Missing Part: 5 Rules for Placement Verification

A missing part is an empty footprint on a finished assembly, and it usually reaches the customer because every check between the machine and the shipping box assumed that something else would catch it. The placement machine fed a component, the inspection looked for solder defects, and the electrical test did not notice a gap that its program never covered.

Preventing an escape means proving each stage separately. The pick and place has to confirm that it picked and placed, the feeder setup has to avoid running dry in the middle of a panel, the vision system has to reject rather than skip, and the aoi inspection has to be verified with a known missing part sample instead of being trusted because it passed last time.

How a Missing Part Reaches the Customer

The common route is a skip that nobody recorded. The machine fails to pick, reports a placement error, and continues to the next component so that the line does not stop. The panel keeps moving, and unless the error is displayed and acted on, the empty footprint travels with the rest of the order.

The second route is a feeder that runs out. A reel that ends part way through a panel leaves the last few references empty, and the operator notices only if the machine reports it or if the count is tracked. The third route is program data. A reference that is disabled, has a zero quantity or points to the wrong feeder position is placed by nobody, and no machine alarm is raised because nothing failed.

Feeder Setup and Feed Failures

Most feed failures come from the tape rather than from the feeder mechanism. A splice that is too thick, a pocket that is not sealed, or a cover tape that releases at the wrong force will cause the component to be presented late or not at all. The nozzle then closes on empty tape and reports a pick error.

Setup discipline removes a large share of these events. Every feeder should be verified against the setup list before the run, and the remaining component count should be checked so that a reel with two hundred parts is not loaded onto a job that needs four hundred. Where the same feeder is used for the whole order, a run out sensor is worth more than an operator’s attention.

Nozzle Vacuum and Pick Errors

The nozzle is the only place where the machine can confirm that a component was picked. A vacuum sensor or a pressure switch detects the change when a part seals the tip, and a machine that has the sensor disabled or the threshold set too loose cannot tell an empty nozzle from a loaded one.

Thresholds drift as the nozzle wears and as the vacuum generator ages. The nozzle vacuum check should be part of the scheduled maintenance, and the value should be recorded so that a slow loss of vacuum is visible before it becomes a pick failure on the smallest chip on the panel.

Vision Verification Before Placement

Vision has two jobs on a modern placement machine. The first is to centre the component, which requires a successful image. The second is to decide whether the part is present and within shape limits. A component that is not there cannot be centred, so the machine should report a pick error rather than a centring error.

The setting that matters is what the machine does next. Options range from stopping the line to skipping the placement and logging a defect. Whichever the shop chooses, the log has to be visible to the operator and to the person who releases the panel, because an error that nobody reads is the same as no error at all.

Program and Bill of Materials Control

Program data is where a missing part can be created without any hardware fault. A reference that has been disabled during a revision, a component that is marked as optional, or a position that has been left blank in the placement file will produce an empty footprint on every panel.

Control comes from comparing the released program against the released fabrication and assembly data. The reference designator list in the program should match the assembly drawing exactly, and the comparison should be repeated whenever the program or the drawing changes. A check of this kind belongs at first article rather than after the order has shipped.

Manual Assembly and Hand Placement

Hand placed parts are missed more often than machine placed ones, because there is no vacuum sensor and no vision check. The operator follows a drawing or a screen, and a reference that is listed in the wrong order or that sits under a component can simply be overlooked.

The practical controls are a counted kit, a marking on the board that shows the hand placement area, and a second check by someone other than the person who placed the parts. Where only a few parts are hand placed, the drawing should show them in a separate view so that the operator does not have to search the panel.

Automated Optical Inspection Limits

An AOI system detects a missing part by comparing the image against a reference. The comparison fails when the component is small, when its colour matches the pad or the mask, or when the reference was learned from a defective board. A program learned from production panels carries whatever was on those panels into its definition of normal.

Detection should be proven with a sample that has a known missing part at each of the smallest footprints on the panel. Where a footprint sits in shadow or under a tall neighbouring part, the camera may not see the pad at all, and the escape will pass every panel until someone reports it. The AOI lighting setup decides whether those positions can be inspected.

Electrical Test and Functional Coverage

In circuit test catches a missing part only if the part is required for the measurement to pass. A missing series resistor opens the net and is found easily. A missing decoupling capacitor may change nothing that the test measures, and a missing pull up resistor may leave a signal that the test treats as a valid logic level.

Boundary scan covers joints on the devices that support it, so a missing component between two scan devices is detected, while a missing part on a net without scan access is not. Coverage should be reviewed when the assembly changes, and the boundary scan coverage analysis should be repeated rather than assumed to still apply.

Records, Rework and Containment

When a missing part is found, the first question is how many others passed the same point. The answer comes from records. Placement error logs, feeder load records and inspection results should be searchable by panel serial number, so that a suspect window can be defined and the affected boards pulled.

The rework record matters as much as the detection. A part that is added by hand should be documented with the operator, the date, the part number and the inspection that followed, because a hand added part that was never verified is not the same product as one placed by the machine. Quarantine and release decisions should follow that record rather than the memory of the shift.

FAQ

Why did AOI not catch the missing part? Either the reference for that footprint was too permissive, or the camera could not see the pad clearly enough. Both are program problems rather than hardware problems. Adding a known missing part sample to the verification set shows immediately whether the program would have caught the escape.

Should the line stop when a pick error occurs? Stopping on every error is expensive, so most shops skip and log instead. The important part is that the log is reviewed before the panel is released and that the skipped reference is repaired. A skip that is logged and ignored is the same as no control at all.

Can in circuit test replace AOI for missing parts? No. Electrical test sees the net, not the board, so it finds the parts that the net needs in order to pass and misses the ones that it does not. The two checks cover different failures, and a shop that relies on one of them should know exactly which missing parts fall through the other.