SMT Adhesive Dispensing and Cure for Wave Soldering

Adhesive holds surface mount components in place while the board passes over a wave of molten solder, and it does the job only if the dots are the right size, in the right place and cured to the right degree. A dot that is too small releases the part into the solder pot; a dot that is too large lifts the component off its pads. Both faults are expensive, and both are prevented by controlling the process rather than by inspecting the result.

Why Adhesive Is Used on a Wave Soldered Board

Wave soldering applies molten solder to the underside of a board, and anything attached there will float off unless it is held. Adhesive, dispensed as a small dot under the body of the component, keeps the part in contact with the board until the solder has solidified around its leads.

The alternative is to place the parts after the wave, or to glue the whole assembly, and neither suits a mixed technology board with fine pitch devices on the top surface. Adhesive is therefore the standard solution for the bottom side of a double sided assembly.

Adhesive Types and Their Behaviour

Most adhesives used in this way are epoxy or acrylic based, and they are supplied as a paste that cures with heat. The important properties are the uncured viscosity and its change with time, the curing profile, the strength after cure and the behaviour of the cured material at solder temperature.

An adhesive that is too fluid spreads under its own weight and reaches the pads, while one that is too stiff will not wet the board and forms a dot that sits on the surface and releases the part. The viscosity also changes as the material stands in the dispenser, so the working life of a syringe is part of the specification.

Dispensing Methods: Pin Transfer, Stencil and Jet

Pin transfer uses a plate with pins that pick up adhesive from a bath and deposit it on the board, and it is fast for a product with a stable pattern. The dot size depends on the pin diameter and on the depth of the dip, so it is set by hardware rather than by a program.

A stencil can be used to print adhesive in the same way as solder paste, which gives a very repeatable volume but requires a dedicated stencil and a print cycle. Jet dispensing uses a valve to fire a measured dot and is the most flexible, since the pattern can be changed in software for every product.

Dot Size and Placement Accuracy

The dot has to be large enough to hold the component through the wave and small enough that the part still sits on its pads. The window between those two limits is narrower for a small chip component than for a large one, which is why fine pitch parts are the hardest to hold.

Placement accuracy is the other half of the requirement. A dot placed off centre exerts a force that can rotate the component, and a dot that touches a pad will contaminate the joint and prevent the solder from wetting the termination.

Cure Profiles and Their Consequences

The cure profile is chosen to develop the strength the adhesive needs without damaging the components or the board. Most materials cure in an oven at a moderate temperature for a period of minutes, and the profile is written from the adhesive data sheet with a margin for the assembly mass.

An undercured dot is soft, and it can move during the wave, which shows up as a shifted or lifted component after soldering. An overcured dot becomes brittle and can crack when the board flexes during depanelling or handling, leaving the part held by its leads alone.

Adhesive Height and Component Clearance

The height of the dot after placement decides whether the component sits flat. A component such as a small outline transistor has a standoff under its body, and the dot has to fit within that space without pushing the leads away from their pads.

Where the clearance is tight, the dot is placed at the edge of the body rather than in the middle, or the volume is reduced. The dimensions involved come from the component drawing and from component tolerance limits, and the process should be checked against them rather than adjusted until the part looks right.

Defects: Missing Dots, Squeeze Out and Popcorn

A missing dot shows up as a component in the solder pot or a part standing at an angle after the wave. Squeeze out occurs when the dot is too large or the placement force too high, and it appears as adhesive on the pad, which then refuses to wet.

Voids and bubbles in the cured adhesive are usually a sign of moisture or of a cure that began too quickly, and they weaken the bond without being visible from the outside. The wider family of these faults is described in the guide to solder defects and board failures.

Compatibility with Cleaning and Coating

The adhesive has to survive whatever the board will see next. Aqueous cleaning can soften an adhesive that is not fully cured, and a solvent based cleaner can attack it outright, so the cleaning process has to be checked against the material before the product is released.

Conformal coating is the other consideration, because a coating that does not adhere to the adhesive will leave a gap around the dot. Where the coating matters for reliability, the combination of adhesive and coating should be qualified together rather than separately.

Process Control and Verification

Control starts with the material: storage temperature, working life after removal from the refrigerator and a record of the batch in use. It continues with the dispenser, where the nozzle diameter, the pressure, the time and the height above the board all affect the volume delivered.

Verification uses a glass slide or a piece of bare laminate to check the dot pattern at the start of a run, followed by a visual check on the first boards after cure. The pattern and the volume should be part of the setup record so that a change can be detected, and the general approach to control on the line is described in the guide to the production process flow.

FAQ

How large should an adhesive dot be? Large enough to hold the component through the wave and small enough to keep it flat on its pads, which is usually a figure between a half and two thirds of the pad pitch for a small chip part. The exact size is qualified on the product.

Can adhesive be omitted for a light component? It can for a part with leads that will be soldered and that is heavy enough to stay in place, but the decision is made from experience with the wave and with the board, not by assumption.

Why does a component fall off in the wave? Usually because the dot was missing, too small or undercured, or because the wave height and temperature were higher than usual. Checking the dot pattern first is the quickest route to the cause.