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Press About Moisture-Sensitive-Components Robert Rowland (SMT) The topic of moisturesensitive components is rather boring but very important- and frequently misunderstood. Increased moisturesensitive component use, such as thin fine-pitch devices an grid arrays (BGA), has increased concern for this failure mechanism. When components are exposed to the elevated temperatures that occur during reflow soldering, moisture trapped inside plastic surface mount devices (SMD) produces enough vapour pressure to damage or destroy the device. Common failure modes include internal separation (delamination) of the plastic from the die or leadframe; wire bond damage; die damage; and internal cracks that do not extend to the component surface. In extreme cases, cracks will extend to the component will bulge an pop (referred to as the "popcorn" effect). The IPC-Association Connecting Electronic Industries Created and released IPC - M - 109, Moisture-sensitive Component Standards and Guideline Manual. It includes the following seven documents:  The Original moisture-sensitive component document, IPC-SM-786, Procedures for Characterizing and Handling of Moisture/Reflow Sensitive ICs, no longer is Active. IPC/JEDEC J-STD-020 defines the classification procedure for moisture-sensitive components, i.e. non hermetic packages made from moisture-permeable materials such as plastic. The process includes exposure to reflow soldering temperatures followed by detailed visual inspection, scanning acoustic microscopy, crosssectioning and electrical testing. The results are based on component body temperature, because the plastic molding is the main concern. The standard reflow temperature is 220C + 5C / -0 C, but reflow experiments discovered that small volume components reach temperatures as high as 235C when the board is temperature profiled for largevolume components. When the higher temperature is a possibility as, would be the case on boards that contain both small- and largevolume components, a reflow temperature of 235C is recommended for the evaluation. Convectiondominant, infrared (IR)-dominant or vaporphase reflow equipment can be used, as long as it can achieved the desired reflow profile according to J-STD-020. The eight moisture classification levels and floor life are listed below. For details regarding soak time criteria, reference J-STD-020. (For Level 6, the components must be backed before use and reflowed within the time limit specified on the moisture-sensitive caution label.) Weightgain analysis (reference J-STD-020) establishes an estimated floor life, and weightloss analysis establishes the bake time required to remove excess component moisture. J-STD-033 provides detail information regarding bake temperature and time. IPC/JEDEC J-STD-033 provides recommendations for handling, packing, shipping and baking moisture-sensitive components. The emphasis is on packing and preventing moisture absorption - baking or desiccation should be used as a last resort after excessive exposure has occurred. Dry packing involves sealing moisture-sensitive components in moisture-barrier bags with desiccant, a humidity indicator card and moisture-sensitive caution labels. The labels contain information regarding shelf life at specific temperature and humidity ranges, peak package body temperature (220C or 235C), expose time after bag is opened, details about when baking is required, the baking procedure, and the bag seal date. Level 1. Drying before bagging optional, bagging and desiccant are optional, and labeling is not required unless the components are classified at the 235 C reflow temperature. Level 2. Drying before bagging is optional, bagging and desiccant are required, and labeling is required. Level 2a a through 5a. Drying before bagging is required, bagging and desiccant are required, and labeling is required. Level 6. Drying before bagging is optional, bagging and desiccant are optional, and labeling is required. Component drying uses one of two methods: desiccation or baking. Roomtemperature desiccation, an option for components that were exposed for less than 8 hours to conditions not exceeding 30C and 60 percent RH, uses standard drypack methods or a dry box capable of maintaining 25 + 5 C with less than 10 percent RH. Baking is a bit more complicated than many people realize. There are baking recommendations for pre- and postdry pack based on a level and package thickness. Pre-baking is used to prepare components for dry packing, while postbaking is used to recondition components after floor life expiration. Review and follow the time/temperature recommendations for baking in J-STD-033. Baking temperatures can decrease lead solderability by oxidizing the leads or causing excessive intermetallic growth. Do not store components in an oven at baking temperature. Remember, high temperature trays can be baked at 125 C, while low temperature trays cannot be baked at temperatures higher than 40 C. IPC pre-bake recommendations prior to dry pack are: Package thickness less than or equal to 1.4 mm: For Levels 2a through 5a, bake times ranges from 8 to 287 hours at 125 C, or 4 to 14 hours at 150 C. Package thickness less than or equal to 2.0 mm: For Levels 2a through 5a, bake time is 48 hours at 125 C hours at 150 C. IPC postbake recommendations after floor life has expired are: Package thickness less than or equal to 1.4 mm: For Levels 2a through 5a, bake time ranges from 4 to 14 hours at 125 C, or 5 to 19 days at 40 C. Package thickness less than or equal to 2.0 mm: For Levels 2a through 5a, bake time is 48 hours at 125 C, or 21 to 68 days at 40 C. Package thickness less than or equal to 4.0 mm: For Levels 2a through 5a, bake time is 48 hours at 125 C, or 67 or 68 days at 40 C. Avoid problems with moisture sensitive by becoming familiar with the contents of IPC-M-109, Moisture-sensitive Component Standard and Guideline Manual. -------------- ROBERT ROWLAND is an SMT Editional/Board member, instructor and coating book Applied.... Contaxt him at Tel: (503) 615-1354; E-mail: rob.rowland@radisys.com. Note 1.: Ab July.2002 new norm JEDEC-JTD- 033A Note 2.:Ab July.2006 new norm JEDEC-JTD-020C led free. |
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