Saunders Technology, Inc. has developed a technique for thermal profiling BGAs for process development that does not require the use of adhesives for thermocouple attachment. Because there are fundamental differences between large surface mount packages such as QFPs and BGAs, process development for attachment or removal/replacement of BGAs is more complex than it might typically be for a QFP.

For example, whereas rework of QFPs 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 package. Therefore, process development for BGA profiling must involve recording the temperature of other areas than simply the outer lead attach area, as is often done with traditional SMT packages. Since reliable reflow or rework of BGAs requires close control of the thermal profile, the challenge is to establish the correct profile near the center of the BGA, where heating is slowest due to the surrounding thermal mass.

To obtain an accurate profile, a thermocouple must be placed on a pad near the center of the BGA. One technique is to slide a fine-gauge thermocouple under the BGA. There are several problems with this technique, beginning with the fact that many BGAs do not have sufficient clearance beneath them to slide in a thermocouple. Another problem is the uncertainty of the thermal connection of the thermocouple junction to a ball near the center of the BGA. It is very difficult to manually thread a delicate wire thermocouple through the narrow corridor between rows of solder spheres and be assured that it will make contact with a ball (or "sphere") near the center of the BGA.

Often, when one is in the process of poking the thermocouple in and trying to engage a sphere, the result is a bending of the thermocouple wires near the tip which will usually cause them to touch and form a new thermocouple junction that is clearly not in contact with the ball. If this 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. This air temperature will show a much quicker thermal response than the solder spheres, simply because it takes time for the hot air to heat the spheres. This results in a thermal profile with a higher rate of rise and higher peak temperature than would be obtained if the thermocouple was in good thermal contact with the sphere.

The most reliable way to profile a BGA is to drill through the back side of the PCB beneath a ball near the center of the BGA, and attach a thermocouple to the bottom side of the pad. This technique has the added advantage that the thermocouple leads are not exposed to the convective heating from the rework or hot air soldering unit's nozzle, which conducts heat down the leads to the junction and elevates its temperature.

There are several techniques for mounting 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 gage wire thermocouple through the bottom, carefully solder it to the pad, keeping the junction .002" to .004" above the pad, epoxy the wire into the hole and replace the BGA. An easier but less reliable method is to drill through the bottom of the board into one of the center pads and use a high temperature adhesive to secure the thermocouple to the pad. The risk is that inaccurate readings can result if the thermocouple lifts off the pad slightly during adhesive cure. A disadvantage of both of these techniques is that it is necessary to retain the instrumented test board and reprofile it whenever the profile must be checked.

The technique advocated by Saunders Technology was developed in conjunction with a customer and is used by them on their rework stations and for reflow process development. It also begins with drilling or end milling a 1/32" diameter hole through the back side of the PCB to a pad beneath a central ball as shown in the Figure to above. Then, however, a thermocouple probe (TEMPROBE(TM)) is clipped to the edge of the PCB with the probe on the bottom side, and its thermocouple tip placed in the hole and preloaded against the pad. The key to this technique is a second thermocouple probe, clipped to the PCB with its probe on the top side, and its thermocouple tip preloaded on a pad or component leg near the BGA.

The instrumented PCB is then profiled, and heating parameters adjusted until the correct profile is recorded by the probe under the BGA, and a simultaneous reference profile recorded by the top side probe. This method is quick, easy and very reliable, as the preloaded thermocouple probes maintain secure thermal contact.

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, including directly on component legs, solder paste or the components themselves. The device is clipped to the edge of the circuit board and the thermocouple placed in the desired location, preloaded to maintain secure thermal contact and locked in position with a twist of the locking knob. TEMPROBEs eliminate the need for soldering, bonding or taping thermocouples to the PCB. This speeds up rework process development and eliminates the additional step of removing bonding material from the PCB.

For more information about the TEMPROBE or other products from Saunders Technology Inc., contact ECD at the number below

This article first appeared in Mass High Tech. ©The copyright for this article is owned by Roger I. Saunders.