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68 SMT Magazine • January 2017 A properly sized nozzle, approximately 2mm larger around the periphery of the pack- age, will allow heat to flow directly onto the PWB and reduce the intensity of the profile. A less "intense" profile, one with lower maximum temperature and lower flow rates, will help to prevent the temperature differentials that can cause the BGA to bow downward creating cor- ner shorts. Using an infrared heating source for BGA rework will reduce, or eliminate, the long lead time for custom nozzles. Multizone bot- tomside heating will also help mitigate device warpage problems by keeping the temperature differential on the part surface to a minimum. Control of the parts with respect to mois- ture sensitivity control guidelines will keep the warpage impact of moisture absorbed by the package to a minimum. The to-be-reworked boards will need to be baked out in order to reduce the chance for MSD damage. In addi- tion, the parts that will be placed back on to the board will need to be kept in dry boxes or a moisture barrier bag which has been properly sealed. Close tracking of "floor time" will help insure that moisture ingress is limited through- out the process. If during the development of the rework process you are discovering opens and shorts and have optimized the other parts of the re- work process, then solder volumes for different areas of the array may be in order. For example, if shorts are being noted at the corners of the re- worked BGA—then print volume may need to be reduced at those locations. This can be ac- complished by using either smaller balls at the corners or shrinking the aperture sizes of the quent thermal/mechanical damage, during the assembly reflow attachment and/or repair op- erations. The thinning of the device bodies has moved them to higher MSD levels and short- er floor life, thereby exacerbating the warpage problem. JEDEC Standard Qualification Levels Time-to-market pressure for repair depots, as well as leaner inventories, have caused addi- tional turnaround stress on PCB rework depart- ments. Many times, this time pressure on BGA rework means taking short cuts when using hot air rework systems. Matching nozzle size to the BGA is important in minimizing the part warp- ing during rework. Using too small of a nozzle requires all the heat to pass thru the part and into the solder joint. This can cause large tem- perature differentials and result in BGA warp- ing. There are several process conditions which need to be controlled in order that warped de- vice packages do not become a problem. The greatest areas to control include: profile devel- opment, MSD controls and solder paste volume adjustments made during printing. Good reflow profile management, in terms of extending the heating/cooling profiles lon- ger, will minimize the impact of device warp- age. Although this will impact throughput. During the cool down process, if you go too slowly, it will create coarse-grained structures in the solder joint. Additionally, the proper con- trol of temperature differential, across the part during reflow by using a multizone bottomside heater, will reduce the warpage impact. REDUCING WARPAGE ON BGAS DURING REWORK Figure 4: JEDEC Standard Qualification Levels for Moisture Sensitivity.

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