Profiling technique improves PCB assembly

Hollis, NH--Printed circuit board assembly is a complex process, but a recent develop in temperature-sensing technology aims to ease such work by eliminating the use of adhesives for thermocouple attachment.

In large surface-mount packages such as quad flat packs (QFPs), leads (the physical/electrical connections) are located around the components' periphery. Rework of QFPs typically involves applying heat on the peripheral lead attach areas.

But with ball grid arrays (BGAs)--grids of solder "balls" located beneath components that constitute both a physical and electrical connection--attachment is more complex. "BGA processing requires concurrent heating and reflow of all underside solder spheres," explains Roger Saunders, president of Saunders Technology Inc. (Hollis, NH).

"Since reliable rework of BGAs requires close control of the thermocouple profile, the challenge is to establish the correct profile near the center of the BGA, where heating is slowest due to surrounding thermal mass," adds Saunders.

To obtain an accurate temperature profile, a thermocouple must be placed against a pad (located beneath each solder sphere) near the center of the BGA. One way to do this is to slide a fine-gauge thermocouple under the BGA. Potential problems: Many BGAs do not have sufficient clearance beneath them, and the thermocouple's connection to a ball near the center of the BGA is uncertain.

Other methods involve drilling or end milling a hole through the back of the pc board to a pad near a central ball, and attaching a thermocouple to the bottom side of the pad. Several techniques are available for mounting a thermocouple in a hole under a BGA (see sidebar), but Saunders promises to simplify the process and save time.

His technique also begins with drilling a 1/32-inch hole through the back of the PCB near a central ball. But that's where the similarity ends. A thermocouple probe--called a Temprobe--is clipped to the edge of the PCB, with its probe on the bottom side. The Temprobe's thermocouple tip is inserted into the hole against the pad. The key to Saunders' technique: a second thermocouple probe, clipped to the PCB with its probe on the top side, and its thermocouple tip on a pad near the BGA.

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

To verify the BGA profile, simply clip a Temprobe to a production PCB, place its tip on the reference point, run it through the profiling process, and then compare the results to the original reference profile.

Saunders' technique eliminates the need to solder, bond, or tape thermocouples to the PCB, and the additional step of removing those bonding materials later. The result: faster, easier rework.

Additional details...Contact Saunders Technology at (603) 881-3330, visit www.saunderstech.com