Selective Soldering Fixture Design Requirements

Selective soldering solders one joint or one group of joints at a time, so the board has to be in the same place for every one of them. That is the fixture job, and it has to do it while the board is heated locally, while the nozzle approaches from below, and while the assembly is still stiff enough to be handled. A fixture that is merely a carrier produces a process that works on the first board and drifts afterwards.

Locating the Board Repeatably

The board is located by tooling holes that engage pins on the fixture. The hole diameter tolerance and the pin wear together determine the repeatability, and a pin that has worn by a tenth of a millimetre moves every joint on the board by that amount. The pins should be checked against a diameter specification rather than by feel.

Where a board has no tooling holes, the outline can be used with edge stops, but that method inherits the outline tolerance and is less repeatable. The tooling holes and their positions belong in the fabrication data, so the fixture can be built from the design rather than from a sample board, as described in PCB fabrication notes.

Support Points and Warpage Control

The support pins hold the board flat so the distance from the nozzle to the joint is constant. Because the nozzle contacts the joint from below, a board that sags changes the wave contact and the fill. Support should be placed under every connector block and at intervals across the board, with the density set by the measured flatness rather than by convenience.

The board already has a flatness value from fabrication, and heating it during soldering changes it further. A fixture that supports the board in its cold state may leave a gap when the board is hot, which is why the support plan should be validated at temperature. The material behaviour behind this is the same as that in PCB warpage control.

Nozzle Clearance and Access

The nozzle has to reach the joint without touching the fixture. The clearance around each soldering area should be defined in the design, because a support pin that sits close to a connector leaves the nozzle nowhere to go. The clearance is normally a few millimetres around the pad and the pin, and it should be shown on the fixture drawing.

Access also has to allow the nozzle to move between joints without colliding with a tall component on the underside. Where a board has components on both sides, the fixture has to provide cut outs for the parts and still support the board around them. A fixture that is built without the assembly drawing will collide on the first production run.

Fixture Material and Thermal Behaviour

The fixture material has to hold its dimensions at soldering temperature, resist flux attack and be light enough to load by hand. Composite materials are common because they do not conduct heat, which keeps the fixture cool and limits the heat drawn from the board, and because they are dimensionally stable at the process temperature.

A metal fixture conducts heat away from the joint and can act as a heat sink under a connector, which produces incomplete fill at the joints nearest the fixture. Where metal is used for stiffness, the contact area should be minimised and the thermal effect measured rather than assumed, because the fill requirement is a specification rather than a preference.

Clamping and Board Retention

The board has to be held down during soldering so that thermal movement and nozzle contact do not lift it. Clamps should press on the clearance zone rather than on a component or a joint, and the force should be enough to hold the board without bowing it. A clamp that is too strong creates a local curvature that changes the nozzle distance.

Where the board is thin, vacuum or edge clamping may be better than point clamps, because it distributes the load. The retention method should be verified by measuring the board position before and after clamping, which is a quick check that catches an over tightened clamp before it produces a fill problem.

Programming Interaction

The fixture determines the coordinate system used to program the machine, so a fixture change invalidates the program. Where several fixtures are used for the same product, they should be built to the same drawing and measured against it, because a fixture that is slightly different in the pin position will shift every joint and the program will not be valid.

The program should be recorded with the fixture serial number, so that a re validation can be limited to the affected fixture rather than repeating the whole product. Where a spare fixture is made, it should be verified against the master fixture before being released, and the verification recorded with both serial numbers.

Maintenance and Wear Checks

Fixture wear is the gradual failure mode. Pins wear, clamps lose travel, support surfaces compress and composite frames absorb flux and swell. The symptoms are a slow drift in the joint position and a slow change in fill, which is why the checks should be scheduled by cycle count rather than by defect rate.

A practical interval includes a diameter check on the locating pins, a flatness check on the support surface and a visual check of the clamp travel. A fixture used for a high volume product should be checked monthly, and the record should be kept so that a drift can be related to the check history. The condition of the board itself is assessed against PCB quality assessment at the same time, which separates board problems from fixture problems.

First Article and Validation

The first article confirms the fixture, the program and the process together. It should include the position of every soldered joint, the fill on a sectioned sample of the most difficult joint, and a check that the nozzle does not touch the fixture or an adjacent component. The fill measurement is the one that requires a cross section, since a visual check of the top fillet does not show the barrel.

The validation should be repeated after any fixture repair, any change of nozzle and any change of board supplier, because each of those can move the process. The acceptance criteria applied to the joints are the same as those in solder joint acceptance criteria, and the record of the first article belongs with the fixture so the two travel together.

Verification and Records

Where the supplier and the user both measure the same property, they should agree on the method before the first delivery. A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record.

FAQ

Can a wave solder pallet be used as a selective fixture? It can be adapted, but the support and clearance requirements differ. A selective fixture needs clearance around each joint and support close to each connector, which a masking pallet does not provide.

How many support pins are enough? Enough that the board flatness measured under the fixture stays within the process tolerance. The number follows the measurement rather than a rule.

Why does fill vary between identical fixtures? Usually because the locating pin position or the support height differs slightly. Both fixtures should be measured against the same drawing.