to form a good solder joint. Alloy Selection The time that the solder is molten should The most common solder is tin/lead-based. be kept to the minimum required to acheive The eutectic version, Sn63/Pb37, has a melt- a good bond. Excessive time and tempera- ing point of 183°C; the Sn60/Pb40 variation ture will cause intermetallic formation within has a melting range of 183°–188°C (183°C the joint to proliferate. This typically leads to is the solidus; 188°C is the liquidus). Higher a brittle solder bond. Depending on the pro- lead versions of this alloy system have higher cess, the time above liquidus (TAL) of the melting ranges. Often, 2% silver is added solder will typically range from a few sec- to the 63/37 mix to strengthen the alloy or onds (for hand soldering with wire) to a min- prevent excessive silver dissolution from sil- ute or more (as with solder paste in a reflow ver-plated circuitry. This alloy, Sn62/Pb36/ oven). When performing multiple soldering Ag2 has a melting range of 179°–188°C. steps (step soldering), it is advisable that the Alloy compositions can accommodate var- first solder alloy have a solidus temperature ious needs: lead-free, gold-containing, in- of 50°C higher than the liquidus of the solder dium-containing for soldering to gold, ther- alloy used in the next step, and so on. mal fatigue resistant, or specific (high or low) Figure 2. An example of good solder wetting (A) and poor melting points. A multitude of solders with wetting (B). Soldering Heat Sources specific physical properties are available to Heat sources for soldering range from low- match specific design requirements. tech handheld soldering irons to high-tech activated) fluxes, or water-washable. lasers. Typical equipments include solder- Fluxes To form a good solder bond, the solder ing irons, hot plates, static convection ov- The purpose of a flux is to remove surface alloy must wet adequately to the substrate ens, belt furnaces (both convection and oxides on substrate metallizations, compo- metal. This means that no surface oxides IR), induction heating apparatuses, resis- nent leads, and the solder itself to allow ade- can be present. Because most non-precious tance heating devices, and lasers. The heat quate wetting. Flux selection is based on the metals and alloys oxidize to some degree, source can be dependent on the form of sol- substrate/component metallization to be sol- the oxides must be removed. Fluxes pro- der used; for example, solder pastes for PCB dered and/or the desired cleaning procedure. vide three basic functions: assembly are commonly used with multi- Metallizations that are prone to forming te- 1) Reducing (chemically dissolving) ox- zone belt furnaces. This is because solder nacious oxides will require a stronger flux. ides on the surface of the substrate metal- pastes are best used with a temperature pro- Fluxes can be no-clean (i.e. the PCB assem- lization and the solder alloy itself, file that incorporates a specific temperature blies do not require cleaning after soldering), 2) Coating the solder joint location. Fluxes ramp-up and cool-down. solvent cleanable, such as RMA (resin mildly displace the air and protect the surface so that oxides do not reform during the Summary soldering process. Since soldering re- Creating a metallurgical solder bond can quires elevated temperatures, there be very easy if the appropriate metalliza- is more energy for oxides to form. tions, alloys, fluxes, and processes are used. Preventing re-oxidation during sol- Although soldering has a wide variety of ap- dering can be just as important as plications, because of its ability to form a removing the initial oxides, and solder joint at low temperatures that circuit 3) Promoting the flow of the sol- boards and electronic components can tol- der. Molten solder alloys are sub- erate, it is a foundation technology for the ject to surface tension physics just electronics industry. Without the marvel of like any other liquid. Using an au- soldering, we would not have mobile phones, tomobile finish as an example, flux personal computers, televisions, and all of can be thought of as an anti-wax. the many electronic devices that we take for On a freshly waxed automobile, granted. SMT water will bead up, while on an un- Figure 3. Cross-section of a large-crystalline waxed automobile finish, the water Chris Nash, TITLE, Indium Corporation, may grain structure solder joint. easily flows. be contacted at NEED INFO.

2 ­­ı July/August 2009 ı www.smtonline.com