Designing Assemblies for Wave Soldering

A wave soldered assembly is designed with the process in mind or it is not designed at all. The board travels through a curtain of molten solder in one direction, and everything that the solder has to reach, and everything it must avoid, is determined by the layout.

The failures that appear on a wave soldered board are usually geometric: a joint behind a tall part that never saw the solder, a pad that drew the solder away from the joint, or a mask opening that let the solder flood an area that should have stayed clear.

What the Process Does to the Board

The board passes over a wave that contacts the underside, and the solder rises into the joints by capillary action and by the movement of the wave. Everything above the surface is shielded by the board itself, and everything in the shadow of a component is shielded by that component.

The direction of travel is therefore a design variable. A feature that is behind another feature in the direction of travel receives less solder, and rotating a component by ninety degrees can move it out of a shadow.

Thermal Shadowing and Its Effects

Thermal shadowing occurs where a tall component or a heavy connector blocks the flow of hot solder from the joints behind it. The joints receive less heat and less solder, and the result is a fillet that does not form or a joint that is only partly wetted.

The remedy is to orient the parts so that their long dimension is parallel to the direction of travel, which reduces the length of the shadow, and to keep the tall components away from the small joints that would suffer behind them.

Component Orientation and Spacing

Orientation also affects the tendency to bridge. A component whose leads are perpendicular to the wave presents small gaps to the solder, while one that is parallel presents a long row that the solder can bridge across.

Spacing matters for the same reason. A part placed close behind another creates a narrow channel where the solder can accumulate, and a part placed close beside another changes the flow locally. The rules that govern this are simple, and they are usually published as a set of orientation preferences rather than as a hard limit.

Solder Thief Pads and Drainage

A solder thief is a small pad placed on the trailing side of a joint to capture the excess solder that would otherwise bridge to a neighbouring pin. It works with the direction of travel, so its position is not optional.

The same principle applies to the drainage of a large area. Solder that flows onto a wide pad can starve a joint further along, and the design can provide a path for it to leave or place the wide copper where the flow does not affect a critical joint.

Keep Out Areas and Mask Openings

The solder mask defines where the solder is allowed to go, and a mask opening beyond the pad is an invitation to flood. Keeping the openings tight around the pads, and avoiding openings that connect two pads, is the first line of defence against bridging.

A keep out area is also needed around the parts that must not see solder at all: surface mount components on the underside of a mixed assembly, test points that have to remain clean and any mechanical feature that the solder would obstruct. Those areas are usually masked by the pallet rather than by the board, and the two have to agree.

Via Placement and Solder Wicking

A via near a joint can wick solder away from it, especially where the via is open and connected to a plane. The result is a joint with less solder than intended, and the countermeasure is a tented or a plugged via, or a position away from the joint.

The interaction with the pallet is worth noting, because a via that is masked by the pallet does not wick. The design and the fixture should be considered together rather than in sequence, which is the point at which the layout and the process planning meet.

Mixed Technology Assemblies

A board with surface mount parts on one side and through hole parts on the other is the common case, and it is where the design decisions have the greatest effect. The surface mount parts have to be protected, the through hole joints have to be exposed, and the layout has to allow the pallet to do both.

Grouping the through hole parts so that their joints are accessible, and leaving clearance around them for the pallet webs, simplifies the fixture. Where the density makes that impossible, part of the work has to be moved to a selective process, which costs time and adds a second thermal cycle.

Pad and Land Design for the Wave

The pads themselves are designed for the process. A pad that is too small gives the solder nowhere to form a fillet, and one that is too large holds a pool that can bridge. The annular ring has to allow for the drilling tolerance and for the solder to rise around the lead.

The land pattern should be the one the connector manufacturer recommends for wave soldering, and it should be checked against the fabricator’s capabilities. Where the two disagree, the decision should be made deliberately, as with any other requirement recorded in the pad design documentation.

Verification and First Article

The verification is the first article, inspected for fill, wetting and bridging, together with the thermal profile of the assembly through the machine. Both should be recorded, because both are needed to reproduce the result.

Where a defect is found, the direction of travel and the orientation of the parts are the first things to review, before the solder temperature or the flux settings are changed. The geometry is the more likely cause, and the alloy and flux decisions that support it are described in the comparison of solder alloys.

Flux Application and the Layout

Flux is applied before the wave, either by spraying or by foaming, and the layout determines how much of it reaches each joint. A tall component casts a shadow in the flux as well as in the solder, and the joints behind it can be under fluxed as well as under heated.

Where the board is spray fluxed, the spray pattern and the conveyor speed set the dose, and the geometry of the board changes the distribution. That is one more reason to keep the layout simple around the wave sensitive area, and it is a consideration that belongs with the process specification rather than only with the layout.

FAQ

Does component orientation really matter? Yes, and it is one of the few design changes that can fix a wave defect without touching the process parameters.

What is a solder thief for? It captures excess solder on the trailing side of a joint so that it does not bridge to the next pin.

Can a pallet compensate for a poor layout? It can mask areas and support the board, but it cannot supply solder to a joint that is shadowed, and it cannot undo a pad that is the wrong size.