Package on Package Assembly: Warpage and Flux Dipping

Package on package assembly stacks one component on another, so the joints that matter are hidden between two bodies that both change shape as they heat. The dominant problem is warpage, and the standard countermeasure is flux dipping, which replaces the printed deposit with a thin film of flux that the top package picks up before placement.

Why PoP Is a Different Assembly Problem

In a normal assembly the joint is between a package and the board, and the board changes shape far less than the package at reflow temperature. In a PoP stack the top package is aligned to the bottom package, and both are thin, both warp with temperature, and their warpage may be in opposite directions at the moment the solder melts.

The result is a joint that depends on the relative shape of two bodies rather than on the flatness of one surface. That is why PoP yield is sensitive to things that do not matter elsewhere: the soak that equalizes the two packages in temperature, the placement force that holds them together, and the specific ball pitch of the top package.

Warpage and Its Temperature Dependence

A package is flat at room temperature and warps as it heats, because the mould compound, the substrate and the die expand at different rates. The shape at 250 degrees Celsius can differ from the room-temperature shape by 100 microns or more, and the direction can change as the stack passes through the melting range.

The countermeasure is a profile that keeps both packages at a similar temperature through the critical window. A fast rise heats the top package before the bottom one has caught up, and the mismatch in timing is what opens joints at the perimeter of the array. Equalizing during the soak is more effective than raising the peak.

Flux Dipping Instead of Printing

The top package usually has a fine ball pitch that is difficult to print, and the space above the bottom package is inaccessible once it has been placed. The solution is to dip the top package balls into a thin flux film held in a shallow tray, which transfers a controlled amount of flux to each ball.

Film thickness is the key parameter, typically 30 to 60 microns, and it is set by the tray depth and the doctor blade rather than by the dipping time. Too little flux and the joint does not wet; too much and the flux boils during reflow, producing voids and solder balls around the stack. The film must also be refreshed before it dries or becomes contaminated.

Placement Force and Stack Settling

The top package is placed onto the bottom one, and the force applied has to overcome the flux film and seat the balls without crushing them. Typical placement forces are in the range of a few newtons, and the machine must be able to hold the stack in position as the head withdraws, because the tack of the flux is the only thing keeping the package aligned.

Settling is a real risk. A package that slides before reflow produces a joint that looks complete on one side and open on the other, which is one of the reasons PoP inspection is done with X-ray from more than one angle.

Reflow Profile for a Stacked Package

The profile has to bring both packages to the same temperature at the same time, and that means a longer soak than a normal assembly, often 90 to 120 seconds in the activation range. The peak is set by the paste and is typically 240 to 245 degrees Celsius, with the time above liquidus kept long enough for the bottom joint to form fully.

Nitrogen is often used because the thin flux film has little capacity to protect the surfaces it must wet, and oxygen in the oven consumes that capacity before the joint forms. The effect is largest on the top package balls, which carry the least flux.

Voiding and Ball Collapse in the Top Package

Voids form in the top joints from flux volatiles that cannot escape between two closely spaced bodies, and the film thickness is the main control. A thick film produces more volatiles than the joint can vent, so reducing the film and lengthening the soak address the cause rather than the symptom.

Collapse is the sign that reflow completed. In a PoP stack the collapse of the top balls also closes the gap between the packages, so a stack that has not fully reflowed is visibly taller than one that has. That height difference is a useful production check, because it can be measured without X-ray.

Inspection Limits and What X-ray Shows

X-ray in the top-down view can show bridging, missing balls and gross displacement, but the interesting joint is the interface between the ball and the pad, which is oriented so that a top-down beam sees it edge-on. An oblique view is what reveals whether the joint formed and whether the void is at the interface.

A section through the stack is the definitive check and is used at first article or when the process changes. It shows the collapse, the interface condition and the void distribution, and it is the only method that shows the metallurgy of the joint between the two packages.

Rework and Its Practical Limits

Reworking a PoP stack means removing the top package without disturbing the bottom one, which is difficult because both are heated together and the bottom package has its own joints to the board. The usual practice is to replace the top package with a dedicated tool and a controlled profile, and to treat the operation as an exception rather than a routine repair.

Repeated rework accumulates thermal cycles in both packages, so a limit is usually set and a stack that exceeds it is scrapped. As with any fine-pitch rework, the decision should be based on the number of cycles already applied rather than on the appearance of the site.

Process Control for a PoP Line

A PoP line is controlled by a small set of parameters that must be held together: dip film thickness, placement force, profile and the flux chemistry. Each of them interacts with the others, so changing one without re-qualifying the stack is how a working process becomes marginal.

The warpage behaviour of the specific packages used is part of the qualification, and it should be confirmed with the supplier data and, where the stack is critical, with a measurement at temperature. Package warpage specifications vary between suppliers even for the same outline, and that variation shows up directly in the joint yield.

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

FAQ

Why is flux dipping used for package on package? The top package pitch is too fine to print reliably and the space above the bottom package is inaccessible, so the balls pick up a controlled flux film by dipping instead.

What dip film thickness should be used? Typically 30 to 60 microns, set by the tray depth and doctor blade. Too thick a film causes voids and solder balls; too thin leaves joints unwetted.

How should a PoP joint be inspected? X-ray in an oblique view for the interface, plus a section at first article. A top-down view alone sees the critical interface edge-on and can miss an open joint.