Board Cleaning After Reflow in Mixed Assembly Lines

Board cleaning after reflow is a line integration problem as much as a chemistry problem. The cleaner has to sit between the reflow oven and the next operation, accept the same panel size and throughput, and deliver a dry board at the end. When one of those three is out of step, the cleaning result suffers or the line slows down, and the compromise usually appears as residue left in the wrong place. Board cleaning is therefore designed with the line rather than added to it, and the conveyor interface is often the deciding factor.

Why Clean After Reflow

Reflow leaves flux residue on the board, and whether it has to be removed is decided by the product rather than by preference. A board that will be coated, that operates at high impedance or that will be used in a humid environment usually has to be cleaned.

Cleaning after reflow is also the last opportunity before the assembly is finished. A residue that survives to the coating step becomes a permanent defect under the coating, where it cannot be removed or inspected. That timing is the reason cleaning is treated as a critical step rather than as a final housekeeping operation.

In Line Versus Batch Cleaning

An in line cleaner is built into the conveyor and processes each board as it arrives, which suits high volume and gives a consistent cycle for every unit. A batch cleaner loads a group of boards into a chamber, which suits lower volume and awkward shapes. Where the product mix changes frequently, a batch machine gives more flexibility at the cost of handling labour.

The in line machine demands a conveyor compatible with the rest of the line and a throughput that matches the printer and the placement machines. A cleaner that is slower than the oven becomes the bottleneck of the whole line. Our production flow guide places the cleaning step in the wider sequence. The machine should also be able to accept the largest panel the line produces, because a cleaner that is one size too small is a permanent constraint.

Chemistry and Water Quality

An aqueous cleaner uses water at a controlled temperature with an optional additive. The additive dosage has to be maintained, because a bath that drifts becomes either ineffective or aggressive towards the mask and the labels. A daily concentration check and a weekly full analysis are the usual level of control on an aqueous line.

The rinse water has to be at least as good as the cleaning water, because the final rinse is what determines the residue left on the board. Deionised water with a monitored conductivity is the normal specification for the last stage. A conductivity meter on the final rinse is one of the cheapest and most informative instruments on the line.

Spray and Nozzle Coverage

The spray pattern decides where the cleaning energy reaches. Nozzles that are partly blocked leave a shadow on the board, and a shadow under a tall component is where the residue survives.

Nozzle condition should be checked on a schedule rather than when a defect appears, and a coverage test with a pattern card makes the check objective. The pattern card also reveals a nozzle that is spraying in the wrong direction, which a visual check would not show. Our quality documentation describes how these results are classified at gopcb.

Rinsing and Drying

Rinsing removes the chemistry and the loosened residue, and it has to be as thorough as the cleaning itself. A cascade arrangement uses far less water than a series of independent tanks and achieves a cleaner final result. The rinse stages should be arranged so that the cleanest water meets the cleanest board, which is the principle behind the cascade.

Drying is where the line most often fails, because water is trapped under components, in connectors and in vias. Air knives, a heated zone and an adequate dwell time together remove it, and the dwell should be set from the heaviest assembly on the line rather than the lightest. A drying zone that is adequate for a simple board will leave water in a heavy assembly without any visible sign.

Cleaning Under Components

The hardest places to clean are beneath an area array package and inside a connector. Water has to enter and leave the same narrow gap, and the flow through a small standoff height is limited by viscosity rather than by pressure. Increasing the pressure does not help, and it may force water into places that are even harder to dry.

Where a component cannot be cleaned, the choices are to clean before it is fitted, to use a low residue flux that does not need removal, or to specify a no clean process for that area. The decision has to be made at the design stage. Our component guide covers the handling and storage factors that affect it. The decision should be recorded on the drawing so that the assembly house does not have to guess.

Verifying the Result

The verification is the ionic contamination test on a cleaned sample, supported by a visual check under a low angle light and, where required, a surface insulation resistance measurement.

A simple water break test on the surface also gives useful information, because a surface that repels water is not clean. Our solder defects guide describes the residue related defects that appear after cleaning. A defect that appears after cleaning rather than before it is usually caused by the cleaning itself, which is worth remembering.

Common Problems

The common problems are residue under a component, water left in a connector, mask or legend damage from an aggressive chemistry, and labels that lift. Each of them has a different answer, and the answer is usually in the chemistry or the drying rather than in the mechanical part of the machine.

A defect that appears on one side of the board only points at the spray coverage, while one that appears everywhere points at the chemistry or the rinse. That distinction saves a great deal of investigation time. Where the pattern is unclear, running one board through with a known good setting is a quick way to establish the baseline.

Process Control Points

The controls are the bath concentration and temperature, the spray pressure and nozzle condition, the rinse water quality and flow, the dryer temperature and dwell, the conveyor speed and the contamination test result on a sample.

Each of those is recorded per run, and the combination is what makes the cleaning process repeatable. A cleaner that is set once and never checked will drift towards either a residue problem or a damage problem, and the drift is invisible until a defect appears. A short weekly check of every parameter is enough to keep the process inside its window.

FAQ

Is cleaning always required after reflow? No. It depends on the residue, the coating requirement and the operating environment. Where cleaning is required, it should be specified in the same way as any other process step.

Why does residue remain under a large component? Because the water cannot flow through the gap between the package and the board quickly enough. Cleaning before the component is fitted is often the practical answer.

How is a cleaned board verified? The ionic contamination test on a sample is the standard measure, supported by a visual inspection and, where specified, a surface insulation resistance measurement.