A printed circuit board will take a scratch from anything harder than a fingernail, and it will flex under a test probe. Most handling defects happen between machines, not inside them.

Support the board, not just the edges

SMT conveyors carry panels on the edge rails because the underside is being printed or reflowed. After reflow, the same edge support is often kept all the way to test, and that is where problems start.

A 1.6 mm board spanning 300 mm between edge rails will sag under its own weight at reflow temperature and will flex when a probe presses on it. The flex cracks solder joints on fine-pitch components long before the board fails a functional test.

For any board wider than 200 mm, add centre support: a support pin block, a vacuum plate or a belt under the board. For ICT, the fixture should support the board directly under the probe field.

The transfer gap

The gap between two conveyors should be no wider than a third of the shortest board edge, and no more than 25 mm on a standard SMEMA interface. A wider gap lets the leading edge drop, which chips the edge connector and, on thin boards, cracks the corner.

Check the height match at the transfer. A 0.5 mm step is enough to catch a board edge. Use a straight edge across the joint, not the machine's own height indicator.

Magazine loaders and unloaders

Magazine slots are sized for the panel plus clearance. If the slot is too tight, the panel is dragged along the slot wall; if it is too loose, the panel tilts and the next one jams. The usual specification is 0.5 to 1.0 mm clearance per side, with the magazine rack checked for squareness every six months.

Set the pusher stroke so the panel is pushed from the centre of its edge, not from one corner. An off-centre push rotates the panel and the trailing corner catches the slot.

ESD control that survives production

Static damage is invisible until it is not. Four measures do most of the work:

  1. Conductive or dissipative belting with a surface resistivity of 10^6 to 10^9 ohms, bonded to the frame at both ends.
  2. A grounded frame, with the earth continuity tested at commissioning and annually.
  3. Ionising bars at the points where boards leave a tunnel or a reflow oven, where charge accumulates fastest.
  4. Operator grounding at every manual station, checked by the line supervisor daily rather than by an auditor annually.

Note that a grounded conveyor does not discharge a board that is sitting on an insulating surface. If the board rests on a plastic nest, the nest is the problem.

Field noteA power supply plant had a 2 percent failure rate at functional test that disappeared when the boards were re-tested the next morning. The cause was charge accumulated on the belt at the unloader. An ionising bar at the tunnel exit reduced the failure rate to 0.3 percent.

Cleanliness between processes

Flux dust and solder balls travel on belts and rollers. A conveyor that runs from reflow to test without a cleaning station will carry particles into the test fixture, where they cause false failures and scratched boards.

Fit a tacky roller or a brush before the test station, and clean the belt weekly. If the line runs conformal coating, separate the coating area entirely; over-spray on a conveyor is impossible to remove completely.

Specification checklist

  • Board thickness range and maximum warp the conveyor must tolerate.
  • Edge clearance available on the panel, which sets the rail width.
  • SMEMA compatibility for each interface.
  • Surface resistivity and grounding method for every belt and nest.
  • Access for cleaning between the rails.