标题: BGA焊接--生产与维修的双重标准 [打印本页] 作者: 张先生 时间: 2009-1-7 15:49 标题: BGA焊接--生产与维修的双重标准 THE NEW PARADIGM — DUPLICATING THE THERMAL PRODUCTION PROFILE:
As mentioned earlier, today's PC board assemblies (PCBA's) are initially manufactured under tightly controlled thermal conditions. The new paradigm for reworking those boards should be to duplicate the thermal curve under which the board was originally manufactured. As the late Dr. Charles L. Hutchins wrote in his text book on surface mount technology, "The time-temperature profile (in SMT rework) should be similar to that used for the original production reflow." By duplicating the processes and thermal conditions under which every board is manufactured, rework can be accomplished effectively and reliably so the PCBA can be put back into service with the same confidence as when the electronics device was first put into service. In other words, the model for good rework equipment has always been in the production processes. (See Figure 1).
In 1995, two top design engineers (David Jacks and Randy Walston) from two of the world's largest soldering equipment firms began to publicly chastise the industry at large for not including pre-heating in the rework regimen. Challenging the old paradigm, David Jacks took Hutchins' words seriously and began lecturing nationally on what he called "The Great Double Standard" between the thermal profiles demanded in initial production and those tolerated in any subsequent rework. In 1996, Jacks and Walston formed Zephyrtronics to develop and market low temperature rework equipment for the electronics industry. The paradigm was shifting.
In hindsight, it is really quite remarkable that it took so long to happen! And to the shame of the larger, established rework equipment manufacturers, it is even more remarkable that this small, upstart entrepreneurial company in Southern California would be rewriting all the rules for electronic rework. Rework tools should duplicate the thermal conditions found in the reflow oven. The PCBA should be ramped up at 2-4°C per second and preheated to a temperature of 300°F for a period of up to 1 minute; just like the reflow oven. By inserting a preheating step in the rework cycle, the benefits of preheating can be realized. "Preheating the assembly activates the flux, removes extraneous volatiles from the flux, brings the metals to be soldered up to solder wetting temperatures, and elevates the temperature of the assembly in order to prevent thermal shock during exposure to the molten solder." Accordingly, this cleansing from the activation of the flux just prior to reflow will enhance the wetting process
