SMT Red Glue Stencil Aperture and Process Control

Red glue is the adhesive used to hold surface mount parts on the underside of a board that will later pass over a wave solder bath. It is applied through a stencil or dispensed as a dot, then cured so that the components stay in place while the top side is soldered. The aperture design of that stencil decides whether the process works or produces missing parts.

Why Adhesive Is Used at All

On a double sided assembly, the second side is soldered by passing the board over a wave, which means the components on the first side are held only by their own joints. Parts that are small or heavy enough to be at risk are bonded with adhesive before the wave, so that the solder wave cannot wash them off.

Adhesive is also used where a component has no solderable termination on the underside, or where a mixed assembly places through hole parts and surface mount parts on the same side. In each case the requirement is the same: enough adhesive to hold the part through the thermal and mechanical load of the wave, and not so much that it interferes with the joint.

Stencil Aperture Design

Apertures for adhesive are smaller than the pad and are placed between the pads rather than on them. The dot has to sit on solder mask or on bare laminate, not on a solderable surface, and it must not bleed into the pad area where it would contaminate the joint.

Aperture diameter is typically 0.3 to 0.6 mm larger than the dispensing needle equivalent, and the deposit height should place the adhesive in contact with the component body after placement. Two or three small dots are usually better than one large dot, because they allow the part to settle evenly instead of rocking on a single blob. Dot placement relative to the component centre is checked on the first article by placing a part and inspecting the squeeze out.

Stencil Manufacture and Wall Quality

Adhesive stencils are usually laser cut from stainless steel. Chemical etching was the original method, but its dimensional consistency depends on operator control, while laser cutting works directly from data and holds the aperture shape far more closely. The laser process also produces a natural taper in the wall, which helps the adhesive release rather than clinging to the sides.

Release can be improved further by electropolishing or nickel plating the walls to reduce friction. Stencil thickness for adhesive work is usually 0.12 to 0.18 mm, thicker than a paste stencil, because the deposit has to build height rather than spread across a pad. The mesh or foil must also be tensioned properly in its frame: a distance of 50 to 75 mm from the aperture area to the frame is a normal guide, so that the foil is held past its elastic limit and behaves consistently across the print.

Printing Versus Dispensing

Printing applies all the dots in one stroke, which is fast and consistent and suits high volume. It does require a stencil for each product and a printer setup, and the accuracy of the dot position depends on the same registration that a paste print depends on.

Dot dispensing places adhesive one dot at a time with a needle or a jet valve. It needs no tooling and adapts easily to a new design, but on a board with many dots it becomes the bottleneck of the line, adding time for every additional part. The choice is usually governed by volume and by how many dots the design needs. Where a board has few components, dispensing is simpler; where it has hundreds, printing wins.

Material Handling and Storage

Adhesive is more sensitive to storage than solder paste. It cures slowly at room temperature, and moisture absorbed from the air changes its viscosity and its behaviour during cure. A board that has been printed with damp adhesive may lose parts even though the print looked correct.

Keep the material sealed and refrigerated, allow it to reach room temperature before opening, and observe the working life after it has been opened. Printing rooms are usually held at a controlled temperature and humidity for this reason, and the same discipline that applies to paste and process documentation applies here.

Curing and Its Effect on Yield

Cure takes place in an oven, usually before the wave solder step, and the profile has to reach the temperature at which the adhesive develops its strength without overheating the components. Under-cured adhesive holds weakly and fails in the wave; over-curing makes it brittle and difficult to rework.

Because the components are placed before curing, placement accuracy and deposit height interact. A tall deposit lifts the part, a short one leaves it floating until the wave pushes it sideways. Checking the settled height on a sample after placement is a simple way to catch a drifting process.

Defects and Their Causes

Missing parts after the wave are the classic failure. They usually come from too little adhesive, an under-cured deposit or adhesive that has absorbed moisture. Cylindrical and glass bodied components are the most difficult to hold, because the contact area between adhesive and body is small and the surface is smooth.

Adhesive on a pad is the opposite problem. It prevents the formation of a solder joint, and it is often only discovered after the wave, when the joint fails inspection or fails in the field. Careful aperture placement relative to the pads prevents it, and the same reasoning about placement accuracy is applied to component shift after reflow.

Comparing Adhesive With Selective Soldering

Adhesive is the traditional answer for holding parts through a wave, but it is not the only one. Selective soldering with a pallet, or a process that solders the second side before placing components on the first, can avoid the need for adhesive entirely. Where a design has only a few parts at risk, a bespoke pallet or a rework step may be cheaper than introducing an adhesive process.

The choice should be made with the assembly house at the design stage, because it affects the stencil set, the process sequence and the tolerance stack. Decisions of that kind are part of the wider picture described in the review of PCB design quality characteristics.

Whichever method is used, the deposit has to be verified before production. Weighing a printed panel gives the average deposit, and measuring the dot height with an optical gauge confirms that the adhesive will contact the component body after placement. That measurement takes minutes and prevents an entire batch of missing parts.

FAQ

How much adhesive should be printed? Enough to hold the part through the wave and no more. Two or three dots that touch the component body after placement are typical, and the settled height is verified on the first article.

Can adhesive and solder paste be printed with the same stencil? Not normally. The two materials need different thicknesses and different aperture geometry, and a combined stencil compromises both processes.

Why do parts fall off after wave soldering? The usual causes are insufficient adhesive, an incomplete cure or moisture absorbed during storage. Glass and cylindrical bodies are the most demanding, because their contact area with the adhesive is small.