Double Sided Reflow Support and Process Control

A double sided assembly is reflowed twice, and the second pass runs with components already attached to the underside. Those components are held only by their solder joints, which are molten again during the second pass, or by an adhesive that has to be dispensed before the first pass. The second reflow is therefore a mechanical as well as a thermal operation, and the failures it produces are different from those of the first.

What the Second Reflow Has to Achieve

The second pass has to form good joints on the newly placed side while not disturbing the components on the other. The underside parts experience the full profile again, so their joints are remelted and their bodies are heated a second time. The design and the process both have to accommodate that.

The thermal exposure is cumulative. Each component sees two profiles, so the intermetallic layers grow more and the thermal load on a sensitive part is doubled. Where a product is reworked as well, the accumulated time at temperature becomes a reliability consideration rather than only a process one.

Holding the Underside Components

Small chip components are usually held by the surface tension of their own joints, which is strong enough for a light part with two terminations. Heavier components, parts with leads and parts with a high centre of gravity are not reliably held and need an adhesive. The adhesive is dispensed before the first reflow and cured either during that cycle or in a separate step.

The adhesive selection has to survive both reflows. A material that softens at the first peak will not hold the part during the second, and one that cures only at a high temperature may not cure sufficiently during the first pass. The cure behaviour and the thermal limits of the material are therefore part of the design decision rather than a detail.

Support Tooling for the Second Pass

The board has components on both sides, so it cannot be supported on a flat surface. Support has to be provided at points that do not touch a component, which usually means a custom fixture with pins placed to avoid the populated areas. A fixture that touches a component on the underside applies a load to a molten joint, which is a reliable way to produce a defect.

The support also has to hold the board flat through a second thermal cycle, because the board has already been through one and may have taken a set. Where the board is large and thin, the sag during the second pass can exceed what the first pass produced. The general support considerations are described in PCB warpage control.

Profile Design for the Second Pass

The second profile should be as gentle as the alloy allows, because the underside components are being reheated. Where a low temperature alloy is used for the second side, the profile can be run lower, which reduces the exposure of the first side. Where the same alloy is used for both, the profile has to be the same, and the underside joints are fully remelted.

The cooling rate deserves attention on the second pass. A fast cooling applies thermal stress to joints that are being cooled while the board is supported at a few points, and the stress is concentrated at the supported areas. A slower, controlled cooling reduces the stress and gives the underside joints time to solidify without disturbance. The profile development follows the same method as any other reflow, described in reflow oven profile verification.

Defects Specific to the Second Reflow

Components falling from the underside are the most visible failure. They occur when the adhesive is inadequate, when the part is too heavy for the joint tension or when the profile is too hot for too long. The first two are design issues and the third is a process issue, and the defect record should distinguish them.

A second category is damage to the underside joints without a fall. A joint that is remelted and then disturbed by support contact, vibration or a fast cooling can crack or become porous, and the damage is not visible from the top side. Where a unit fails after assembly, the underside joints adjacent to support points should be inspected.

Design Rules for Double Sided Assembly

The layout should place heavy components on the side that is reflowed last, so they are not remelted, and should use adhesive on any part that cannot be held by surface tension alone. The support points should be reserved during layout so that the fixture has somewhere to sit, and the adhesive keep out areas should be defined.

The gopcb assembly group reviews the double sided plan with the layout before release, because the decisions that make the second pass reliable are made at design time. Where a product cannot avoid a heavy part on the first side, the adhesive and the support plan are validated on a first article that includes a thermal profile measurement on the underside component.

Acceptance and Its Evidence

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. Handling between operations is part of the process, and the damage it causes is often attributed to the operation that preceded it.

A change that is not recorded is a change that cannot be explained when the result moves, which is why the record is part of the process. 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.

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

FAQ

Do all bottom side components need adhesive? No. Small chip parts are usually held by their own joints. Heavier parts, leaded parts and parts with a high centre of gravity need adhesive.

Can the second profile be the same as the first? It can, but it exposes the first side to another full cycle. A gentler second profile reduces the exposure where the alloy allows it.

What causes a component to fall during the second reflow? Inadequate adhesive, excessive thermal mass for the joint tension, or a profile that keeps the alloy molten for too long.