2005 Articles

Low-tech, single-sided boards can be complicated to solder.

Machine soldering of PCBs with non-plated through-holes is often more difficult than soldering PCBs with plated through-holes. Bridging may occur more readily due to the comparatively narrower gaps between the solder joints. The pad dimensions on a single-sided board are often larger than those on a PCB with PTHs in order to provide sufficient adhesive strength between the base material and copper cladding to which joints are soldered.

Another problem often seen are joints that have not been filled completely with solder – “open joints.” Even knowing the reasons open joints occur, there is no easy way to fix these problems during soldering through the process settings. In fact, it is often remarkable that fully soldered joints will result when soldering boards with non-plated through holes, if we take a look at what can go wrong.

If solder bridging between joints occurs because of too narrow a space between these joints, using a solder-resist screen with smaller solder spot apertures may provide a solution for future runs.

When soldering joints on PCBs with non-plated through-holes, it may occasionally be difficult, or even impossible, to create a completely filled joint. Here’s why: the formation of a solder joint is the result of wetting forces and capillary forces.

These forces can only work on metal parts with good solderability, and also when the solder is in a liquid state. As soon as the equilibrium of wetting forces, or capillary forces, is disrupted, complete joint filling will be compromised.

When PCBs with a HASL coating are used, we might see, after lengthy storage, a reduction in solderability of the inner and outer areas of the solder pad. This is because the coating is very thin at the edges of the pad. If the coating is less than 3 µm thick, migration of the intermetallic layer can bring intermetallics to the surface of the coating. When these intermetallics oxidize, they result in poor solderability. This can even increase the gap between the lead and the solderable part of the pad, making the joint more unstable during its formation.

Poor solderability can also be caused by “bleeding” soldermask or misprinting of the soldermask. During the insertion and bending of the component lead, part of the solderable lead finish may be removed by the bits of the insertion tool. This may create a less solderable or even non-solderable area on the lead, possibly causing an open joint.

Every joint on single-sided PCBs will have an air gap between the lead and the pad to be soldered; thus we begin with a obstacle. Because air is not solderable and no solderable hole plating exists, this gap will weaken the force between the lead and the solder pad at the gap position.

This obstacle can be overcome if the difference between hole diameter and lead diameter is <0.3 mm. Due to the surface tension of the solder, this small air gap can then be bridged, but it will be a unstable situation as long as the solder is not solidified.

If the holes are drilled, often a burr around the hole is formed due to a drill that is blunt or not in good shape. A burred pad, having a sharp edge near the hole, increases the risk of open joints.

A damaged solder pad edge may negatively affect this unstable situation. The sharp burr on the pad edge disturbs the capillary gap between the pad and the lead, and will thereby cause an open joint.

The presence of foreign non-wettable parts on the lead, pad or between lead and pad could cause an open joint, since such parts disturb the capillary gap between pad and lead.

If the topside of the hole is covered by the component body, for example, or if there is a flux film between the component and the PCB, flux vapors cannot escape via the top side. These vapors will then escape via the bottom side through the liquid solder in the joint and create a hole in the joint.

If the PCB material has absorbed too much humidity, e.g., during storage in a humid environment, this may cause outgassing of the PCB via the holes during soldering. This is often the case with materials that have a relative “open” structure in the holes, such as FR-2 material. This will increase the risk of open joints.

When humidity is high, PCBs can be stored in a dry cabinet to prevent moisture absorption. If PCBs have already been exposed to a humid environment, they should be pre-dried before soldering. Pre-drying will take more time than the general preheating time in the soldering machine.

Gerjan Diepstraten is a senior process engineer with Vitronics Soltec BV
(vitronics-soltec.com); gdiepstraten@nl.vitronics-soltec.com. His column appears monthly.

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