Moisture-sensitive Components T - McDRY

1 ROBERT ROWLANDhe topic of Moisture-sensitive Components is rather boring but very important andfrequently misunder-stood. Increased mois -ture- sensitive compo-nent use, such as thin fine-pitchdevices and ball grid arrays (BGA),has increased concern for this fail-ure mechanism. When compo-nents are exposed to the elevatedtemperatures that occur duringreflow soldering, moisture trappedinside plastic surface mountdevices (SMD) produces enoughvapor pressure to damage ordestroy the device. Common fail-ure modes include internal sepa-ration (delamination) of the plas-tic from the die or lead-frame;wire bond damage; die damage;and internal cracks that do notextend to the component extreme cases, cracks willextendto the component surface; in themost severe cases, the componentwill bulge and pop (referred toas the popcorn effect).

2 The IPC Association Con-necting Electronic Industries cre-ated and released IPC-M-109, mois -ture- sensitive component Standardsand Guideline Manual. It includesthe following seven documents: IPC/JEDEC J-STD-020, moisture /Reflow SensitivityClassification for Plastic Inte-grated Circuit (IC) SMDs IPC/JEDEC J-STD-033, Stan-dard for Handling, Packing,Shipping and Use of MoistureReflow sensitive SMDs IPC/JEDEC J-STD-035,Acoustic Microscopy for Non-hermetic Encapsulated Elec-tronic Components IPC-9501, PWB Assembly Pro-cess Simulation for Evaluationof Electronic Components (Pre-conditioning IC Components )

3 IPC-9502, PWB Assembly Sol-dering Process Guideline forElectronic Components IPC-9503, moisture Sensitiv-ity Classification for Non-ICComponents IPC-9504, Assembly ProcessSimulation for Evaluation ofNon-IC Components (Precon-ditioning Non-IC Components ).The original moisture -sensitivecomponent document, IPC-SM-786, Procedures for Characterizingand Handling of moisture /ReflowSensitive ICs, no longer is J-STD-020 definesthe classification procedure formoisture- sensitive Components , ,non-hermetic packages made frommoisture-permeable materials suchas plastic.

4 The process includesexposure to reflow soldering tem-peratures followed by detailed visualinspection, scanning acousticmicroscopy, cross-sectioning andelectrical results are based on com-ponent body temperature, becausethe plastic molding is the mainconcern. The standard reflow tem-perature is 220 C +5 /-0 C, butreflow experiments discovered thatsmall volume Components reachtemperatures as high as 235 Cwhen the board is temperatureprofiled for large-volume compo-nents. When the higher temper-ature is a possibility, as would bethe case on boards that containboth small- and large-volume com-ponents, a reflow temperature of235 C is recommended for theevaluation.

5 Convection-dominant,infrared (IR)-dominant or vapor-phase reflow equipment can beused, as long as it can achieve thedesired reflow profile according eight moisture classificationlevels and floor life are listed details regarding soak time criteria, reference J-STD-020. Level 1 unlimited floor lifeat less than or equal to 30 C/85percent RH Level 2 one year floor life atless than or equal to 30 C/60 percent RH Level 2a four week floor lifeat less than or equal to 30 C/60percent RH Level 3 168 hour floor life atless than or equal to 30 C/60 percent RHMoisture- sensitive Componentssurface mountManufacturing component moisture sensitivity is a complex subject that should be understood clearly TReprinted with revisions.

6 From the October 2000 edition of SURFACE MOUNT TECHNOLOGYC opyright 2000 by PennWell Corporation Level 4 72 hour floor life at less thanor equal to 30 C/60 percent RH Level 5 48 hour floor life at less thanor equal to 30 C/60 percent RH Level 5a 24 hour floor life at less thanor equal to 30 C/60 percent RH Level 6 time on label floor life at lessthan or equal to 30 C/60 percent RH.(For level 6, the Components must bebaked before use and reflowed within thetime limit specified on the Moisture-sensitive caution label.)

7 Weight-gain analysis (referenceJ-STD-020) establishes an estimatedfloor life, and weight-loss analysis estab-lishes the bake time required to removeexcess component moisture . J-STD-033 provides detailed information regard-ing bake temperature and J-STD-033 provides rec-ommendations for handling, packing, ship-ping and baking Moisture-sensitive com-ponents. The emphasis is on packaging andpreventing moisture absorption bakingor desiccation should be used as a last resortafter excessive exposure has packing involves sealing Moisture-sensitive Components in moisture -barrierbags with desiccant, a humidity indicatorcard and Moisture-sensitive caution labels contain information regardingshelf life at specific temperature and humid-ity ranges, peak package body temperature(220 or 235 C)

8 , exposure time after bagis opened, details about when baking isrequired, the baking procedure, and thebag seal 1. Drying before bagging is optional,bagging and desiccant are optional, andlabeling is not required unless the com-ponents are classified at the 235 C 2. Drying before bagging is optional,bagging and desiccant are required, andlabeling is 2a through 5a. Drying before bag-ging is required, bagging and desiccant arerequired, and labeling is 6. Drying before bagging is optional,bagging and desiccant are optional, and label-ing is drying uses one of two meth-ods: desiccation or baking.

9 Room-temper-ature desiccation, an option for Components that were exposed for less than8 hours to conditions not exceeding 30 Cand 60 percent RH, uses standard dry-packmethods or a dry box capable of maintain-ing 25 5 C with less than 10 percent is a bit more complicated thanmany people realize. There are baking rec-ommendations for pre- and post-dry packbased on level and package thickness. Pre-baking is used to prepare Components fordry packing, while post-baking is used torecondition Components after floor life expi-ration.

10 Review and follow the time/temper-ature recommendations for baking in J-STD-033. Baking temperatures can decrease leadsolderability by oxidizing the leads or caus-ing excessive intermetallic growth. Do notstore Components in an oven at baking tem-perature. Remember, high-temperature trayscan be baked at 125 C, while low-temper-ature trays cannot be baked at temperatureshigher than 40 pre-bake recommendations priorto dry pack are:Package thickness less than or equal to mm:For levels 2a through 5a, bake time rangesfrom 8 to 28 hours at 125 C, or 4 to 14 hoursat 150 thickness less than or equal to mm:For levels 2a through 5a, bake time rangesfrom 23 to 48 hours at 125 C, or 11 to 24hours at 150 thickness less than or equal to mm.