Package Warpage and Solder Ball Coplanarity Risk Control

A package that is perfectly flat at room temperature can be significantly warped at reflow temperature, and that is the moment that matters. Warpage changes the gap between the ball and the pad, which changes how the joint forms, and the result can be an open joint that passes every inspection after the fact. Package warpage is therefore a property of the whole assembly rather than of the component alone. This article explains why packages warp, how coplanarity is measured, and how to control the risk through design, profile and incoming checks.

Why Packages Warp

An area array package is a composite of materials with different expansion rates: the silicon die, the substrate, the mould compound and the solder balls. When the assembly is heated, each part expands by a different amount, and the mismatch has to be absorbed by the package, which bends.

The construction decides how much. A thicker substrate resists bending, a large die spreads the mismatch over a longer span, and the mould compound thickness changes the balance between the two sides. This is why package warpage is normally quoted with the body size and the die size attached, and why two packages of the same body size can behave very differently.

Warpage Over Temperature

Warpage is not a fixed quantity. It changes continuously through the reflow profile, and the shape can change with it: a package that is convex at room temperature can be concave at the peak, or the reverse. The condition that matters is the shape and the amplitude at the moment the solder is molten.

That is why a room temperature coplanarity figure is a poor predictor on its own. The measurement has to be made over temperature, or the behaviour has to be taken from the supplier data that was measured that way. Package warpage at the peak of the profile is the figure the process has to accommodate. Our BGA inspection notes describe how the resulting joints appear after the fact.

Coplanarity Measurement

Coplanarity is the deviation of the ball array from a reference plane, and it is measured optically or with a laser scanning system that maps the height of each ball. The figure is reported as the distance between the highest and the lowest ball, or as a deviation from the seating plane.

The measurement conditions matter as much as the value. Shadow moire and digital image correlation systems can measure the package at temperature, which is the only way to see the shape at the peak of the profile rather than at the bench. A supplier figure without the measurement conditions cannot be compared with anything.

The Effect on the Solder Joint

Where the package is warped towards the board, the ball is pressed into the paste and the joint forms with a thin, wide fillet. Where the package is warped away, the ball may not touch the paste at all before the solder melts, and the joint depends on the paste bridging the gap.

A gap that is too large produces an open joint or one that is connected by a thin bridge, which is mechanically weak. A gap that closes completely squeezes the paste out, which produces a narrow joint and can trap flux. Both failure modes come from the same warpage, which is why a localised open joint and a localised squeezed joint can appear on the same package.

Head in Pillow and Open Joints

Head in pillow is the condition where the ball and the paste both melt but do not coalesce, leaving a joint that looks connected in an X-ray image but has a boundary in the middle. It is associated with warpage that opens the gap at the critical moment, and with a paste that has oxidised on the surface. The package involved is usually a ball grid array, and the joint that fails to form is buried underneath it.

The defect is difficult to detect in production because the X-ray image can look normal, and it often fails only after a thermal cycle or a mechanical shock. The control therefore has to be preventive. Our solder defect notes group the failures that produce this kind of hidden joint.

Shadowing and Insufficient Paste

Warpage also changes how the paste is deposited, because a package that is warped before placement can shadow the stencil and prevent the paste from transferring to the pads in some areas. The result is a joint that is starved of solder even though the print looked acceptable.

Our land pattern notes describe how the pad and the aperture are designed for a given ball pitch, and the paste volume is the first thing to check when a starved joint appears in a localised area. A stencil that has been shadowed leaves a print that looks normal in the areas that were not affected.

Package and Board Warpage Together

The board also warps during reflow, and the two warpings add or subtract depending on their direction. A board that bows upwards under a package that bows downwards opens the gap between them, which is the worst case for an open joint.

The board warpage depends on the copper distribution, the stack up symmetry and the support in the oven. That is why a package that assembles reliably on one board can fail on another with the same profile. The comparison between the two boards usually points at the stack up or the support.

Mitigation: Stiffeners, Underfill, Profile

The profile is the first lever. A longer soak brings the package and the board to a more uniform temperature and reduces the differential, and a slower ramp reduces the gradient through the package. A peak at the low end of the paste window reduces the amount of warpage because the package does not reach the temperature at which it warps most.

A stiffener or a thicker substrate helps at the package level, and underfill helps after the joint has formed by transferring the load away from the solder. None of those changes the fact that the gap has to be small enough at the moment of melting. The profile is the only lever that applies to both the package and the board at the same time.

Incoming Control and Specification

The specification should state the coplanarity at temperature rather than only at room temperature, and it should state the measurement method. An incoming check on a sample confirms that the delivered parts match the data that the design was based on, which is the assumption the whole process window rests on.

At gopcb the package and the board are considered together in the assembly review, and the acceptance of the resulting joints is judged in our quality documentation so that a warpage related defect is classified consistently.

FAQ

Is room temperature coplanarity enough? It is not, because the package warps over temperature and the shape can change. The figure that matters is the one at the moment the solder is molten.

Can warpage be corrected by the reflow profile? It can be reduced, because a gentler profile lowers the temperature differential and therefore the bending. It cannot be eliminated, so the package and the board design still have to work together.

Does underfill prevent head in pillow? It does not, because the defect forms before the underfill is applied. Underfill improves the reliability of a joint that has formed correctly.