Wave Solder: Preparation, Placement and Process Control

Wave solder is the process that solders a whole board in a few seconds by dragging it across a standing wave of molten alloy. The board design has a larger effect on the result than most engineers expect, because the solder has to flow into every hole from one direction while the board moves. Features that look harmless in the drawing become a shadow, a bridge or a cold joint on the line.

How the Wave Process Works

The board passes over a fluxer, then through a preheat zone, and then across the wave. The flux removes the oxide, the preheat brings the board and the flux to temperature, and the wave supplies the solder and the heat for the joint.

The wave is turbulent at its surface and the board is in contact with it for a few seconds. During that time the solder has to fill every hole, form a fillet on both sides and drain away from the surface without leaving a bridge.

The process is used for through hole components and for mixed technology boards that have already been reflowed on one side. The second pass has to be protected from the first, which is where the pallet and the adhesive enter the design.

The temperature and the contact time are set by the machine, while the ability of the solder to reach every joint is set by the layout. That is the part the designer controls.

Component Orientation and Shadowing

Shadowing happens when a tall component stands between the wave and a joint that is behind it, relative to the direction of travel. The solder cannot reach the joint, and the result is an unfilled hole or a joint with a poor fillet.

The direction of travel is along the long axis of the board in most machines, so the designer should know which edge leads. Components are then oriented so that their bodies do not stand upstream of a joint, and long parts are placed parallel to the wave rather than across it.

A connector with a row of pins across the direction of travel is a classic shadowing case. Rotating it by ninety degrees, or placing it at the trailing edge of the board, removes the problem without a process change.

Where a tall part cannot be moved, the pad spacing and the hole size can be adjusted to make the joint easier to fill. A larger hole and a longer pad give the solder more opportunity to reach the barrel.

Solder Thieves and Dummy Pads

A solder thief is a pad that is placed at the end of a row to collect the excess solder that would otherwise bridge the last two pins. It is connected to nothing, or to a net that does not care, and it gives the solder somewhere to go.

Thieves are used on connectors, on fine pitch through hole parts and on any row of pads that ends abruptly. The thief is placed beyond the last pin, in the direction that the solder drains.

The size of the thief matters. A thief that is too small does not collect enough and the bridge remains, while one that is too large steals solder from the joint next to it and leaves a starved fillet.

The thief is not needed on a well spaced row, because the surface tension of the solder is enough to keep the joints separate. It becomes necessary when the pitch falls below about 2.5 mm, or when the pads are large relative to the gap between them.

A similar trick is used with dummy pins on a connector, where an unused position at the end of the row performs the same function. The solder defect guide describes the bridge and the starved joint as the two failure modes that thieves address.

Pallets, Fixtures and Masking

A pallet is a carrier that holds the board and masks the areas that must not be soldered. It also protects the already reflowed side of a mixed technology board from the wave.

The pallet has to be designed with the board, because it affects the thermal profile and the flow of solder. A pallet that covers too much of the board cools it and leaves cold joints, while one that covers too little lets solder reach parts that must stay clean.

The openings in the pallet should match the areas to be soldered with a margin, and the fingers that hold small parts should not block the flow. A finger that crosses a row of pins is a shadow that no amount of machine tuning will fix.

The pallet material and its thermal mass also determine how often the profile has to be re-measured. A new pallet is a process change, and the profile should be verified before the first production run.

Through Hole and Mixed Technology

A through hole joint has to be filled from the top of the pad to the top of the barrel, and the solder has to wet both the pin and the barrel wall. The hole size and the pin diameter set the gap that the solder has to fill.

A gap that is too small traps flux and gas, which forms a blowhole or a void in the joint. A gap that is too large needs more solder than the wave can deliver in the contact time, and the result is a joint that is filled on one side only.

The pad on the solder side has to be large enough to form a fillet and small enough to avoid bridging to its neighbour. The dimension depends on the pin pitch, and the hole and pad types are chosen together with the process.

A thermally heavy plane connected directly to a through hole pad draws heat out of the joint, so a thermal relief is used on the plane. The relief is one of the few features that helps both the wave and the hand soldering process.

Design Review

The review should start with the direction of travel and a list of the tall components. Every joint that sits downstream of a tall part is a shadow risk, and the fixes are a rotation, a move or a larger pad.

The second item is the row ends, where a thief or a dummy pad should be added. The third is the pallet, which should be drawn and reviewed with the assembly house.

The fourth is the solder mask and the finish, since a HASL finish is the traditional companion of a wave process and the mask must not cover a pad that has to be soldered.

Finally, the first article should be inspected for fill, bridging and shadow, and the result recorded. The quality check on a wave soldered board is mostly about the joints that the process could not reach.

FAQ

Why does a connector bridge on the last two pins? The solder has nowhere to drain at the end of the row. A solder thief or a dummy pin beyond the last position gives it somewhere to go.

Does the pallet change the soldering temperature? It does, because it adds thermal mass and shadows the board. The profile should be measured with the production pallet in place.

Can surface mount parts go through the wave? They can on the underside if they are bonded with adhesive, and the layout has to keep them out of the shadow of the pallet fingers.