New Technique for Area Array Package Profiling Speeds Rework Process Development

A new method for thermal profiling of array packages does not require bonding, soldering or taping thermocouples to the PC board.

By Roger Saunders, Saunders Technology Inc., Hollis, New Hampshire

A recently-developed technique for thermal profiling BGA and area array chip-scale packages can be performed without employing adhesives for thermocouple attachment and also eliminates the need for bonding, soldering or taping thermocouples to the PC board.

The technique was developed due to the fundamental differences between the most common large surface mount packages, specifically BGAs and QFPs. Process development for attachment, removal or replacement of BGAs is more complex, typically, than for a QFP.

For example, while QFP rework usually involves focusing or directing heat on the peripheral lead attach areas, BGA processing requires concurrent heating and reflow of all underside solder spheres. This is usually achieved by heating the entire BGA or CSP package. Therefore, process development for BGA profiling must involve recording the temperature of areas other than simply the outer lead attach area, as is often done with traditional peripherally-leaded SMT packages.

Since the reliable reflow or rework of area array packages requires close control of the thermal profile, the challenge has been to establish the correct profile at a corner and near the center of the BGA. These locations typically represent the temperature extremes during rework. With smaller plastic BGAs, the center is usually slower to heat than the corners due to the relatively poor thermal conductivity of plastics. However, with large ceramic BGAs, which have much better thermal conductivity, the center is often quicker to heat, peaking as much as 25°C above a corner.

Accurate Profiles

For accurate BGA profiles, a thermocouple must be placed in firm contact with a pad or ball to assure a good thermal reading. One technique is to slide a fine-gauge thermocouple under the BGA package. There are, however, several problems with this technique.

For example, many BGAs do not offer a sufficient clearance beneath them to slide in a thermocouple. Another problem is the uncertainty of the connection to the thermocouple's junction. It is very difficult to thread a delicate wire thermocouple manually through the narrow corridor between rows of solder spheres and be confident that the junction will make good contact with a ball (sphere) near the center of the BGA.

Often, in the process of poking the thermocouple in and trying to engage a sphere, thermocouple wires near the junction will become bent. This will usually cause the wires to touch and form a new thermocouple junction that is clearly not in contact with the sphere, as desired.

If that happens, or if the thermocouple junction does not make good thermal contact with a sphere, the thermocouple will indicate the temperature of the hot air from the rework nozzle as it passes under the BGA. The air temperature will show a much quicker thermal response than that of the solder spheres, simply because it takes time for the hot air to heat the spheres to melt temperature. The result is a thermal profile with a higher rate of rise and higher peak temperature than would be obtained if the thermocouple junction were in good contact with the sphere.

The most reliable way to profile a solder joint on a BGA package is to drill through the backside of the PC board beneath a ball and attach a thermocouple to the bottom side of its pad.

There are several ways to mount a thermocouple in a hole under a BGA pad. An elaborate but effective way is to remove the BGA, drill through the board at one of the center pads, insert a fine gauge wire thermocouple reference point, run it through the process and compare the resulting profile with the original reference profile. This technique is equally effective for establishing and monitoring a reflow oven process.

This procedure is designed around Saunders Technology's TEMPROBE, a simple temperature-sensing instrument with a subminiature thermocouple tip that can be placed anywhere on a circuit board, even directly on component legs, solder paste or the components themselves. The Saunders method also begins with drilling or end milling a 1/32" diameter hole through the backside of the PC board to a pad beneath a ball.

The TEMPROBE™ speeds up rework and eliminates the additional step of removing bonding material from the PC board.

Then a TEMPROBE thermocouple, as shown in the figure, is clipped to the edge of the PC board with the probe on the bottom side and the thermocouple tip placed in the hole and preloaded against the pad to maintain secure thermal contact. A second TEMPROBE is clipped to the PC board with its probe on the top side and its thermocouple tip placed on a pad near the BGA. The TEMPROBE eliminates the need for bonding, soldering or taping thermocouples to the PC board.

The instrumented PC board is then profiled and heating parameters are adjusted until the correct profile is recorded by the probe under the BGA package, with a simultaneous reference established by the topside TEMPROBE.

Conclusion

A new technique for assuring accurate thermal profiling of BGAs during reflow or rework has been developed. The method should help to speed process development not only for BGAs but for next generation packages such as CSPs.

Mr. Saunders is president of Saunders Technology, which he founded after 30 years service at what is now a Lockheed subsidiary. He is the designer of the TEMPROBE. Contact him at service@saunderstech.com or by phone at 603.881.3330.