Current international studies reinforce the global trend to the use of area array packages. BGA, CSP and Flip Chip technology not only offer significantly more I/Os (input/output) per mm² of PCB real estate, but they also have distinct electrical, mechanical, and unit cost advantages. Increased density, reduced feature size and packaging all add up to shorter distances for signals to travel, hence increasing speed and performance.  Advances in production equipment have allowed for an acceptable ppm failure rate during the production process. For many, however, the concept of quality repair remains an expensive nightmare. A more thorough understanding of the area array package and the production parameters can reduce fears of BGA repair, guarantee process control, and greatly save in rework costs.

While the desoldering process can be handled with the majority of hot air equipment available, it is the re-soldering process that is most difficult to control. In rework, as in production, quality is the ultimate goal. Quality BGA reflow can be achieved for production in the enclosed environment of a reflow oven. Rework, unfortunately, cannot be done in a completely enclosed environment since the heating conditions required for BGA reflow are difficult to achieve when blowing hot air through a nozzle. Success depends on uniform heat distribution across the package and PCB land pattern without blowing or moving the component during reflow. A convective heat transfer in a repair situation involves blowing heated air through a nozzle that has the shape of the component.  Air flow dynamics, encompassing the effects of laminar flow (high and low pressure zones and circulation rate), is a complicated science in and of itself. When combining these physical effects with those of heat absorption and distribution, it is clear that the construction of a hot air nozzle for localized area heating is a difficult task at best. Any pressure fluctuations or problems with the compressed air source or pump required by hot air systems would radically decrease the performance of the machine.  A viable repair alternative to the numerous convective heat transfer problems listed above is the use of  medium wavelength infrared.

Infrared is not new to the reflow oven and repair equipment arena. It has, however, lost some of its  popularity due to the limiting physical effects of previously used short wavelength IR. The thermal radiation, while uniformly distributed, is unevenly absorbed and reflected by objects lighter or darker in color. Although such a heat source is perfectly acceptable for PCB preheating, the use of short wavelength IR for reflow often results in overheating of the dark component body and FR4 substrate material before the reflecting leads reach proper reflow temperatures.  Medium wavelength or “dark” IR radiators (2-8 μ on the electromagnetic wavelength spectrum) not only transfer heat perfectly uniform across a surface, they also reveal an even absorption/reflection ratio between dark and light materials. With an optimal design of a medium wavelength IR BGA repair system, K-type thermocouples can be easily placed on the board in order to monitor and document precise thermal profiles during the actual reflow process.

Dark IR as a heating source alternative is an ideal solution for micro BGA, CSP, and Flip Chip repair applications. As there is no required air flow which could blow away or  vibrate the component during reflow, the IR can be uniformly focused, directed, and transferred to an exact component size and location on the PCB. Adjacent component heating can be reduced or completely eliminated by covering components with reflective foil. Even components situated as close as 0.5 mm from the component to be repaired can be safely blocked from the heating source. Due to the escaping air and/or required nozzle thickness, this is impossible with a hot air system. Additionally, a top and bottom medium wave IR repair system can also act as an ideal “mini-reflow oven” for process controlled re-balling of all types of area array packages.

Medium wavelength IR is an ideal heat source alternative to hot air applications.  It provides the most ideal heat transfer and distribution, is controllable, flexible, user-friendly, while also staying cost-effective.

For additional information on the above article or to schedule a demonstration of the ERSA equipment located at the EMPF, contact Robert N. Berta, 610-362-1200 ext 253 or via e-mail at rberta@aciusa.org.