Dip and Look Solderability Testing: 7 Rules

Dip and look is a solderability test in which a sample is fluxed, dipped into molten solder and inspected visually. It answers a simple question: does the surface wet, and does the solder cover it evenly. The method does not produce a number, it produces a judgement against a written criterion, which is why the acceptance rules matter as much as the equipment.

The test is used on incoming components, on bare boards and on finished plated surfaces, and it is fast enough to run on every lot. Where it is run with consistent conditions it catches oxidation, contamination and plating faults that a visual inspection at goods inwards would miss. This note covers the method and the way it is judged.

What the Dip and Look Test Shows

The test reveals the condition of a surface rather than the strength of a joint. A surface that wets freely is a surface that will form a joint when the process is correct. A surface that dewets has been contaminated or oxidised, and no amount of profiling will fix it, because the problem is at the interface before the alloy arrives.

It also reveals coverage, which is a separate property. A pad can wet at the edges and stay dry in the centre, and the pattern of coverage tells you where the problem sits. Edge wetting with a dry centre points at a surface film, while patchy wetting points at particulate contamination or at a plating fault.

Standards and Acceptance Criteria

The method and the acceptance conditions are defined in the IPC solderability standards, which specify the flux, the solder temperature, the immersion depth and the duration. Following them matters because a test run hotter or longer than the standard will pass a surface that would fail in production.

The criterion is normally written as a minimum percentage of coverage together with a rule about where gaps may fall. A typical requirement is 95 percent coverage with no gap on a critical area. Writing the criterion in the work instruction, rather than leaving it to the operator, is what makes the test repeatable between shifts and between sites.

Preparing the Sample

The sample must be representative and it must be handled with care. Take components from the reel or tray as they are, without cleaning them, because cleaning removes the very contamination the test is looking for. Handle with gloves, hold the part by the body and keep the test surface away from fingers, tape and packaging material.

Boards are sampled from the lot and cut so that the test area is at a defined position, usually including a pad and a plated hole. Where a board has several surface finishes, each has to be tested separately, because an organic coating and a metallic finish behave differently at the same temperature and need different preparation.

Flux, Solder Bath and Immersion Conditions

The flux is specified by the standard and should be a fresh batch at the correct density. A flux that has been standing open absorbs moisture and loses activators, and the result is a false failure that sends a good lot back to the supplier. Store the test flux in a small container, replace it on a schedule and record the batch.

The bath temperature is normally held at 245 degrees for lead free alloy, with the surface skimmed immediately before the dip. Immersion depth and dwell are timed from the moment the sample touches the surface, and both are short enough that a slow operator can change the result. Use a fixture or a controlled dip mechanism, and check the bath temperature with a calibrated probe rather than trusting the controller. The behaviour of a coating before soldering is covered in our notes on OSP coatings and storage.

Reading the Result: Wetting, Dewetting and Nonwetting

A good result shows a thin, smooth, continuous layer of solder that has spread over the surface at a low contact angle. Wetting of that kind means the alloy reacted with the metal and formed an intermetallic layer, which is the condition the process needs. The surface looks bright and the solder edge fades rather than stops.

Dewetting shows as solder that has covered the surface and then pulled back, leaving thin patches with visible metal underneath. Nonwetting shows as solder that never spread at all, standing in a dome with the original surface untouched. The three conditions need different responses, and the difference between a genuine dewetting result and a residue left by the flux is one of the reasons the sample is examined under magnification.

Common Causes of a Failure

Oxide growth is the most common cause on stored parts and on boards kept in a humid room, and it is the reason storage controls exist. Contamination from handling, from packaging or from a plating line is the second, and it shows as a patchy pattern that follows the contamination rather than the geometry. The third group is the plating itself, where thickness, porosity or an organic co-deposit produces a surface that cannot wet. Each surface finish has its own failure mode, and the comparison is set out in our notes on PCB surface finish.

A failed sample should be retained and photographed, because the response from a supplier is far more useful when it includes an image and a batch number. Retaining the sample also allows a second test after a cleaning step, which separates a surface film from a genuine plating defect.

Using Dip and Look at Goods Inwards

A dip and look station at goods inwards costs little and catches problems before they reach the line. The routine is a sample per lot, a photograph of the result, and a record of the batch number, the flux batch and the operator. A lot that fails is quarantined and the supplier is contacted with the evidence while the material is still in the building.

The test is also useful as a check on storage. Where the same part number passes on receipt and fails three months later, the storage conditions are the variable, and the difference between two tests on the same lot is the measurement. The relationship between storage and solderability is covered in our notes on component storage and solderability.

Limits of the Method

Dip and look produces a judgement, not a measurement, so it distinguishes gross problems better than marginal ones. Where a process is running close to a limit, a quantitative method such as a wetting balance gives the trend that visual inspection cannot. The two are complementary, and our notes on the wetting balance solderability test describe the instrument side.

The test also consumes samples, which matters for expensive components, and it destroys the very surface it inspects, so the same part cannot be retested after the fact. That is why the sample should be photographed and stored, and why the test conditions have to be recorded in enough detail that a second opinion is possible. The reference conditions are defined in the IPC solderability standards.

FAQ

Can the test be run without the specified flux? It can, and the result will be meaningless. The flux is part of the method because it defines how much oxide the test is designed to remove. Using a stronger flux masks a contamination problem that would fail in production.

How many samples should be tested per lot? The standards give a sampling plan, and most shops use a small number of pieces from across the lot rather than several from one reel. Where a supplier has a history of variation, increase the sample and test the beginning and end of the reel.

Does a pass guarantee a good joint? No. It confirms that the surface is solderable, which is one of several conditions. Stencil volume, reflow profile and component fit are all outside the scope of the test, and each has to be controlled separately.