Warpage and Reflow: Where the Stress in a Board Comes From

Warpage is a change in the shape of a board, and it appears when the layers that make up the board expand or contract by different amounts. The movement is small in absolute terms and it is large relative to the tolerances of assembly, because a stencil sits on the surface and a nozzle has to reach a joint. A board that meets specification at fabrication can still be outside it after the thermal excursion of assembly.

The reflow profile takes part in the problem twice: it determines the temperature difference between the layers during the excursion, and it determines how much of the resulting strain is locked into the board when it cools. Both effects are predictable enough to be controlled. The profile is therefore a fabrication variable as well as an assembly one.

What Warpage Is

Warpage is described by two measurements. Bow is a curvature of the board as a whole, and twist is a rotation of one corner relative to the others. Both are expressed as a maximum deviation from a reference plane. A panel that bows by a millimetre over three hundred millimetres is common and serious at fine pitch.

The measurement is only meaningful with a stated method, because a board resting on a surface under its own weight takes a different shape from one supported at three points. The method and the reference plane belong with any figure quoted as flatness. A figure without the method makes comparison between suppliers meaningless.

Thermal Expansion Mismatch

Copper expands about five times as much as the laminate for the same rise in temperature, so the two layers of a board disagree about how much they want to grow. The mismatch is a material property and it cannot be removed, only balanced. It is also why a board with no copper on one side and a plane on the other is a design that will bow.

Where copper appears on one side and not the other, the mismatch produces a bending moment. The direction is set by which side carries more copper, and the magnitude is set by the temperature rise and by the thickness of the two materials. The moment is proportional to the rise, so a board that sees a higher peak moves more.

Stack-Up Symmetry

Symmetry in the stack-up is the primary defence against warpage. If the copper distribution and the prepreg thickness above the centre line mirror those below it, the bending moments cancel and the board stays flat. Symmetry is a design decision and it is easier to achieve before the artwork is released than after.

The centre line matters rather than the middle layer, because what is being balanced is the product of thickness and expansion on each side. Thickness and symmetry are controlled together, and the rules are described in the notes on layup symmetry control. A stack that is symmetric in layer count and asymmetric in copper weight is a common mistake.

The Reflow Profile as a Cause

The reflow profile sets the peak temperature and the time above it, and both feed into the amount of movement. A higher peak expands the layers more, and a longer excursion allows more stress relaxation in the laminate. The profile is one of the few variables the assembly engineer can use to reduce a fabrication problem.

The profile also sets the temperature gradient across the board. A board that is hot at one end and cool at the other during the ramp is a board whose layers are at different temperatures at the same moment, which is a different loading case from a uniform rise. A slower ramp reduces that gradient and costs cycle time the product may not have.

Cooling Rate and Locked-In Stress

Cooling is where stress becomes permanent. Above the glass transition temperature the laminate is soft and the layers can move against each other, and below it the shape that was reached is frozen in. The glass transition temperature of the laminate is the number that governs this behaviour.

The cooling rate therefore determines the temperature at which the shape becomes fixed, and how far above the glass transition that happens. A fast cool locks the shape in at a higher temperature, and the board then distorts further as it continues to cool after leaving the oven. The rate is recorded with the profile, because it is part of the same schedule.

Copper Balance and Plane Distribution

A ground plane on one side and a signal layer on the other is a balanced stack in terms of copper area and an unbalanced one in terms of copper distribution. Local areas with no copper behave differently from areas that are dense with it. Balancing copper area on each side is a starting point and not sufficient on its own.

The result is a board that is flat overall and locally dished. The effect appears when the board is measured across the panel rather than along a single axis, and it grows as the panel gets larger. The remedy is usually a copper thieving pattern rather than a change to the stack.

Effect on Assembly

Assembly is where the warpage is noticed. A gap between the stencil and the board lets paste spread sideways, a raised area presses the stencil into the board, and either error shows as a volume variation across the panel. The error is larger on a thin board, where the same absolute bow is a larger fraction of the thickness.

The conveyor also has an effect. A board that does not sit flat on the conveyor supports can shift as it crosses a gap, and the rail clearance that accommodates that is described in the notes on conveyor edge clearance.

Measuring Flatness

Flatness is measured before and after reflow, and the difference is more useful than either reading alone. The before measurement is a fabrication result, and the after measurement is what assembly actually sees. The measurement is quick when a flat reference and a height gauge are available.

The measurement is taken at a defined temperature as well as at room temperature where the product is sensitive. A board that passes at twenty degrees and fails at two hundred is a board whose reference plane was measured at the wrong condition.

Process Control and Records

Control starts at the stack-up, continues through the lamination cycle and ends with the reflow profile. Each of the three can move the result, and the lamination side is described in the notes on lamination cycle parameters. The reflow profile is the last of the three and the easiest to adjust, which is why it is often blamed first.

The record holds the stack-up revision, the lamination cycle, the profile and the flatness results. With those fields a change in flatness can be attributed to the board rather than the oven, or the other way round, instead of being argued about.

FAQ

Can warpage be fixed at assembly? It can be supported by pins or by a vacuum table, and the underlying board remains warped. The support masks the cause rather than removing it.

Does a slower cool reduce warpage? It reduces the stress locked in at the glass transition and it can also change the grain of the laminate. The effect is measured rather than assumed.

Is a symmetric stack-up always flat? Not always. Local copper density differences persist even when the layers balance overall.