--Solder Quality

There is no one universal solder alloy. Tin-lead solder combinations were formerly used widely. These solder combinations melt between 180 and 190 °C, which is close to or over the glass transition temperature for a large number of PCB laminates. Due to the negative health consequences of lead, the industry is transitioning away from lead and toward lead-free solder in compliance with the European Union’s Restriction of Hazardous Substances in Electrical and Electronic Equipment (RoHS) Directive. Lead-free and silver alloy solders melt at much greater temperatures (about 220 and 450 degrees Celsius, respectively).

Regardless of the kind of solder used, the temperature and ability of the solder to wet a surface will impact the strength of the PCB solder bond. Additionally, each kind of solder may generate oxides when the improper type of flux is applied, reducing the strength of the PCB solder junction.

--Temperature

When solder is heated, the metals combine to generate a hot eutectic, which wets the surface of a pad (for surface-mount components) or vias (for through-hole components). The surface tension of the molten solder draws it into a convex “fillet” form along the component’s edge during wetting, and the solder cools after the heat source is withdrawn. However, if the solder temperature is too low during heating, the eutectic will not be properly blended and will harden as a cold junction, in simpler words, the solder joint strength won’t hold.

A cold joint has a dull, pockmarked surface and poor strength as a result of inadequate eutectic mixing. The absence of heat on the junction during soldering may be attributed to one of two factors:

1.Inadequate iron temperature: The soldering iron was set to a temperature that was too low for the kind of solder being used.

2.Excess heat dissipation: An excessive amount of heat was evacuated from the solder spot, lowering the eutectic temperature during soldering.

In reference to the second point above, if pads or traces are linked to a large ground plane, a cold joint may occur. Without thermal relief, a pad attached directly to a plane through a via will conduct heat from the soldering iron to the plane. If not handled effectively during the design process, fractures in the joint might develop over time, finally resulting in failure.

--Mechanical Stress-Related Events

Mechanical overstress failures occur when a solder joint is subjected to excessive loading as a result of a mechanical event such as a shock, drop, in-circuit testing, board depanelization, connection insertion, or PCBA insertion.

Overstress failures may be difficult to avoid since they are often unpredictable. According to shock testing studies, the optimal answer is a random distribution of such failures.

Overstress failure of a solder joint often expresses itself as a pad crater or joint fracture along the intermetallic connection (IMC). A pad crater is a break in the laminate layer underneath the copper pad of a solder junction that is formed like a crater. IMC refers to the area in which the copper pad and solder join to generate Cu3Sn or Cu6Sn5. It is the most fragile section of the solder junction, which makes it particularly prone to overstress.

This sort of failure occurs more often with finer pitch components — notably BGAs — or with the usage of very brittle laminates. Pad cratering is a major problem, since it often results in trace fractures. In contrast to fatigue cracks, which often run the length of a solder junction, mechanical overstress failures appear as joint fractures along the IMC.

Due to the significant degree of dependence on PCB boundary conditions and geometries on mechanical event failures, FEA is often suggested for predicting mechanical overstress risk. Other approaches have difficulty predicting complex loading circumstances or board designs. Additionally, FEA enables the measurement of strain and curvature.

--Solder Tip Health

Maintaining a clean soldering tip is critical for the resulting connection’s visual quality.

Soldering tips are coated with a unique material that enables them to rapidly heat metals and adds temperature sensing to the tip. As you solder, slag (unusable metal) and burned particles (by-products of the burning process) accumulate.

Drag your soldering point over on both sides using a natural sponge dampened with water. This creates an impeccably clean tip for the subsequent application. Apply a little amount of new solder to it and proceed as usual.

--Oxidation and Flux

While the majority of PCB assemblers are not chemists, it helps to grasp some fundamental chemistry in order to appreciate the importance of solder flux. While soldering, molten solder generates a hot eutectic, which readily reacts with oxygen to form oxides. Metal oxides have a lower mechanical strength than pure metals.

Flux is used to prevent oxidation and to attract solder to the target location during assembly. Flux also has the advantage of aiding in the removal of any greasy leftovers from the region. During PCB construction, the proper flux and an adequate quantity of flux must be matched to the correct kind of solder.

--Which Flux Can be Used for Soldering?

Occasionally, contaminants such as grease, dirt, or oxidation form at the joint site; the flux helps prevent oxidation and may occasionally

chemically clean the metal. Rosin flux is employed, which improves the mechanical strength and electrical contact of electrical junctions. 

Occasionally, a ‘wetting agent’ may be used to lower the surface tension.