Line Audit Checklist for PCB Assembly

A line audit is a structured look at a running line to confirm that the process is being executed as documented. It is not a quality inspection of the product and not a search for defects; it is a check of the arrangements, the records and the practices that produce the product. Done well it finds the small drift that accumulates between procedures and reality, before that drift produces a defect that is expensive to explain. The findings feed process control directly, because they describe the state of the process rather than the state of the product.

What a Line Audit Is For

The purpose is to test the process, not the people. An audit finding that a reflow profile has not been re-measured since a heater was replaced is a finding about the system, and it is useful. An audit finding that an operator was not paying attention is not actionable and does not prevent a recurrence, because the next operator will behave the same way in the same arrangement.

The audit also provides the evidence that a customer or a certification body will ask for. A shop that can show regular audits with findings and closed actions is demonstrating control, and the record is worth more than a perfect audit with no findings, because a perfect audit usually means the checklist was not sharp enough. The scope should cover the whole route from material receipt to packing, with the depth adjusted to the risk of each stage. The inspection stages of the route give the natural structure for the checklist.

Documents and Revision Control

The first check is whether the documents in use are the current ones. The work instruction at the machine should match the revision in the master list, the bill of materials should match the one the planner issued, and the drawing should be the revision the customer approved. Where a document is found to be a different revision from the one in the system, the finding is serious, because every unit made with it is suspect.

The check should include the records rather than only the documents. A traveler that has not been signed at a completed step, a first article sheet with no measurement values, and a profile record with no date are all findings of the same kind. The audit should sample the records from the last few days rather than the ones prepared for the visit, because the prepared ones are not representative. The document control arrangements are what keep the two in step.

Materials, Storage and Floor Life

Material checks cover identity, condition and exposure. The reel on the machine should match the part number in the program, the paste should be within its working life and at the correct temperature, and the moisture sensitive parts should be within their floor life. The check is quick and it is the one that most often finds something, because the pressure of production encourages a slackness in the handling of materials.

Storage conditions belong in the same check: the temperature and humidity of the paste store, the condition of the dry cabinets, the state of the labels on the reels and the accumulation of exposure time. The records should be sampled against the physical items, because a record that has been filled in from memory is worse than no record. The shift handover record often reveals the state of the material storage, since it is the point at which materials are returned or left out.

Equipment, Setup and Parameters

Equipment checks confirm that the machine is set as the program requires and that the settings are the ones that were qualified. The stencil, the squeegee pressure, the placement program, the feeder positions and the reflow setpoints should all be compared against the current setup sheet. Anything adjusted by the operator without a record is a finding, even if the adjustment was correct, because the next shift will not know what was changed.

The condition of the equipment belongs in the same check. A worn squeegee, a feeder with a damaged sprocket, a nozzle with caked flux and a conveyor belt with low tension are all visible on an audit walk and all affect the product. The audit should not attempt to diagnose them, but it should record them so that they reach the preventive maintenance schedule. Photographing a finding is often faster than describing it and leaves less room for argument later.

First Article and In-Process Verification

The first article is the point at which the setup is confirmed before the batch proceeds, and the audit should check that it was done, that it was measured rather than judged, and that the results are within limits. A first article sheet with no values, or one signed before the measurements were made, is a finding that invalidates the evidence for the whole batch.

In-process verification follows the same logic: the checks the process plan calls for at defined intervals should be present, with values and times. Where a check is described as visual, the audit should ask what the acceptance criterion is, because a check without a criterion is not a check. The criteria should be available at the station, not in an office. Sampling of the records should cover the period since the last audit so that the pattern is visible rather than a single instance.

Operator Practice and Training Records

Practice is observed rather than interrogated. How the operator handles a board, whether the wrist strap is worn and tested, whether the paste is left open, whether the stencil is wiped on schedule: these are visible in a few minutes of watching. The observation should be recorded as what was seen rather than as a judgement about the operator, and it should be repeated at different times of day because practice changes under pressure.

Training records should confirm that the operator has been qualified for the task being performed and that the qualification is current. Where a task requires periodic requalification, the date should be checked. Where a new operator is on the line, the audit should confirm that the qualification was completed before the task was assigned rather than being scheduled afterwards, since the second arrangement is a common way for a training system to look complete while being ineffective.

Recording Findings and Grading

Findings should be recorded with enough detail to be actionable: what was found, where, when, and what the requirement is. A statement that the profile record was incomplete is weaker than a statement that the profile for a named product on a named oven had no date and no belt speed. The requirement should be quoted from the document, which also confirms that the requirement exists.

Grading findings by severity helps the response. A finding that affects the product already made, such as a wrong revision in use, requires an immediate containment and a review of the affected units. A finding about a record that is missing but where the process is otherwise correct requires a corrective action but not a containment. Mixing the two leads either to over-reaction or to under-reaction, and the grading rule should be written before the audit rather than applied afterwards.

Closing the Loop

An audit without a closed loop is a ritual. Each finding should have an action, an owner and a date, and the actions should be reviewed at a defined interval until they are closed. Where an action is not closed by its date, the reason should be recorded and the date revised rather than the action quietly dropped. The closure should be verified by checking that the condition no longer exists, not by checking that a form was signed.

Recurring findings are the most valuable output, because they indicate that the arrangement rather than the execution is at fault. Where the same finding appears in three audits, the corrective action has not worked and the approach should change from a reminder to a change in the equipment, the layout or the procedure. Reviewing the findings over a year, rather than one audit at a time, is what shows whether the line is actually improving.

FAQ

Should the audit be announced? Announced audits get prepared for, which is useful for checking the documented state and useless for checking the normal state. The practical compromise is a scheduled audit of the records and an unannounced walk of the line, with both using the same checklist.

Who should perform the audit? Someone who does not work on the line being audited, and who knows the requirements well enough to quote them. An operator from another line is often better than a manager, because the operator recognises the shortcuts that a manager would not see.

How long should an audit take? A focused audit of one line takes two to three hours including the record sampling. A shorter audit tends to check the visible items only, which are the ones least likely to be wrong. The depth should follow the risk of the product rather than the time available.