Profiling technique improves PCB assembly 22739

April 6, 1998 Design News

ENGINEERING PRODUCTIVITY KIT: FASTENING. JOINING. & ASSEMBLY 

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.

Right-angle clinch fasteners weigh in with strength 

East Providence, RI--When Fillon Pichon USA wanted to create strong right-angle attachment points in two-piece enclosures for its precision table-top paint scales, the company turned to R'ANGLE^TM fasteners from Penn Engineering & Manufacturing Corp. (Danboro, PA). The result: strong attachment points, half the amount of hardware, assembly time cut from 11/2 hours to 15 minutes, and the elimination of two major fabricating steps.

Fillon Pichon's FP7000 digital electronic paint scales weigh automotive paint for quick color mixing. "The scale is an extruded metal chassis that would normally require tabs punched and then bent to create right angles," explains Emile R. Charette, an engineer at fabricator RESH Inc. (Franklin, MA). "Those steps are no longer necessary with the R'ANGLE fasteners."

RESH fabricates the enclosures and installs the hardware prior to final assembly. The R'ANGLE fasteners, painted with a black finish, are pressed into 0.375- x 0.312-inch rectangular mounting holes. They clinch into the metal for permanent installation, leaving the reverse side flush.

Fillon Pichon then installs the load sensors and electronic components, and completes the final assembly--which requires only standard #6-32 thread-forming screws.

When the six R'ANGLE fasteners and their compatible thread-forming screws are installed, notes Art Bernier, systems manager at Fillon Pichon, the result is an extremely tight thread fit that resists vibration and has excellent strip-out characteristics. Even with repeated removal and re-installation of the screws during service, R'ANGLE fasteners hold tight, he adds. Reusability proved to be a big advantage.

"We were looking for production economies, standardized hardware, and performance," says Bernier. "We found them with R'ANGLE fasteners."

Adhesive improves production efficiency

St. Laurel, NJ--Like any other business, Productive Plastics, a manufacturer of heavy-gauge thermoformed parts, has to maintain efficiency when it comes to meeting deadlines, saving money, and avoiding excessive waste.

So when local distributor John Petrauskas of Ellsworth Adhesive Systems (Audubon, PA) learned that Productive Plastics was not as efficient as it could be, he contacted Manufacturing Engineer Scott Downing with an idea.

Petrauskas suggested that Productive Plastics could increase efficiency and reduce costs by re-evaluating its current adhesive product. He recommended using a single adhesive--Plastic Welder^TM from ITW Devcon (Danvers, MA)--in place of the three the company had been using.

With Devcon's 400-ml pneumatic applicator guns and pre-packaged cartridges filled with Plastic Welder, Productive Plastics improved productivity by eliminating manual mixing. Also, mixing nozzles that attach to the guns enable workers to control the amount of adhesive applied, reducing waste.

When the application process is complete, workers use clamps to bind the parts together. Fixturing time is only ten minutes, so little work time is lost. Once the adhesive dries, the product is ready for additional machining or shipment to customers.

Downing says there are many benefits to using Plastic Welder. Working with a single product decreases the potential for human error, eliminates application steps, and requires a shorter drying time. In fact, Plastic Welder takes 30% less time to dry than Productive Plastics' previous adhesives--cutting overall assembly time by nearly 15%. In addition, the quick drying time enables workers to handle finished products faster, maintaining an efficient, even flow of production. The result: less idle time for employees and fewer delays in packaging the final product. However, the most important benefit to using Plastic Welder, notes Downing: an annual savings of more than $20,000.

Adhesives hold at high speeds

Black Rock Desert, NV--When the Thrust Supersonic Car (SSC) shot across the desert to break the land speed record and the sound barrier, it needed something stronger than rivets, bolts, and welded joints to keep it from breaking apart under the severe stresses of its 763-mph performance.

The car remained intact, thanks in part to adhesive technology from the Permabond division of National Starch & Chemical Company (Bridge-water, NJ).

Two years ago former world record holder Richard Noble approached Permabond for adhesives and technical assistance. One reason: The company's adhesives can bond the many different materials that make up the Thrust SSC, including titanium, carbon- and glass-fiber composites, aluminum, and steel. The adhesives allow for better stress distribution than welds (which would have been incompatible with the car's range of materials), reducing the effects of the enormous pressures on the car. Adhesives also provide smoother and cleaner connections of the body panels on the car, resulting in better aerodynamics.

Permabond's adhesives had to withstand strong vibrations from the 50,000 lbs of thrust generated by two Rolls Royce engines that power the car, as well as stresses caused by supersonic speeds. They also had to maintain their strength under the intense heat of the engine's afterburners.

In the end, Permabond supplied three adhesives (A130 threadlock, E32, and ESP110) and a pretreatment product (SIP, Self Indicating Pretreatment), as well as technical support for the car's design, notes Paul Sennett, marketing manager at Permabond U.K. The company's epoxy resins were used in several critical areas, including fabrication of the jet-engine support structure, and the securing of external panels to the car body's framework.

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