Work Instruction Content for PCB Assembly
A work instruction is the document that tells a qualified operator how to perform one operation and how to know that it has been performed correctly. It is written for the person at the machine, not for the customer or the auditor, and it fails when it is written for either of those audiences. The test of a work instruction is whether an operator who has never built the product can complete the step and reach the same result as the person who wrote it.
What a Work Instruction Has to Answer
Four questions have to be answered for every step: what to do, with what, to what tolerance, and how to tell whether it worked. A document that answers only the first two leaves the operator to infer the rest, and the inference will differ between people and between shifts. The third and fourth answers are the ones that make the process repeatable, and they are the ones most often missing.
The scope also has to be clear. A work instruction covers one operation, and the boundary between it and the next instruction should be unambiguous. Where a step is shared between several products, it may be written once and referenced, but the reference should be explicit rather than implied, so that a change to the shared step is reflected everywhere it is used. The assembly documentation structure defines how those references are arranged.
Structure and Sequence
The instruction should follow the order of the work. Each process step is numbered and carries a single action, and the materials and tools for that step are listed with it rather than in a table at the front. Where a step depends on a previous one, the dependency should be visible, as it is when the steps are in sequence and the numbering is continuous.
The level of detail should match the risk. A step that can be performed in only one way does not need a paragraph, and a step that has several failure modes needs the failure modes described. Where the whole instruction is written at the same level of detail, the important steps are buried and the trivial ones are given undue weight, which is how operators learn to skip sections.
Parameters and Their Tolerances
Every parameter that the operator sets should be given with a value and a tolerance, and the tolerance should be the one that matters rather than the one that is convenient. A temperature given as 245 °C without a tolerance leaves the operator to decide whether 250 °C is acceptable, and the answer depends on the component ratings rather than on the operator’s judgement.
The parameter list should also say which parameters are recorded and which are only set. A parameter that is recorded needs a place on the record sheet and an interval; one that is only set needs to be verifiable by looking at the machine. Mixing the two produces record sheets that are filled in from memory, which is worse than no record. The document control arrangements should ensure that the parameter list matches the machine settings sheet for the product.
Acceptance Criteria at Each Step
Every step that changes the product should have a criterion, and the criterion should be something the operator can check at that step rather than something that will be checked later. Where the check is visual, the instruction should say what is being looked for and under what light; where it is dimensional, it should give the value and the limit.
The criterion should also say what to do when it is not met. An instruction that says a joint must be free of bridging and stops there leaves the operator to decide whether to rework, to escalate or to continue. A short statement covering the three options, with the point at which the decision moves to a supervisor, removes the hesitation and makes the response consistent between shifts.
Records the Operator Must Make
The record should be the minimum that is needed to demonstrate the step was performed. A signature and a date against a step, with values for the parameters that were measured, is usually enough. Where a record requires a value, the instruction should state the interval at which it is taken, because a value taken once and copied down the sheet is not a record of the process.
The record should be designed so that it cannot be completed without doing the work. A sheet with a column for the first article result and no place for the measurement invites a tick, while a sheet with a space for the value and the limit invites a measurement. The design of the record is part of the instruction and should be reviewed with it. The shift handover record is a good example of a document whose design determines whether the information is useful.
Photographs and Visual Aids
A photograph of an acceptable result and one of a rejected result is worth a page of text. The photographs should be taken under the same lighting and at the same magnification that the operator will use, because a photograph taken under laboratory conditions does not describe what the operator sees. Where the criterion is dimensional, a dimensioned illustration is more useful than a photograph.
The visual aids should be revised when the product changes, and they should be dated so that an old printout in a plastic sleeve can be identified. Where a boundary sample is kept as a physical reference, the instruction should say where it is kept and when it is replaced, because a sample that has been handled for a year no longer represents a new board. The workmanship standards provide the acceptance criteria that the photographs should illustrate.
Revision Control and Change Marks
The instruction needs a revision identifier, a revision history and a rule for what happens to superseded copies. A change that is not marked is a change that the operator cannot see, and an operator who has learned the previous revision will continue to follow it. Change bars in the margin, or a highlighted step, make the change visible without reading the whole document.
The rule for superseded copies should be explicit: which copies are withdrawn, who holds them, and how a copy at the machine is replaced. A laminated copy that is not replaced when the revision changes is the most common source of a nonconformance, and it is a document control failure rather than an operator error. The control should include the electronic copy and the printed copies, and the printed copy should carry the revision number in the footer.
Keeping the Instruction Alive
An instruction should be reviewed when the process changes, when a new failure mode appears and at a defined interval even if nothing has changed. The review should include the operators, because they are the people who know which parts of the document are followed and which are worked around. A step that is routinely bypassed is either wrong or unnecessary, and both outcomes are worth knowing.
The review should also check the instruction against the actual process, by watching the step performed rather than by reading the document. Where the two differ, the document should be corrected or the process should be, and the decision should be recorded. Instructions that drift away from the process are worse than none, because they give the appearance of control without providing it, and the drift is only discovered after a defect has escaped.
FAQ
How long should a work instruction be? Long enough to cover the steps and short enough to be read at the machine. Where an instruction runs to many pages, it should be split by operation so that the operator sees only the relevant part, and the sequence between the parts should be carried by the traveler.
Should the instruction include the reason for a step? Briefly, where the reason helps the operator make a judgement. An operator who knows why the preheat is held for a defined time is more likely to notice when the timer is not running, and less likely to shorten the step to catch up.
Who should write the instruction? A process engineer with the operator’s input, and it should be validated by having a different qualified operator perform the step from the document alone. If they cannot, the document is incomplete regardless of how thorough it appears.
