Wave Solder Defects and Process Control

Wave soldering is a process with a small number of variables and a large number of defect modes, and most of the defects are related to the interaction between the flux, the preheat and the wave rather than to the solder itself. The process remains in wide use for through hole assemblies, and it rewards the same discipline as any other process: a defined profile, a controlled flux, a maintained machine and a record that allows a defect to be traced to a setting.

The Process in Outline

A board passes over a fluxer, through a preheat zone and then across one or two solder waves. The flux removes oxide and lowers the surface tension of the solder, the preheat activates the flux and brings the board to a temperature close to the solder, and the wave supplies molten solder that wets the joints by capillary action into the plated holes.

Each stage has a purpose and each can cause a defect if it is wrong. Insufficient flux leaves oxide on the surfaces so that the solder does not wet. Excessive preheat consumes the flux before the board reaches the wave. A wave that is too turbulent produces turbulence related defects, while a wave that is too still fails to push solder into narrow holes.

The two wave arrangement, with a turbulent chip wave followed by a smooth main wave, is common. The first wave ensures penetration into the holes, while the second removes bridges and produces a clean fillet. Where only one wave is available, the compromise is made in the wave shape rather than in the elimination of one of the functions.

Flux and Its Activity

Flux activity is the property that determines how much oxide it can remove, and it is chosen from the board’s requirements rather than from convenience. A high activity flux leaves residues that must be cleaned and may not be permitted for the product, while a low activity flux may not be sufficient for a heavily oxidised board or a surface finish that wets poorly.

The application method matters as much as the chemistry. Spray fluxing gives fine control over the quantity, while foam fluxing applies more but with less uniformity, and wave fluxing sits between the two. The quantity applied should be measured rather than estimated, and it should be checked when the flux type or the board changes.

Flux that is applied to the underside but does not reach into the holes will not protect the joint. Spray systems that cover only the surface can leave the barrel dry, and the resulting defect appears as a poor fill that looks like a wave problem. Checking the flux penetration on a sectioned sample is worth doing when a fill defect appears.

Preheat and Temperature

The preheat has two functions: to activate the flux and to reduce the thermal shock when the board meets the wave. The top side temperature is what matters for activation, and it should be measured rather than inferred from the heater setting, because the mass of the board and the loading of the conveyor change the result.

Too little preheat produces spattering, incomplete flux activation and thermal shock, which appears as poor wetting and occasionally as cracked components. Too much preheat consumes the flux before the wave is reached, which produces exactly the same symptom as insufficient flux, and the two are frequently confused.

The preheat profile should be measured with a thermocouple on the top surface of a representative board, at a point near a heavy thermal mass. The measurement should be taken at the production conveyor speed and with the production loading, since both change the result.

Wave Parameters

The wave height determines how much of the board is in contact with the solder, and it should be set so that the wave touches approximately half to two thirds of the board thickness. A wave that is too high floods the board and produces bridging, while one that is too low fails to reach the joints and produces insufficient fill.

The conveyor speed determines the contact time, and the angle of the conveyor determines how the solder drains away from the joints as the board leaves the wave. A shallow angle gives more contact time and better fill, while a steeper angle drains better and reduces bridging. The two effects are opposed, so the setting is a compromise that should be established for the board rather than for the whole range.

Solder temperature should be maintained within a narrow band, and its measurement should be taken in the pot rather than from a display that may be calibrated differently. The composition of the pot also changes over time as copper dissolves from the boards and as the tin is consumed, and the analysis of the alloy is a periodic requirement rather than an occasional one.

Common Defects

Bridging is the most visible defect, and it comes from excess solder, from a wave that is too high or too turbulent, from insufficient flux so that the solder does not separate cleanly, and from board features such as closely spaced pads with insufficient solder mask. Each of those has a different correction, so the first step is to identify which applies.

Solder icicles and shorts occur when the solder does not separate cleanly from the joint as the board leaves the wave. The causes are similar to bridging, with the addition of a conveyor angle that is too shallow, a solder temperature that is too low and a component lead that is too long. The inspection of the exit point is the fastest way to diagnose them.

Insufficient fill, sometimes described as a blowhole or a partial hole, is usually a flux or preheat problem, and it can also come from a hole that is too small relative to the lead or from contamination on the barrel. Where the defect appears on one area of the board rather than throughout, the cause is usually mechanical rather than chemical.

Machine Maintenance and Control

A wave solder machine needs regular maintenance to perform consistently. Dross has to be removed from the wave surface, the nozzle has to be cleaned or replaced, the fluxer nozzles have to be kept clear and the heaters have to be checked. A machine that is neglected will produce a slow drift in the results that is attributed to the materials.

The alloy in the pot should be analysed periodically for copper and for the other elements, and the results recorded. Copper accumulates from the boards and from the plating, and above a certain level the alloy becomes less fluid and produces more bridging and more icicles. Topping up with fresh solder does not correct the copper content.

The process settings should be recorded per product, together with the flux type and the alloy analysis. When a defect appears, the settings can be compared, and the comparison is much faster than re-establishing the process from scratch.

Practical Rules

Measure the top side preheat temperature on a representative board, set the wave height to touch part of the board thickness, and choose the conveyor angle as a compromise between fill and drainage. Match the flux activity to the board and the finish.

Maintain the machine on a schedule analysis the alloy periodically. Record the settings and the analysis with the build records so that the defect data can be traced, and review the inspection results for the trends that indicate a drifting process rather than an isolated fault.

Additional Considerations for This Build

Practical attention to wave solder defects pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating wave solder defects explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

FAQ

What causes bridging in wave soldering? Excess solder, a wave that is too high or turbulent, insufficient flux, or board features such as closely spaced pads.

Why does preheat matter so much? It activates the flux and reduces thermal shock. Too little causes spattering, while too much consumes the flux before the wave is reached.

How often should the solder alloy be analysed? Periodically and on a schedule. Copper accumulates from the boards and makes the alloy less fluid, which increases bridging and icicles.