Small Volume SMT Assembly from Quotation to Shipment

A development team that has finished a design needs boards assembled in tens or hundreds rather than thousands. Small volume smt work is organised differently from mass production, and knowing the sequence removes most of the delay from a first order.

What Small Volume Assembly Means

Small volume covers the range between a handful of hand built prototypes and a full production order. The panels are processed on the same machines as a large run, but the setup time is spread over far fewer units.

Because the setup dominates the cost, the quotation depends on the number of component types and the number of placements rather than on the number of boards alone. A design with fewer unique parts is cheaper to build at this scale.

Quotation and the Production Package

The order starts with the fabrication data, the bill of materials and the assembly drawing. Where the assembly house supplies the stencil, the paste layer and the panel drawing are needed as well.

The bill of materials has to carry the reference designator, the manufacturer part number, the package and the quantity per board. A list that gives only a value and a footprint leaves the buyer guessing, and guessing produces substitutions.

Material Procurement and Inspection

Components are purchased against the approved list and inspected when they arrive. The inspection looks at the part marking, the packaging and the quantity, and it confirms that the reel matches the part number on the purchase order.

Counterfeit and re-marked parts are a real risk for small buyers, so the sourcing channel matters as much as the price. Where a part cannot be obtained, the substitution has to be approved by the designer before it is fitted.

Component Preparation

Some parts need preparation before they can be placed: connectors may be supplied on a tray, and pin headers may need trimming or forming. Doing that work in advance keeps the machine running when the panel arrives.

Moisture sensitive parts are baked according to their floor life and stored in dry conditions. A plastic package that has absorbed moisture can crack during reflow, and the damage is not visible from the outside.

The Stencil

The stencil is a thin sheet of metal with apertures that match the pads on the board. It is usually laser cut from stainless steel, and its thickness follows the finest pitch on the panel.

Aperture dimensions are the designer’s contribution to the printing process. The area ratio between the aperture opening and its wall has to stay above the printable limit, otherwise the paste does not release cleanly and the joint is short of solder.

Paste Storage and Printing

Solder paste is stored cold and brought to room temperature before it is opened, because condensation inside the container changes its rheology. The printing window, the squeegee pressure and the separation speed are set by recipe.

The stencil is aligned to the panel fiducials by camera, and the first prints are measured. Paste printing is the step that decides the yield of the whole line, and correcting it after reflow is not possible.

The First Article

The first assembled board is inspected in detail before the rest of the run continues. Placement, orientation, polarity and solder joint quality are checked against the assembly drawing.

The first article is also the point at which the design is confirmed for manufacture. A footprint that is too small or a silkscreen that overlaps a pad is far cheaper to change now than after the batch is complete.

Placement

Components are placed by machine at speeds that depend on their size and the required accuracy. Fine pitch parts are aligned by vision before placement, which allows a small package to be positioned reliably.

The placement program is derived from the assembly data, so the coordinates and rotations come from the design rather than being typed in. A consistent orientation convention in the library reduces the number of corrections at this stage.

Reflow

The panel passes through a multi zone oven whose profile was measured on the actual assembly. The profile brings the largest thermal mass on the board to the reflow temperature without overheating the smallest parts.

Nitrogen is often used on a small run to reduce oxidation, which improves the wetting of fine pitch joints. The profile is verified at the start of the run and checked periodically while the line is in production.

Inspection and Rework

Optical inspection compares the assembled board with a program that defines what a correct joint and a correct placement look like. Joints that cannot be seen are checked by X-ray or by electrical test.

Rework is a normal part of a small run, and it has to be controlled. A joint that is touched up with the wrong tip or the wrong temperature is a reliability risk, so the rework station is set up with the same discipline as the line.

Test and Quality Control

Functional testing confirms that the board does what the design intended, and it is usually performed on a fixture built for the product. In circuit test checks the connections, while a functional test exercises the firmware.

Sampling and record keeping belong to the same stage. The inspection standard, the limits and the result for each panel are what allow a fault to be attributed to a process step rather than to a supplier.

Packing and Traceability

Finished boards are packed with the moisture and electrostatic protection the parts require, and each box is labelled with the product, the quantity and the date code.

The record links the boards to the components and the process settings used. On a small run it is the only practical way to answer a question about a specific unit months later. The same discipline is described in our notes on the PCBA development process.

Cost Structure of a Small Run

The cost of a small order is dominated by engineering, stencil making, machine setup and inspection. Material is a smaller share than in volume production, which is why the price per board falls so sharply with quantity.

Reducing the number of unique components and using standard packages reduce the setup time directly. Where the schedule allows, ordering the stencil and the components before the boards arrive keeps the line occupied for the shortest time.

Lead Time

A small run is normally measured in days rather than weeks, provided the materials are available. Long lead components are the usual cause of a delay, and they are visible in the bill of materials well before the order is placed.

Discussing the schedule before the order is confirmed is worthwhile. A run that can move between machines is easier to fit into a busy line than one that needs a specific placement program at a specific hour.

What the Design Should Provide

Fiducials, a panel design that fits the conveyor, and a paste layer that matches the stencil requirements are the three items the assembly house needs from the designer. Without them the work becomes manual.

Test access and a clear assembly drawing matter as much for a small run as for a large one, because the same fixtures are reused. The layout requirements are described in our notes on manufacturability.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

FAQ

Is a stencil needed for a very small run? For surface mount parts it is, unless the quantity is so small that hand dispensing is chosen deliberately. The stencil is a one off cost and it is reused for every build of that design.

Why is a first article inspected before the whole run? Because the setup is what varies on a small order. Confirming the first board removes the risk of repeating a placement or printing error across the batch.

Can parts be sourced by the customer? Yes, and many teams do that for long lead items. The purchased parts still have to be inspected against the bill of materials before they are placed.