Difficulties and how to avoid them when soldering PCBs with non plated through holes

Introduction
Machine soldering of PCBs with non plated through holes is often more difficult than soldering PCBs with plated through holes (PTHs).
Bridging might easier occur due to the smaller gaps in-between the solderjoints, compared to the same component on a PCB wit PTHs. The reason is often that the pad dimensions on a single sided PCB are larger than on a PCB with PTHs. These larger pad dimensions are needed to give a sufficient adhesive strength between the PCB-base material and the copper clad on which the joints are soldered.
Another problem that is often been faced is that of the so called "open joints”. These are joints that have not been filled completely by the solder. Next we will explain how such joints will occur. Knowing the reasons, it is clear that in such cases there is no easy panacea for the process settings to fix these matters during soldering. In fact it is a often a "miracle" that full soldered joints will be the result of soldering boards with non plated through holes, if we see what can go wrong.
Some rules to keep in mind when soldering on PCBs with non plated through holes
If solder bridging in-between joints occurs due to a too narrow space in-between these joints, a solution can be to use a solder resist screen with smaller solderpot apertures.
Of course this is not a solution that can be implemented to solve the acute problem in the solder process, but it will just give a suggestion on how this can be solved for the next series of PCBs to be made.
During soldering joints on PCBs with non plated holes it may sometimes be difficult, or even impossible to create a full joint completely filled with solder. For this there are a few clear reasons.
First of all we have to realize that the formation of a solderjoint is a result of wetting forces and capillary forces.
These forces can only work on well solderable metal parts and when the solder isin a liquid state. As soon as the equilibrium of wetting forces, or capillary forces is disrupted, complete joint filling will be hampered.
When PCBs with a HASL-coating are used we might face after longer storage a reduction of the solderability of the inner and outer area of the solderpad. The reason for this is that at the edges of the pad the coating is very thin. If the coating is less than 3 microns, migration of the intermetallic layer can bring intermetallics up to the surface of the coating. When these intermetallics oxidize they result in poor solderability.
This in fact will even increase the gap between the lead and the solderable part of the pad, making the joint more unstable during its formation. Sheet 047
Poor solderability can also be caused by 'bleeding' solder resist or misprinting of the solder resist mask.
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 give a less- or even non-solderable area on the lead and this may create a open joint.
Since on every joint on single-sided PCBs there will be an air gap between the lead and the pad to be soldered, we start with a difficulty. Due to the fact that air is not solderable and there is no solderable hole plating, this gap will weaken the force between the lead and the solderpad at the gap position.
This difficulty can be tackled 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 than be bridged, but it will always 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 a good shape. Such a burred pad having a sharp edge near the hole increases the risk of open joints
A damaged solderpad edge is an event that may affect this unstable situation in a negative way. The sharp burr on the pad edge disturbs the capillary gap between the pad and the lead and will so cause an open joint.
If on the lead, or the pad, or in-between lead and pad foreign non-wettable parts are present; it will cause an open joint, since such parts disturbs the capillary gap between the pad and the lead.
If the topside of the hole is covered e.g. by the component body, or there is a flux film in-between the component and the PCB, the flux vapors can not escape via the topside. These vapors will then escape via the bottom side through the liquid solder in the joint and so 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 also cause out-gassing of the PCB via the holes during soldering.
This often the case with materials that have a relative 'open' structure in the holes, like FR2 material. This will enlarge the risk of open joints.
In times of high humidity e.g. during the autumn, PCBs can be stored in a dry cabinet to prevent humidity absorption. When the PCBs are already exposed to a humid environment they should be pre-dried well before soldering. This predrying will take more time than the general preheating time in the soldering machine.
Summary of demands for the creation of sound joints on single sided PCBs without PTHs
• The solderability of the lead and the pad must be sound.
• The diameter difference between hole and lead must be < 0.3 mm.
• The rim of the solderpad may not be damaged, e.g. during insertion of the lead.
• No foreign materials may be present in the joint area, e.g. drill residues or lacquer on the lead.
• The component may not block the hole on the topside, so that the flux vapors can not escape via the topside of the PCB.
• The PCB should be stored in a dry environment before soldering to prevent out-gassing of absorbed humidity via the holes during soldering.