Double Sided Assembly: Design Rules and Process Limits

A double sided assembly is reflowed twice, and the second pass is the one that causes trouble. The parts placed in the first pass are already on the board, and they have to survive a second trip through the oven without falling off or degrading.

The profile for the second pass is therefore not the same as the first, and the decision about which side to run first follows from the thermal mass and the component sensitivity.

Which Side Goes First

The side with the heavier components and the larger thermal mass is usually run first, so that the second pass can use a lower peak. The side with the fine pitch parts benefits from the cleaner first pass.

The decision also depends on the process: a board with parts on both sides needs either an adhesive or a reflow that does not disturb the underside.

Holding the Underside Parts

A chip part on the underside is held by the surface tension of its joints once the second reflow begins, and by adhesive before that. The surface tension is often sufficient for a small part, while a heavy part needs adhesive.

The adhesive is applied before the first reflow and cured during it, which is why its cure temperature has to be below the alloy melting point. Its placement is described for component bonding.

Thermal Mass of the Second Pass

By the second pass the board carries copper and components that change its thermal behaviour, so it heats differently from the bare board the first profile was developed on. The profile should be measured with the first side populated.

The difference is largest on a board with a heavy component or a large plane, and it can shift the effective soak and the time above liquidus appreciably.

Component Limits

Every component has a maximum number of reflow cycles, and a double sided assembly consumes two. A part that is limited to one cycle requires a selective soldering or an adhesive attachment for the second side.

The limit should be read from the component data rather than assumed, because it varies widely between package types.

Moisture and the Second Bake

A board that has been through one reflow has been dried by it, so the moisture risk on the second pass is lower. The parts placed in the first pass have also been baked by the process.

Where a long interval passes between the two passes, the board can reabsorb moisture and the consideration returns. The interval should be controlled.

Warpage and Support

A populated board warps differently from a bare one, and the second pass has to support it accordingly. The support positions used for the first pass may not be appropriate.

The support should be reviewed when the second side is introduced, particularly where a large component creates a local stiffening effect.

Profile Development

The second profile is developed with the first side populated and with a representative panel loading. The measurement positions should include a joint on the underside, since that is where the risk is.

The result is compared with the paste specification and with the component limits, and the profile is released for that side only.

Yield Effects

Defects on the second pass are more expensive, because the board already carries value. The defects include parts falling off, tombstoning on the underside and head in pillow on the second side.

Where those defects appear, the profile, the adhesive and the support are the items to review in that order. This is the same reasoning applied in zone verification.

Records

The records should identify the profile for each side, the adhesive used and the support arrangement. A board that is built with two profiles and no record cannot be reproduced.

They belong with the process evidence described for manufacturing processes.

Process Control and Verification

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. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.

Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

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.

FAQ

Does every second side need adhesive? Only where a component is heavy enough that surface tension will not hold it during the second reflow.

Can the same profile be used for both sides? Sometimes, and it should be demonstrated with a measurement on the populated board rather than assumed.

How many reflow cycles can a part take? The supplier states a maximum, and it is often two, which is why a third pass requires a different method.

What is the most common second pass defect? Components displaced or lost from the underside, followed by joint defects in the parts placed second.