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ABS":PQFF packages, prior to board assembly, have a requirement to be "Baked Out7' of absorbed moisture to prevent 'Pop Corning'. This paper identifies key factors that are necessary for ,cont.roIling an efficient and optimized Bake Out process. Moisture absorption and desorption of the PQFP package will be discussed. Methods will be recommended for optimizing and verifying the performance of the Bake Out ovens. blTRODUCIION :The intent of this paper is to present techniques to reduce the bake out process time for PQFP packages. The results of this report are based on using 20,28 and 32 mm sq body sizes.Generally, a high pin count plastic package (160,208,240 PQFP) with a moisture level at or below 0.05% by dry weight is considered dry enough for dry packing. These components are dried below the required safe moisture level to endure a specified time for shipping, handling and storage in a humid environment before the componexlts are mounted in the printed wiring board [I].This paper will address the question how long will it take to bake out a package such that the moisture content witbin the package is effectively &ied to 0.05% of its dry weight. One of the most effective ways to keep the bake out time short is to control the amount of dme a package is exposed to moisture prior to bake out. To insure that this is met within our process, the moisture level targeted for the package was driven to 60 % less than 0.05% by dry weight package as a safety margin. Using this as a guidebe, our cumnt process has successfully shipped without any "pop com" defects. The peak temperature of the solder reflow at board attach is 240 degree C. The bake out time used for achieving this was from a minimum of four hours to a maximum of eight hours by optimizing the key elements mentioned above. BACKGROUND:

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