Requirement Definition Before Choosing an SMT Partner

Enquiries for assembly work usually begin with the board and the quantity, and they usually omit the two things that determine which supplier is suitable: how accurately the work has to be done, and what the deadline actually is. Without those, a quotation describes a supplier’s habits rather than the customer’s requirement, and the comparison between two offers is not a comparison at all.

Requirement definition is the step that comes before selection, and it is short. What has to be placed, how small it is, how many units, by when, and which additional operations are needed. Answering those five questions narrows the field considerably and makes the remaining conversation specific.

How Fine the Work Has to Be

The smallest device and the finest pitch on the board set the placement accuracy that is required, and they should be stated rather than left to be inferred from the files. A line that routinely places generous pitches and a line that routinely places fine pitches are both competent, and only one of them is suitable for a given board.

The distinction between occasional and routine capability matters here. A supplier may have placed an area-array package once; a board that depends on several of them deserves a supplier who does so weekly, because the difference lies in the accumulated adjustments to paste volume, placement force and profile.

It is worth listing the positions that are difficult rather than expecting them to be discovered. The device with a pitch below the general rule, the connector whose footprint is unusual, the component close to a break line: naming them allows the process to be planned for them instead of around them.

How Many Units and in What Pattern

The quantity matters less than the pattern. Ten boards built once and ten boards built every month are different requirements, and so are ten boards of one design and ten boards of three variants.

Where the work will repeat, the value of retaining the stencil, the programme and the parameters is realised on the second order, and a supplier who does this is more useful than one who is marginally cheaper on the first. Where several variants are involved, the ability to run them without mixing them becomes part of the requirement.

Volume therefore determines not only the price but the kind of relationship that makes sense. A single exploratory batch can be placed almost anywhere; a product with a foreseeable future should be placed with a supplier who can follow it.

Special Process Requirements

Most boards need more than placement. Inserted components, a coating, programming, a functional test, separation from a panel and packing are all steps that may be required, and each of them is either available in house or has to be arranged.

Stating them at the enquiry stage allows the sequence to be planned rather than discovered. A coating cannot be applied before a test that needs access to the contacts; assembly into an enclosure may prevent access to a programming header; a test point may be covered by a conformal film if the order of operations is left to chance.

It also changes who can do the work. A supplier who performs the whole sequence holds the board, the programme and the test definition under one control, which shortens both the schedule and the route by which a question is answered.

The Deadline and What It Depends On

A deadline is only useful when it is real, and a real deadline is built from the work. The date depends on the stencil if one has to be made, on the material if a part has a purchasing lead time, on the availability of the line, and on how many operations follow the placement.

Stating which of the dates matters — the sample for a demonstration, the batch for a production start — allows the schedule to be arranged around it. Where everything is urgent, nothing is, and a supplier who is told that the board is needed for an unspecified purpose will schedule it on price.

It is also reasonable to ask what would have to change to bring the date forward, and to decide in advance whether those changes are acceptable. The answer is usually material, sequence or quantity rather than effort.

What the Supplier Should Be Told

The information that produces a useful response is largely the same for every order, and it fits in a short message: the board data, the bill of materials, the coordinates, the quantity, the deadline and the additional operations. Beyond that, three items are commonly omitted and commonly decisive.

The first is the function of the product. A board that will run continuously in an enclosure imposes different requirements from one that will be demonstrated once, and the difference affects the profile, the inspection and the packing.

The second is the substitution rule: which devices are interchangeable and within what range. The third is the acceptance criteria, wherever a test is involved, since a test without limits cannot be performed consistently.

With those, the reply to an enquiry is a plan with assumptions attached rather than a figure with a date. That is the form in which two suppliers can be compared, and it is also the form in which a problem can be anticipated rather than experienced.

What the Requirement Excludes

Defining what is needed also settles what is not, and that is useful in its own right. A board that will be demonstrated once does not need an ageing test; a prototype that will be modified next month does not need a fixture that follows it into production; a product stored for years does not need the same packing as one that enters a line the following morning.

Every requirement has a cost, and requirements that are inherited rather than chosen are the ones that go unquestioned. A test that was defined for an earlier stage of the product, an inspection that was added after a specific incident, a packing specification copied from another project: each may still be appropriate, and each deserves a moment of consideration before it is repeated in the order.

The same applies in the other direction. Where a requirement is real but not yet defined, saying so is better than omitting it. A test method that is still being agreed, or a material decision that depends on an availability check, can be flagged and resolved while the quotation is being prepared instead of stopping the line after the order is confirmed.

Turning the Requirement Into an Order

Once the requirement is written down, the order almost assembles itself. The revision to be built is fixed, the material is confirmed, the sequence of operations is agreed, the inspections are defined and the delivery format is decided.

The operations that follow are the familiar ones: SMT assembly for the placed devices, through-hole assembly where inserted parts are present, PCBA testing for the verification and the controls under quality management that keep each batch comparable. Where protection is required, conformal coating is planned in the correct place within the sequence.

FAQ

Why define the requirement before contacting suppliers? Because the answers determine which suppliers are suitable, and without them every quotation is measured against a different understanding of the work.

What is the most commonly omitted item? The acceptance criteria for any test, which is the information that allows a result to be judged rather than described.

How specific should the deadline be? Specific enough to be planned. A date with a purpose attached can be scheduled; an urgency without one cannot.