Supplier Selection for SMT Assembly: What to Check

Supplier selection for assembly work is usually approached as a comparison of prices, and the comparison is usually made on a quotation that describes a scope nobody has fully defined. The result is a decision based on the least reliable part of the information. A more durable approach is to establish what the product actually requires, and then to check whether the supplier can demonstrate those capabilities rather than assert them.

Start From the Product

The requirements of the product decide what the supplier has to be able to do. A board with a fine pitch device and a bottom terminated package needs a stencil capability, an optical inspection programme and an X-ray facility. A product with a cleanliness or coating requirement needs a cleaning line and the measurement that verifies it. A product that will be shipped as a complete unit needs a box build area and a system test. A product with a safety function needs the traceability that the standard behind it demands.

Writing that list before contacting suppliers changes the conversation from a general enquiry into a specific question. It also prevents the common outcome in which a supplier is selected on price for a product whose difficult requirements were not mentioned, and the difficulty then appears as an unexpected cost or a delivery slip.

What Capability Looks Like as Evidence

A claim of capability is worth very little on its own, because every supplier will claim it. Evidence takes one of four forms: a published limit that can be compared against the design, a sample that can be examined, a record of a similar product that has been delivered, and a process document that describes how the characteristic is controlled.

The most useful of those is the process document, because it describes the mechanism rather than the result. A supplier who can explain how the paste deposit is measured and what happens when the measurement moves has a process. A supplier who can only describe the equipment they own has a set of machines, which is a different thing.

An audit visit answers questions that a document cannot. Whether the inspection equipment is actually in use, whether the records are being completed, whether the work instructions are at the stations and whether the material store is organised are all visible in an hour of walking the floor, and none of them is visible in a brochure.

Process Control and the Measurements

A supplier who controls the process can tell you what they measure and what the limits are. The paste volume after printing, the placement offset after placement, the reflow temperature at defined positions, the impedance of a controlled structure and the functional test results are all characteristics that can be tracked, and a supplier who tracks them has a basis for detecting a change.

A supplier who inspects the finished product but does not measure the process is relying on the inspection to find everything, and inspection finds what it is designed to find. The distinction becomes visible when a defect appears: a process controlled operation can compare the current batch with the previous ones and identify what changed, while an inspection only operation has no baseline to compare against.

Traceability and the Records

The traceability question is simple to ask and hard to answer badly. Given a specific finished unit, can the supplier identify the boards used, the material lots consumed, the programme and profile revisions applied and the test result? Given a material lot with a reported problem, can the supplier identify the orders that consumed it?

A supplier who can demonstrate both directions has a record that is maintained rather than assembled for an audit. A supplier who can demonstrate only one has a system with a gap, and the gap will be found at the moment when the answer is most urgently needed.

Communication and the Working Relationship

The last criterion is the least technical and the most predictive. How does the supplier respond to a question about a difficult requirement, to a change of quantity and to a request for a schedule that is tight? A supplier who answers with a specific question has understood the design. A supplier who answers immediately with a yes has probably not read it.

What matters in production is the exception, not the routine. Every supplier will deliver an ordinary order; the difference appears when a component cannot be obtained, when a first article shows a problem or when a delivery date is at risk. The response to those situations is decided by the working relationship, by how early the information is shared and by whether the conversation is about solving the problem or about allocating the blame.

The practical test is a small order for the actual product, placed and observed. It answers more questions in two weeks than a year of evaluation. Our operation offers that route through low volume PCB assembly and rapid PCBA prototyping, the capabilities are stated on PCB capabilities, the production runs under SMT assembly, the material is handled by component procurement, the testing sits with PCBA testing and the records are held with quality management.

The Questions Worth Asking

A short list of questions separates a supplier who has a process from one who has equipment. What is the smallest package placed in routine production, and how is the placement verified? How is the stencil designed for a fine pitch device and for a thermal pad? What is measured after printing, and what is the limit that triggers an investigation? How is the reflow profile established for a new product? What happens to a batch when the first article fails?

Each of those has an answer that describes a mechanism and an answer that describes an intention. A supplier who says the placement is verified by the vision system has described an intention; one who describes the nozzle, the recognition setting and the first article check has described a mechanism. The distinction is not pedantic, because the intention is the same in every brochure and the mechanism is only present when the work has been done.

The material question deserves the same treatment. How is an incoming lot verified, what happens when a label does not match its contents, and what is the process for proposing a substitute? A supplier who answers those three specifically has a material control system; a supplier who answers them generally has a purchasing department.

Cost in Its Proper Place

Price is a real criterion and it belongs in the comparison, but it belongs at the end and with the scope stated. Two quotations for the same board can differ because one includes the paste inspection, the optical inspection and the X-ray check while the other performs them as optional extras, and the cheaper quotation is only cheaper because it describes less work.

The useful comparison is therefore total cost at the intended quantity, with the scope written out: what is included in the placement price, what is charged once, what happens if the first article fails, and how a change of quantity affects the figure. A supplier who is willing to state those terms clearly is a supplier whose price can be compared meaningfully with another’s, and the exercise of obtaining that clarity frequently changes which quotation looks best.

The other element of cost is the one that never appears in a quotation: the cost of a problem. A delivery that slips, a batch that has to be reworked, a defect that reaches the customer or a question that nobody can answer from the records all carry a cost that is not on the invoice but is paid nonetheless. A supplier whose process controls those outcomes is worth a price difference, and the difference is usually smaller than the cost of the first incident.

FAQ

What is the best indicator of capability? A description of how the characteristic is measured and controlled, rather than a statement that it can be achieved.

Is an audit necessary? It answers the questions a document cannot, particularly whether the systems described are the systems in use.

How should a supplier be tested? With a small order for the real product, observed from data intake to delivery.