Solder Skip: 5 Causes on a Wave Solder Line

A solder skip is a pad or a lead that passes over the wave without being soldered, leaving the metal bright and bare. It is different from a cold joint, because no alloy was ever applied, and different from a missing component, because the part is present and simply not connected.

The defect is usually found at the electrical test rather than at the wave, so the board has already travelled through several operations by the time anyone looks. Reading the pattern of skips on the panel is what identifies whether the cause is thermal, mechanical or chemical.

What a Solder Skip Looks Like on the Board

The classic appearance is an untouched pad with the solder mask and the plating intact, often with a small amount of flux residue around it. Where the skip follows a row of pins, the whole row is affected, which points at a shadow rather than at a thermal problem.

Where a single pad is skipped in the middle of a soldered row, the cause is usually local: a heavy copper plane drawing heat away, a lead that is bent away from the pad, or a bubble of flux gas that formed at the moment of contact.

Flux and the Activity Behind Wetting

Flux does two jobs: it cleans the surface and it protects it until the alloy arrives. A flux applied too thinly, or one whose activity has been reduced by age or by dilution, leaves an oxide film that the solder cannot break through on a difficult pad.

The application method matters as much as the chemistry. A spray fluxer with a blocked nozzle, or a foam fluxer with a low head, gives an uneven coating that produces skips in a repeating pattern along the conveyor. Our wave soldering notes cover the settings that go with the fluxer.

Wave Height and Contact Depth

The board has to touch the wave at the right depth. Too shallow and the contact time is short, while too deep and the board is flooded and the wave pushes components. The height should be set with the pallet fitted, because a pallet changes the effective depth.

The wave also has to be stable. A pump that is drawing air, a nozzle with dross in it, or a pot level that is low all produce a choppy crest that contacts some pads and misses others as the board passes. Our dross control notes describe the maintenance that keeps the crest even.

Thermal Mass and Cold Pads

A pad connected to a plane needs more heat than an isolated one, and the wave has a fixed amount of energy to give. Where the thermal mass is high, the flux burns off before the pad reaches the soldering temperature, and the pad is skipped while its neighbours solder normally.

The counters are a slower conveyor, a higher pot temperature and a design change that reduces the connected copper. The design route is the only one that removes the problem rather than moving it, and it is worth raising with the designer when the pattern repeats on one net.

Pallet Design and Shadowing

A pallet that is thicker than the leads are long will hold the board away from the wave, which is a common cause of a skip on a whole row. The same happens where a pallet finger or a clamp sits under a pad, because the wave cannot reach a shadowed area.

Pallet design should be checked against the board before a run rather than after a failure. Our pallet care notes describe both the cleaning routine and the checks that keep the tooling from causing defects.

Conveyor Speed, Angle and Dwell

Contact time is set by the conveyor speed and by the angle at which the board meets the wave. A board that runs fast may cross the crest before a heavy pad reaches temperature, while a very slow board risks damage to the laminate and to the mask.

The conveyor angle affects how the wave climbs the board and how the solder drains away afterwards. Skips at the leading edge of the panel often indicate an angle or a speed problem rather than a flux problem, and the two are separated by a simple trial with the speed changed.

Component Orientation and Lead Length

A lead that is too short does not reach the wave, and a lead that is bent away from its pad leaves the pad exposed. Both are assembly issues rather than wave issues, and both are visible on a sample before the run if the leads are checked with a simple gauge.

Orientation matters where a tall component sits upstream of a row and shields it from the wave. Rotating the part is a design change, while adding a pallet cut out is a process change, and the cheapest answer is usually found by inspecting the shadowed area first.

Process Checks That Find the Cause

The practical sequence is to check the flux coverage on the board as it enters the wave, then the wave crest with the production pallet, then the contact depth, and only then the thermal settings. Each check removes one class of cause and takes a few minutes.

Where the skip repeats on the same net across many boards, the board is telling you something about the design rather than the machine. Recording the location of every skip for a shift gives a map, and the map usually shows a pattern that a single board cannot.

Design Changes That Reduce the Risk

The changes that help are thermal and geometric: larger pads where the plane connection is heavy, thermal reliefs that reduce the heat being drawn away, longer leads on through hole parts and a layout that keeps tall parts downstream of the solder side.

Where the product uses a pallet, the design should make room for it. A board whose component layout leaves no space for pallet support will be soldered in a shadow, and the defect will be repeated on every build. Our selective soldering notes describe the alternative for a board that wave soldering cannot reach.

Records and Defect Mapping

The record should show the wave settings, the flux type and coverage, the pallet used and the skip location for each board. With those items a repeat defect can be traced in minutes instead of being rediscovered with every run.

Where the same position fails across several batches, the issue is in the design or in the tooling rather than in the machine settings for that day. The IPC assembly standards define the acceptance criteria for the joints that are produced.

FAQ

Is a skip the same as a cold joint? No. In a skip no solder was applied to the pad, while a cold joint has alloy on it that did not wet or did not flow. The two have different causes and different corrections, though both may be found at the same test step.

Can a skip be reworked by hand? It can, and the rework should be recorded like any other. Where skips repeat across a batch, hand soldering them creates a mixed process, and the joints that were reworked should be inspected separately from those that were not.

Does more flux always help? Only up to the point where the flux is fully applied. Beyond that the extra flux costs money, leaves more residue and can create its own defects, while the real cause of the skip is elsewhere on the line.