Second Side Reflow and Support Fixture Design Guide

On a double sided assembly the second reflow pass is the one that causes trouble, because the components placed on the first side are now on the underside and held only by the surface tension of their own solder joints. Second side reflow requires a support fixture, a profile that protects the already soldered joints and a layout that keeps heavy parts on the first side wherever possible. The second pass is therefore planned at the design stage, and the side on which each component is placed is a process decision rather than a layout preference.

What the Second Side Pass Involves

The board is flipped after the first reflow, paste is printed on the second side and the remaining components are placed. The assembly then passes through the oven again, which means every joint made in the first pass is reflowed a second time.

That second exposure has two consequences. The joints on the first side must survive remelting without losing their position, and the intermetallic layers at those joints grow a little more with each exposure, which is a reliability consideration rather than only a process one. Where the product has a strict thermal budget, the number of reflow exposures should be counted and recorded during the process design.

The Gravity Problem

When the board is flipped, the components on the underside are held only by the self alignment force of the molten solder. A light chip component is held securely by surface tension, while a heavy inductor or a large connector may fall off entirely.

The ratio between the component mass and the pad area is the practical guide. Beyond a certain mass, the joint cannot hold the part during the second pass regardless of how good the profile is, and the part has to be moved to the first side or bonded. Where the component cannot be moved, an adhesive or a fixture is required, and the choice should be made before the stencil is ordered.

Support Fixtures and Their Design

A support fixture holds the board flat and supports the components on the underside, so they cannot drop when the solder melts. The fixture may be a machined carrier with pockets, a magnetic pallet with pins, or a set of adjustable supports under a standard frame.

The material has to survive the oven temperature and it must not mark or contaminate the board. Fixtures are usually made from a high temperature polymer or a composite, and they are designed so that they can be handled by the conveyor without catching. The fixture also has to allow the board to be loaded and unloaded quickly, because handling time is a real cost on a high volume line.

Adhesive and Paste Selection

Where a fixture is impractical, an adhesive dot can hold the component through the second pass. The adhesive has to cure at a temperature low enough not to disturb the first side joints, and to hold well enough to survive the trip through the oven. An adhesive that is applied by dispenser is more repeatable than one applied by hand, and the dot size has to be verified on a sample.

The paste used on the second side also matters. A paste with good tack holds the component in place between placement and reflow, which reduces the demand on the fixture and the adhesive. Our surface finish guide describes how the finish affects the paste behaviour. Tack is the property that matters most here, and it should be part of the paste selection criteria for a double sided product.

Profile Adjustments

The second side profile is usually run with a slightly lower peak or a shorter time above liquidus, because the board already carries thermally sensitive components. The reduction has to be small enough that the second side paste still forms a proper joint. A profile that is reduced too far produces a joint that looks formed but has not fully wetted the pad.

The thermal mass also changes between the passes, because the first side components add mass and shield part of the board from the airflow. Measuring the profile on both sides of a real assembly is the only way to know what each side experiences. A thermocouple attached to the second side and another on the first side gives the two thermal histories in one run.

Warpage and Sag

A thin board that carries a heavy component can sag when it is hot, and the sag changes the position of the components on both sides. A support fixture that contacts the board at several points rather than only at the edges keeps it flatter.

Warpage that develops during the first pass makes the second pass more difficult, because the board no longer sits flat on the printer or in the fixture. Where the board is thin and the assembly is dense, the fixture may have to provide support across the whole area rather than at a few points. Controlling the flatness at the first pass is therefore part of the second side problem.

Component Retention and Weight Limits

Component retention can be answered with a simple calculation: compare the mass of the component with the surface tension that the joints can provide, and apply a safety factor. Components beyond the limit need a fixture, an adhesive or a move to the first side. The calculation is approximate, but it is far better than deciding by observation after the first production run.

The calculation is worth doing at the design stage rather than at the line, because moving a component between the two sides is free at that point and expensive later. Our solder defects guide describes the defects that appear when the retention is inadequate. A disturbance that occurs at the moment of melting is invisible afterwards, which is why the retention margin should be generous.

Defects and Detection

The defects associated with the second side are missing components, tombstoning on the second side, disturbed joints on the first side and components that have shifted but not fallen. The last of those is the most dangerous because it passes a visual inspection. A shifted component may still be electrically connected and may still fail a mechanical test in the field.

X-ray inspection and a functional test are the practical ways to find a partially disturbed joint. Our quality documentation describes how these conditions are classified at gopcb.

Process Control Points

The controls are the side assignment of each component, the fixture design and its condition, the use of adhesive where required, the measured profile on both sides and the inspection plan for the second pass.

Each of those is decided at the process design stage rather than during production, and each of them should be recorded with the assembly. Our production flow guide places the second reflow pass in the wider sequence of the assembly line. Process discipline on the second pass is what keeps a double sided product as reliable as a single sided one.

FAQ

Why do components fall off during the second reflow pass? Because they are held only by surface tension once the solder on the first side melts. A heavy component has to be moved to the first side, bonded or supported by a fixture.

Does the second pass damage the first side joints? It remelts them, so the position must be held by surface tension. The extra thermal exposure also thickens the intermetallic layer slightly, which is a reliability consideration.

Can the second side run at a lower peak temperature? Often it can, and it usually should. The reduction has to be small enough that the second side paste still wets and forms a proper fillet.