Documents Needed Before SMT Contract Manufacturing

Every delay at the start of a build traces back to a document that was missing, ambiguous or out of date. SMT contract manufacturing runs on files, and the quality of those files decides how quickly the line can be set up, how many questions the engineer has to ask and how likely the first article is to pass. The list is short, but each item has a purpose that is easy to underestimate.

The BOM and Its Details

The BOM defines what gets placed. A useful one carries the reference designator, the manufacturer part number, the quantity, the package and the acceptable substitutes. Omitting the package is the most common cause of a wrong part arriving, because the same electrical value exists in several footprints.

Lifecycle status matters too. A part marked as not recommended for new designs will create a problem on the second build even if the first one succeeds. Flagging it now lets the engineer approve a substitute before the stencil is ordered, rather than after the boards are populated.

Gerber Data and Fabrication Drawings

Gerber data describes the copper, the solder mask, the silkscreen and the drill. It should be exported as a complete, self-consistent set with a drill file whose units match the artwork. A board outline that exists only in a mechanical drawing is a frequent source of confusion.

The fabrication drawing adds the stack-up, the surface finish, the impedance targets and the tolerances that the artwork cannot express. A current-mode step-down regulator placed close to a via field will behave differently depending on the ground structure underneath, so the drawing is where that intent is recorded.

Assembly Drawing and Reference Designators

The assembly drawing tells the line where each part goes. It should show the polarity of diodes, the orientation of connectors and the pin one marker for every integrated circuit, and it should match the silkscreen that will actually be printed on the board.

When the drawing and the silkscreen disagree, the operator resolves the conflict by judgement, and judgement varies between shifts. Keeping both in step removes that variable entirely. Our SMT prototype BOM checklist walks through the checks that catch this before the first board is built.

Stencil, Tooling and Test Requirements

The paste stencil is generated from the copper and mask layers, but its thickness and aperture design are engineering decisions. A board with fine pitch parts and a large thermal pad needs different aperture ratios in the same stencil, so the requirement should be stated rather than assumed.

Test requirements belong in the package as well. Whether the board needs in-circuit test, functional test or only visual inspection changes the fixtures, the test points and the cycle time. Stating it late forces the test points to be added by hand or the test to be skipped. A disciplined PCB design release checklist keeps this from being forgotten.

Revision Control

Every document needs a revision that matches every other document. A BOM at revision C paired with Gerber data at revision B will produce a board that contains pads for a part the BOM does not list, and the mismatch may not surface until test.

Archive the exact set that was used for the build. When a second batch is ordered months later, the archived revision is the only reliable record of what the first batch actually contained, and it is what makes a controlled change possible. Details that affect the final joint, such as flux choice and profile, are covered in our PCBA soldering requirements notes.

Working With the Manufacturer

The package is a starting point, not a substitute for engineering review. A good SMT contract manufacturing partner will return a set of questions within a day, and the quality of those questions is the best signal of how the build will go. Questions about polarity, substitutes and test access mean the files were actually read.

Answer in writing rather than by phone. A verbal answer that is not recorded becomes an assumption, and assumptions diverge between shifts. A short email thread attached to the revision is enough to keep every party working from the same instruction.

First Article and Approval

The first article is the moment the documents meet reality. It should be inspected against the assembly drawing and the BOM together, not just against the board that came off the line, because the drawing is what the next build will follow.

Record what was measured and what was approved. When the second batch is ordered, the approved first article becomes the reference for judging whether anything has drifted, and it removes the argument about whether the current result is acceptable.

Common Causes of Rework at Introduction

Most early rework comes from a small number of gaps. A missing polarity indicator, a part placed on a footprint that does not match its body, a stencil aperture that delivers too little paste on a thermal pad, or a test point that is covered by a connector.

Each of these is cheap to find on paper and expensive to find on a board. That is the entire argument for spending time on the documentation set before the order is released, rather than treating it as paperwork that accompanies the order.

Panelisation and Breakaway Details

How the board is arranged on the panel affects assembly as much as fabrication. The breakaway method, the tab width and the position of the routed slots all determine whether the panel survives the placement and reflow stages intact.

Include the panel drawing with the rest of the package. A breakaway that leaves a rough edge near a connector will fail inspection, and the fix is a change to the panel rather than to the process, which means it has to be raised before the tooling is cut.

Solder Paste and Profile Expectations

State any alloy or flux constraint explicitly. A product destined for a harsh environment may require a specific alloy, and a board with fine pitch parts may require a no-clean chemistry that behaves differently during the soak phase of the profile.

Where the product has a thermal limitation, such as a component rated for a low temperature, that constraint belongs in the documentation. The line can only build to the profile it is given, and discovering the limitation after the first batch wastes the entire run. Our component tolerance and reliability notes cover how these limits are set at design stage.

Where the Responsibility Sits

The customer owns the design intent and the manufacturer owns the process. Ambiguity about which party decides a substitution, a profile or a test limit is what turns a routine build into a dispute. Writing the boundary into the package, even in a single sentence, removes the gap where a wrong assumption can hide until the boards are already populated.

FAQ

What causes a late package in SMT contract manufacturing? Usually a BOM whose package column is empty or a Gerber set exported with mismatched units. Both are found during engineering review, which is exactly when they are cheapest to fix.

Is a native CAD file needed, or are Gerbers enough? Gerbers plus a fabrication drawing and assembly drawing are sufficient for assembly. A native file helps when the engineer needs to move a part or add a test point.

How often should the package be resubmitted? On every revision change, and once more before a repeat build so that the archived set can be confirmed against the current one.