Assembly Documentation And Work Instructions

A board is built from documents. The assembly drawing says what goes where, the fabrication drawing says what the bare board is, the work instruction says how each operation is performed and what the result should look like, and the traveler records that it was done. Where any of those is missing or out of date, the decision is made on the line by whoever is standing there, and the result is a product that matches the intention of the operator rather than the intention of the designer.

This article explains what the document set contains, why documentation fails in practice, how an instruction is written so that it is followed, and how the records are kept.

What The Set Contains

The assembly drawing carries the component locations, the polarity and orientation marks, the bill of materials, and the notes that apply to the assembly. The fabrication drawing carries the stack up, the material, the finish, the tolerances, and the acceptance class. The process documents then convert those into operations: the stencil and print specification, the placement program, the reflow profile, the inspection criteria, and the packing requirement.

The work instruction is the document the operator actually uses. It describes one operation, the equipment it needs, the settings that are used, the acceptance criteria, and the action to take when the result is outside them. Where an instruction covers several operations, or where it refers to a setting without stating its value, the operator supplies the missing information, and the process becomes different on each shift.

The same set has to travel with the product when it moves. A contract manufacturer building a design that was developed elsewhere needs the drawings, the bill of materials, the approved supplier list, the process specifications, and the acceptance criteria, and a gap in that transfer becomes a question on the line during the first build. The transfer is a deliverable in its own right, and it is usually accompanied by a first article inspection, in which a small number of units are built and measured against every requirement in the documents. That inspection is where a missing or contradictory requirement is discovered at a cost of a few boards rather than a production lot.

A document set is only as good as its index. Where the drawings, the specifications, and the instructions are numbered independently and stored in different places, finding the revision that applies to a given build becomes an exercise in reconstruction. A single document list, referenced by the assembly part number, is the cheapest control that can be added, and it is the first thing an auditor will ask to see.

Why Documentation Fails

The most common failure is a revision that does not reach the line. An engineering change is made on the drawing, the drawing is reissued, and a copy of the old revision remains in a folder or on a machine. The second is a document that is copied from a similar product and not fully edited, so that it names the wrong component or the wrong quantity. The third is an instruction that describes the equipment rather than the result, which leaves the acceptance judgement to the operator.

A fourth failure is the loss of knowledge when a person leaves. A step that was never written down because everyone knew it disappears with the operator, and the defect that appears afterwards is attributed to the equipment rather than to the missing step. Documentation is the mechanism that makes a process independent of the individual who happens to be running it.

Writing An Instruction That Works

The structure should be consistent across products. The operation is named, the equipment is identified, the materials and their quantities are listed, the settings are given with units, the sequence is numbered, and the acceptance criteria are stated with the inspection method. The final section describes what to do when the result does not meet the criteria, which is the part that is most often left out and most often needed.

Diagrams and photographs carry more information than text for placement and inspection. A picture of an acceptable joint beside a picture of a reject removes the discussion that otherwise happens at the bench, and a marked photograph of the board showing the orientation of the polarized components is faster to use than a table of rotations. The instruction should also state the conditions that require a stop and an escalation, such as a missing component, a damaged pad, or a repeated defect.

Revision Control

Every document has a number and a revision, and the revision in force is the only one that may be used. The change process records what changed, why, and when, and the new revision is distributed to the locations where the old one was used, with the old copies withdrawn. In a production environment the withdrawal is the step that fails, so the control is usually physical: documents are held in a controlled location, printed copies are stamped and collected, and the line references a single controlled copy.

The product itself should carry the revision. A marking on the silkscreen or on a label links the unit to the documentation that was in force when it was built, which is what allows a later investigation to reconstruct how it was made. The marking conventions are described under board outline and mounting design as part of the outline documentation.

The Traveler And The Record

The traveler follows the product through the line and records what happened at each step: the lot number, the material batches, the equipment used, the settings applied, the operator, and the result of the inspection or test. It is the record that answers the question of what was done when the product is examined months later, and its value is entirely in its completeness.

Electronic records have largely replaced paper in volume production, and they offer the advantage of being queryable, of linking automatically to the equipment settings, and of preventing a step from being signed off out of sequence. The retention period is set by the industry: a consumer product may need a few years, while a medical or aerospace product may need decades, which means the system that holds the record has to outlive the product and the equipment that made it.

The Customer Requirement

The customer requirements are the acceptance class, the traceability level, the certifications, and the reporting. Those requirements determine the depth of the documentation rather than the other way round, and an audit normally begins with the documentation because that is where a system either holds together or does not. The wider quality framework is described under PCB design quality characteristics, and the fabrication documents that feed the assembly under PCB design and fabrication.

FAQ

What is the difference between a work instruction and a drawing? The drawing defines the product and what it contains, while the instruction defines how it is produced and how the result is judged. Both are needed, and neither substitutes for the other.

Why does the traveler matter if the process is controlled? Because it is the evidence. A controlled process that leaves no record cannot demonstrate what was done, and it cannot support an investigation when a unit fails later.

How often should instructions be reviewed? Whenever the product, the equipment, or the process changes, and periodically in between. An instruction that has not been read by anyone for a year is usually out of date in several places.