Screening a Contract Assembler: A Purchasing and Engineering Checklist

Screening a contract assembler is a different exercise from comparing prices, and it is usually split between two people who ask different questions. Purchasing wants to know what the order will cost and when it will arrive. Engineering wants to know whether the process can build the board at all. The checklist below is the overlap between the two, and working through it before the first order costs an hour and prevents most of the surprises.

Ask for the Documents, Not the Claims

The first screen is documentary. A contract assembler that can produce a capability list, a process flow and a list of its inspection equipment is describing a system. One that answers the same questions in conversation is describing the person who answered the phone.

The capability list should state the minimum and maximum component size, the placement accuracy, the smallest pitch handled in production, the panel size the line accepts, the reflow capability and the inspection equipment. The process flow should show where the checks happen: incoming component verification, paste inspection, first article, optical inspection, X-ray where required, and functional test.

If the assembler also handles component procurement, ask for the process for approved alternatives, because a substitution decision made without the customer is a design change.

Minimum Quantity and Line Configuration

Ask what the smallest order the factory will accept is, and then ask which line it will run on. A factory that accepts a small order and runs it on a line configured for high volume will change the line over for it, which is expensive for them and slow for the customer. A factory with a line or a configuration intended for small and medium batches can accept the order without disrupting anything.

This is the question that separates a quotation that will be honoured from one that will be renegotiated. The follow-up is worth asking as well: whether the small order and a later production order would use the same process and the same equipment, because a product that changes process between the sample and the volume build changes its own behaviour.

The Quotation Structure

Two quotations for the same board can differ by more than the price. The items that belong in the comparison are the stencil, the tooling, the programming and test fixture, the inspection level, the component handling fee if the customer supplies parts and the packaging.

A quotation that excludes the stencil is not necessarily cheaper; it may mean the charge appears later. A quotation that excludes inspection is cheaper in a way that matters. Ask for the price with the inspection level stated, so that the comparison is between two complete offers.

The other item worth defining in advance is what happens to a board that fails. Whether the assembler repairs, replaces or reports, and whether the components consumed in a repair are charged, should be agreed before the first batch rather than during a dispute about one.

The Technical Screen

The engineering questions narrow the list quickly.

Ask which packages the line has built in production: ball grid arrays with a stated pitch, quad flat no-lead devices, chip scale packages, connectors with fine pitch leads. Ask how the paste volume under a QFN thermal pad is controlled, and how the joints under a ball grid array are verified. Ask how the reflow profile is developed for a new board, and whether the profile is measured on the actual assembly with a profiler or taken from a similar product.

Ask how the flexible or irregular parts of a board are supported, because a carrier or fixture requirement is one of the most common reasons a quote changes after the design is seen. And ask about the maximum board size and the panel arrangement, because a design that does not fit the line has to be re-panellised, which is a design change. Ask as well how moisture sensitive parts are handled, from the labelling of the floor life to the baking step where a reel has been open for longer than the part allows.

The Commercial and Administrative Screen

The remaining questions are about how the relationship will be managed.

Who is the engineering contact, and what is the response time for a technical query. What happens when the supplied data is ambiguous: is a query raised before the batch, or is an assumption made. Who signs off the first article, and is the customer’s approval required before the batch continues. How is confidentiality handled, and is a non-disclosure agreement acceptable. How is a delivery date set, and what happens if the date slips.

None of those are unusual requirements, and an assembler that finds them difficult is telling you something about the rest of the relationship. The answers also identify the one or two items that need to be written into the agreement, which is the point of the exercise.

Verifying the Answers

The last screen is a sample. Ask to see a board of similar complexity that the factory has built, with its inspection data, and examine it: the joint shapes, the placement alignment, the flux residue, the cleanliness of the assembly. A board that is presented as a sample is the factory’s own choice of what represents it, and it is a fair basis for a first impression.

Where the product is destined for a regulated application, ask for the documentation the factory keeps: the process record for a batch, the traceability from components to units and the results of the tests performed. Our quality management system produces that record for every build, and the data package that supports it is the subject of our order process review. Where the boarding and assembly come from the same organisation, the handover between them is short, which is how our turnkey PCB assembly programmes are structured.

What the First Order Should Prove

The first order placed with a new assembler is a test of the arrangement rather than of the boards. It should establish four things. That the data package was read, which shows in whether the questions arrive before the build or after it. That the first article is reviewed with the customer rather than internally, because that is the stage at which an orientation or package error is still free to correct. That the inspection results are reported in a form the customer can file, because a result that exists only as a conversation cannot be compared with the next batch. And that a discrepancy, when one occurs, is reported with the step at which it was found, because that is what makes the next batch better rather than merely different. An assembler that does those four things on a first order will do them on a production order.

FAQ

How many assemblers should be screened? Three is usually enough to see the range of answers. The differences appear in the engineering questions rather than in the prices.

What is the strongest single indicator? Whether the factory asks its own questions about the design before quoting. A factory that quotes without reading the data has not read the data.

Should a sample build be placed before choosing? It is the most reliable test, provided the sample is a real production order with the same inspection level as the intended volume build.