Engaging a PCB assembly house: panels, paste and the parts nobody can get

Panelisation and rails belong to the assembler

Machines grip a board by its edges, need clear space for conveyor rails, and place small boards far more efficiently in arrays. Your outline therefore acquires a border, tooling holes and fiducials, and several copies stepped into a panel with breakaway tabs or scoring. Deciding all that yourself without knowing their equipment usually produces a panel that has to be redone. Send single-board data and let them propose the array, then check that tabs do not sit where a connector must reach an edge and that separation will not stress a component sitting near a break.

The paste layer is not the copper layer

Stencil apertures derive from the paste layer, and the correct aperture is frequently not the same shape as the pad. Fine-pitch parts, thermal pads on power devices, connectors with large ground tabs and small passives each want different treatment, and getting it wrong produces bridging, tombstoning or voids beneath a component you cannot inspect. Assemblers routinely modify the paste layer for this reason, which is ordinary engineering rather than a change to your design. Ask to see what they altered and keep it with your files, or the next assembler starts from your original data and reproduces the original problem.

Approved parts and the substitution question

A parts list naming a manufacturer and part number on every line is an instruction. A list describing values and packages is a suggestion, and the assembler resolves it from whatever is available. For passives that is usually harmless; for regulators, connectors, crystals, memory and anything with firmware implications it is not. Mark which lines permit an alternate without approval, which need your written agreement, and which are fixed. Reviewing what was actually fitted against what was specified, on the first build and periodically afterwards, catches substitutions that were made politely and never mentioned.

Moisture, storage and parts that spoil

Many packages absorb moisture from the air and crack or delaminate on reaching reflow temperature, so they arrive sealed with desiccant and an indicator and carry a limited exposure period once opened. Boards left part-built over a weekend, parts consigned by a buyer who stored them in an office, and reels reopened for a second build all create failures that surface as intermittent faults much later. Consigning components means asking what the handling requirements are and following them. Where the assembler buys, ask how they control exposure and what they do with parts that exceeded it.

Reels, minimums and the leftovers

Components arrive on reels in fixed quantities, and machines want reels rather than cut tape. A small build therefore buys far more of certain parts than it uses, and the surplus ends up somewhere. Establish who owns those leftovers, whether they are held against your next order or invoiced now, and how they are stored, because attrition, mixed reels and unlabelled bags of parts are a familiar source of build errors. The same question applies to the stencil and any programming fixtures. None of it is expensive individually, and all of it becomes messy after several builds if nobody decided at the start.

Frequently asked questions

Why does the assembler want to change our board outline?

Because their equipment needs somewhere to hold the board and somewhere to see reference marks. Rails, tooling holes and fiducials are not cosmetic requests; without them a board cannot be conveyed, aligned or populated accurately. Small boards also get panelised into arrays because placing them individually is uneconomic. Provide the single-board design, let the assembler define the panel, then review their proposal against mechanical constraints such as connector positions and mounting points.

How do we stop parts being substituted without our knowledge?

Specify manufacturer part numbers rather than descriptions, mark each line as fixed, approval-required or free to substitute, and require written agreement for anything in the first two categories. Then verify: request the actual parts used on a build and compare against the specification, at first article and occasionally thereafter. Substitution usually happens under time pressure and with good intentions, so a stated rule plus an occasional check beats a warning.

What should we ask about a board that failed in the field?

Which build it came from, what test data exists for that serial number, which parts were actually used on that build, and whether any process change occurred between builds. That chain only exists where traceability was specified up front through serialisation, retained test records and identification of the reels used. Where it was not, diagnosis becomes guesswork and the same failure usually reappears on the next production run.