The key factor of soldering technology

Throughout history, as society has evolved, so has the need for bond- ing metals to metals. Whether the need for bonding metals is mechani-cal, electrical, or thermal, it can be accom- plished by using solder.Metallurgical Bonding Processes Attachment of one metal to another can be accomplished in three ways: welding, brazing, and soldering. The most signif- icant difference between these methods is the processing temperature and melt- ing temperatures of the metal. Welding  is the highest temperature process, often performed at temperatures above 1500°C. While brazing is accomplished at temper- atures above 450°C, only soldering can be performed at low enough temperatures to take place in the presence of organic mate- rials, such as printed wiring boards (PWBs) and the plastics in electronic component packages. Thus, soldering is a fundamen- tal technology that enables electronic as- sembly. Without soldering, we would not have an electronics industry.

Soldering uses a filler metal and, in most cases, an appropriate flux. The filler metals are typically alloys (al- though there are some pure metal sol- ders) that have liquidus temperatures below 350°C. Elemental metals com- monly alloyed in the filler metals or solders are tin, lead, antimony, bis- muth, indium, gold, silver, cadmium, zinc, and copper. By far, the most common solders are based on tin. Fluxes often contain rosin, acids (or- ganic or mineral), and/or halides, de- pending on the desired flux strength. These ingredients reduce the oxides on the solder and mating pieces.

Basic Solder Metallurgy
As heat is gradually applied to sol- der, the temperature rises until the alloy’s solidus point is reached. The solidus point is the highest temper- ature at which an alloy is completely solid. At temperatures just above solidus, the solder is a mixture of liquid and solid phases (analogous to ice mixed with water). Further temperature increases bring the liquidus point, the lowest tempera- ture at which the alloy is completely molten. The solder remains in the fully liquid or molten state at tem-
peratures above the liquidus point. Upon removal of the heat source, the cycle is reversed, i.e. the solder’s physical form changes from completely liquid to liq- uid and solid to completely solid. Graphs that plot temperature vs. composition are known as phase diagrams and are widely used to determine the phases and inter- metallic compositions of solder at a given temperature.

Soldering  101  —  A   Basic   Overview The temperature range between the soli- dus and liquidus is known as the plastic zone of the solder. If the solder joint is mechanically disturbed while the assem- bly is cooling through its plastic region, the solder crystal structure can be dis- rupted, resulting in a high electrical resis- tance. Such solder joints with high elec- trical resistance are referred to as cold solder joints and are undesirable. To avoid this problem, it is best to select a solder that has a narrow plastic range, typically less than 10°C. There are some solder alloys that have no plastic region (liquidus = soli- dus). These are known as eutectic alloys. As heat is applied to a eutectic alloy, the sol- der passes directly from solid to liquid in- stantaneously at the eutectic melting point of the solder.

Alloy Selection
The most common solder is tin/lead-based. The eutectic version, Sn63/Pb37, has a melt- ing point of 183°C; the Sn60/Pb40 variation has a melting range of 183°–188°C (183°C  is the solidus; 188°C is the liquidus). Higher lead versions of this alloy system have higher melting ranges. Often, 2%  silver  is  added to the 63/37 mix to strengthen the alloy or prevent excessive silver dissolution from sil- ver-plated circuitry. This alloy, Sn62/Pb36/ Ag2 has a melting range of 179°–188°C. Alloy compositions can accommodate var- ious needs: lead-free, gold-containing, in- dium-containing for soldering to gold, ther- mal fatigue resistant, or specific (high or low) melting points. A multitude of solders with specific physical properties are available to match specific design requirements.

Fluxes
The purpose of a flux is to remove surface oxides on substrate metallizations, compo- nent leads, and the solder itself to allow ade- quate wetting. Flux selection is based on the substrate/component metallization to be sol- dered and/or the desired cleaning procedure. Metallizations that are prone to forming te- nacious oxides will require a stronger flux. Fluxes can be no-clean (i.e. the PCB assem- blies do not require cleaning after soldering), solvent cleanable, such as RMA (resin mildly

activated) fluxes, or water-washable.
To form a good solder bond, the solder alloy must wet adequately to the substrate metal. This means that no surface oxides can be present. Because most non-precious metals and alloys oxidize to some degree, the oxides must be removed. Fluxes pro- vide three basic functions:
1)Reducing (chemically dissolving) ox- ides on the surface of the substrate metal- lization and the solder alloy itself,
2)Coating the solder joint location. Fluxes displace the air and protect the surface so that oxides do not reform during the soldering process. Since soldering re-
quires elevated temperatures, there is more energy for oxides to form. Preventing re-oxidation during sol- dering can be just as important as removing the initial oxides, and
3)Promoting the flow of the sol- der. Molten solder alloys are sub- ject to surface tension physics just like any other liquid. Using an au- tomobile finish as an example, flux can be thought of as an anti-wax. On a freshly waxed automobile,

Soldering Temperature
As a rule of thumb, the soldering tempera- ture should be 30°–50°C higher than the liq- uidus temperature of the alloy. This ensures that enough heat energy is available to form a good metallurgical bond between solder and substrate. With lower temperature solders, it is better to gravitate toward the 50°C end of this range. In a lower temperature process, there is not much  energy inherently present to form a good solder joint.
The time that the solder is molten should be kept to the minimum required to acheive a good bond. Excessive time and tempera- ture will cause intermetallic formation within the joint to proliferate. This typically leads to a brittle solder bond. Depending on the pro- cess, the time above liquidus (TAL) of the solder will typically range from a few sec- onds (for hand soldering with wire) to a min- ute or more (as with solder paste in a reflow oven). When performing multiple soldering steps (step soldering), it is advisable that the first solder alloy have a solidus temperature of 50°C higher than the liquidus of the solder alloy used in the next step, and so on.

Soldering Heat Sources
Heat sources for soldering range from low- tech handheld soldering irons to high-tech lasers. Typical equipments include solder- ing irons, hot plates, static convection ov- ens, belt furnaces (both convection and IR), induction heating apparatuses, resis- tance heating devices, and lasers. The heat source can be dependent on the form of sol- der used; for example, solder pastes for PCB assembly are commonly used with multi- zone belt furnaces. This is because solder pastes are best used with a temperature pro- file that incorporates a specific temperature ramp-up and cool-down.

Summary
Creating a metallurgical  solder  bond  can be very easy if the appropriate metalliza- tions, alloys, fluxes, and processes are used. Although soldering has a wide variety of ap- plications, because of its ability to form a solder joint at low temperatures that circuit boards and electronic components can tol- erate, it is a foundation technology for the electronics industry. Without the marvel of soldering, we would not have mobile phones, personal computers, televisions, and  all of the many electronic devices that we take for granted.