Surface Mount Adhesive for Double Sided Assembly

Adhesive is used on a double sided assembly to hold the bottom side components during the second reflow and through the wave. It is a small dot with a large influence on whether the parts stay in place.

Why Adhesive Is Used

A bottom side chip component held only by the surface tension of molten solder will fall off during the second reflow or in the wave. The adhesive holds it until the solder solidifies.

It also holds a component that would otherwise be moved by the wave or by the handling between the two processes. Our defect notes describe the failures that appear when it does not.

Dispensing Methods

The adhesive is applied by a dispenser with a needle or by a stencil print. Dispensing is more flexible and slower; printing applies many dots at once and needs a dedicated stencil.

A printed adhesive pattern gives a more consistent volume than a dispenser that is affected by the viscosity and the needle wear. Where the volume matters, the print is the better method.

Dot Placement and Size

The dot should be placed where it does not interfere with the pad or the solder joint. A dot under the termination reduces the solderable area; a dot that bridges two pads can hold the part above the board.

The size should be enough to hold the component through the process and small enough not to lift it. The window is narrow on a small chip component, which is why the volume is usually specified rather than the pressure.

Cure

An adhesive that is only partly cured holds the part and it also outgasses during the reflow. The outgassing produces voids and it can move the component.

The cure should be completed before the second reflow, usually during the first reflow or in a dedicated step. The profile should be checked against the adhesive data rather than assumed to be covered by the solder profile. Our profile notes describe the measurement.

Adhesive and the Wave

In a wave process the adhesive also has to survive the wave temperature and the flux. An adhesive that softens at the wave temperature releases the component even though it survived the reflow.

The temperature rating should be checked against the highest temperature the adhesive will see, which is the wave rather than the reflow.

Removal and Rework

An adhesive that has cured is difficult to remove without damaging the pad. Rework of a bottom side part often means the adhesive has to be cut away and the site cleaned.

Where the rework is expected, the adhesive should be chosen for removability as well as for strength. Our repair notes describe the trade.

Verification

The verification is the dot volume and position measured on a sample, the component displacement after the second reflow, and the fall off rate in the wave.

The fall off rate is the number that matters, and it should be monitored rather than discovered at the end of the line. Our yield notes describe how the data is used.

Process Control and Verification

A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.

The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.

Verification and Records

The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting. Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record.

Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected.

FAQ

Is adhesive needed on every bottom side component? It is needed where the mass to pad area ratio is unfavourable. A large part with many joints may not need it.

Can adhesive replace a stencil? No. It is applied in addition to the paste, and the two patterns must not overlap.

What does gopcb provide for bottom side adhesive? We provide method selection between dispense and print, dot volume and position rules that keep clear of the pad and the joint, cure completed before the second reflow with profile verification, temperature rating checked against the wave, removability for expected rework, and fall off rate monitoring.