BGA Soldering and Rework: Controlling Temperature and the Profile

Ball grid array packages fail in a narrow temperature window that is easy to miss and hard to diagnose. The solder balls are underneath the package, so the joint cannot be inspected visually, and the difference between a good joint and a marginal one is a matter of a few tens of degrees in the profile. Controlling the BGA soldering temperature is therefore not a refinement, it is the process.

Why Preheat Decides the Outcome

Preheating the board before the package is heated serves two purposes. It reduces the thermal gradient across the assembly, which limits the differential expansion that causes a board or a package to warp, and it raises the starting temperature from which the reflow stage begins, so the profile does not have to deliver all of the energy in a short time.

The preheat temperature is not a fixed number. It has to be adjusted for the ambient temperature and for the thickness of the board, and for the distance between the board and the preheat element of the equipment. A thin board needs a higher preheat because it has less thermal mass and loses heat faster, and a low ambient temperature has the same effect. The figure that matters is the temperature the board actually reaches, not the number set on the controller.

Fixturing and the Thermal Mass

The board must be held so that it cannot move during the process. A board that shifts while the package is being placed will produce a misaligned joint even if the profile is correct, and a board that is free to bow will lift or drop a corner of the package. Clamping the board at both ends, tightly enough that it does not move under light finger pressure, is the standard practice.

The fixture also conducts heat away from the board, so it changes the profile. A metal fixture in contact with a large copper area is a thermal load in the same way that a ground plane is, and the profile has to be adjusted for it. This is one reason a profile that works on one product does not transfer to another.

<img src="https://www.gopcba.com/wp-content/uploads/2026/08/Turnkey-PCB-Assembly.jpg-1.webp" alt="BGA package on a PCB during a controlled reflow profile” />

Building a Profile in Stages

A workable reflow profile is built from a small number of stages, each with a rate of temperature rise, a target temperature and a dwell time at that temperature. A rise rate of around three degrees per second is a common starting point, and a hold at the end of a stage of around forty seconds gives the assembly time to equalise before the next step.

The stages are not simply a ramp to the peak. The soak stage gives the package and the board time to reach the same temperature, which is what prevents the package from warping relative to the board. The reflow stage then has to exceed the melting point of the alloy by enough to form a reliable joint, and the final stage must not overshoot the maximum temperature the package and the laminate can tolerate.

Choosing the Peak Temperature

The starting point is the melting point of the alloy: around 217 degrees Celsius for the common lead-free alloys and around 183 degrees for tin-lead. That is the temperature at which the solder melts, not the temperature at which the joint forms properly, because the ball is not fully liquid throughout the package at its theoretical melting point.

A practical target is a peak of roughly 235 degrees for lead-free and around 200 degrees for tin-lead at the joint, which allows the balls to become fully molten before cooling. The way to establish the real number is to measure: run the profile with a thermocouple between the package and the board, read the temperature at the point where the reflow stage completes, and adjust the setpoints by the difference. If the measured value is well below the target, raising the setpoints is the correction; the amount should be proportional to the deficit, and the upper element should be raised less than the lower one, because excessive top-side heating damages the package.

Staying Under the Maximum

Above the peak, the constraint changes from forming the joint to surviving the process. Packages have a maximum temperature and a maximum time above liquidus, and exceeding either produces damage that may not be visible immediately. The usual guidance is to keep the peak below about 245 degrees, and to reduce the setpoints if the measured maximum approaches that figure.

The measurement to make is the highest temperature reached during the final stage, not the setpoint of the final stage. A profile with a modest setpoint can still exceed the limit if the assembly has more thermal mass than expected or if the previous stage ended hotter than intended. Our BGA reballing notes cover the rework case, and the BGA inspection material explains how the result is verified.

Where BGA Soldering Problems Appear Later

A joint that forms but is not properly alloyed behaves correctly at first and fails after thermal cycling. The failure mode is a crack that begins at the interface between the ball and the pad and propagates across the joint, and it appears in the field as an intermittent connection that changes with temperature. Because the joint cannot be inspected optically, the defect is usually found by X-ray or by functional test, and by then the assumption is that the problem is thermal rather than process-related.

That is the practical reason to invest in profiling rather than in inspection. The profile that produces a reliable joint is documented, repeatable and verifiable, whereas the joint that is merely acceptable on the bench is not. The solder defect notes describe the faults that follow an inadequate profile, and the thermal management material covers the heat path that a package relies on afterwards.

Documenting the Profile

A profile that is established once and written down is an asset; a profile that lives in the head of the operator is a risk. The record should include the stage setpoints, the measured temperature at the joint, the fixture used and the board thickness. When the product is built again months later, or when a new operator runs the equipment, the recorded profile is what makes the result repeatable, and it also provides the baseline against which a change can be judged.

FAQ

Can a BGA joint be inspected after reflow? Not optically, because the balls are underneath the package. X-ray inspection is the standard method and it can detect bridging, voids and joints that are too large or too small. Where the package is not an area array device, or where the joints can be seen from the side, optical inspection is possible and useful, but for a true ball grid array the verification has to be radiographic or electrical.

Why does the profile have to be measured rather than set? Because the temperature that matters is at the joint, and the relationship between the controller setpoint and the joint temperature depends on the thermal mass of the assembly, the fixture and the equipment. Two boards with the same setpoints can reach different joint temperatures. Measuring with a thermocouple at the joint converts the settings into a result, which is what a process needs to be repeatable.

What does gopcb recommend for a BGA assembly? Decide the assembly and rework method before the package is selected, since the pitch and the package size determine whether the process can be controlled. We review the land pattern and the thermal relief around the pads, because a pad that conducts heat away too quickly changes the profile that the assembler has to use.