Ask the Experts
January 18, 2018
Floor Life of MSD Parts
We run small jobs so a reel of MSD components could be opened and closed a number of times. Each time we put a new desiccant pack and new humidity indicator card in the MSD bag and seal it. If we total all the times we opened the bag it may exceeds 168 hours. Example: 8 hrs open then sealed for one or more days before we open the bag again for 10 hrs then sealed etc. If the humidity indicator card is not pink, do we still need to bake the components?
M.B.
Expert Panel Responses
YourHIC is only indicating the RH in the bag. You explain that the componentshave exceeded their MSL floor life exposure and therefore they are in jeopardyof damage during reflow. The clock needs to be reset, but there arebetter, more modern ways than high temperature baking that do not promoteoxidation and intermetallics.
Ultra low humidity desiccant technology isnow available that can sustain a low rest-humidity of 0.3% RH (0.05 grams H20/m3)effectively a "moisture vacuum." Components stored in ultra low RHcabinets utilizing such technology are thus dehumidified, even at ambienttemperature.
Increasing the temperature to 40C (the point at which mostalloys will not oxidize) while maintaining the ultra low RH can furtheraccelerate the drying time of components without oxidation or inter-metallicgrowth, and at 10% of the operating cost of high temperature baking.
Richard Heimsch
Director
Protean Marketing
Now a director at Protean Marketing, Mr. Heimsch has worked in the electronics industry 25+ years in a wide variety of international sales, marketing and operations roles. Rich spearheads Protean's international business development, specializing in Brand Management and Strategic Communications.
J-STD-033C has a provision for repeated short term exposure(open bag) of parts in paragraphs: 4.1.2.1 and 4.1.2.2. Repeated exposure is OK, so long as the exposure time is lessthan 12 hours for MSL2, 2a, 3 and less than 8 hours for MSL 4, 5, 5a.
This exposure is acceptable ONLY if you follow the exposure by adrying period to "reset" the floor-life clock. The drying period is differentfor the following two MSL groups: For MSL 2, 2a, 3 this drying period must be in a dry cabinet atless than or equal to 10%, or resealed in a MMB for at least 5X the exposuretime. For MSL 4, 5, 5a this drying period must be in a dry cabinet atless than or equal to 5%, or resealed in a MMB for at least 10X the exposuretime. You must wait this drying period or longer BEFORE you can re-exposethe parts.
Paul Austen
Senior Project Engineer
Electronic Controls Design Inc
Paul been with Electronic Controls Design Inc. (ECD) in Milwaukie, Oregon for over 39 years as a Senior Project Engineer. He has seen and worked with the electronic manufacturing industry from many points of view, including: technician, engineer, manufacture, and customer. His focus has been the design and application of measurement tools used to improve manufacturing thermal processes and well as moisture sensitive component storage solutions.
Sinceyou specify 168 hours as the floor life, I will assume that your devices areMSL 3. You probably do not need to re-bake, per J-STD-033, Table4-3. From Table 4-3:- For MSLs 2 to 4, the exposure time must be less than 12 hours at30 degrees C, 60%RH and the time in dry pack must be >5x the exposuretime to avoid re-baking
- For MSL 5 and 5a, the time in the dry pack must be 10x theexposure time or greater to avoid re-bakingNotethat the J-STD-033 specification is written assuming shop floor conditions of30 degrees C, 60%RH, and if your shop floor is cooler and/or drier, your allowableexposure time may be longer; see Table 7-1 in J-STD-033.
Fritz Byle
Process Engineer
Astronautics
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
Assumingby the information provided I believe you are talking about an MSDclassification of 3 which amounts to 168 hours floor time. According toIPC, once the 168 hours are up, the components will require a bakingperiod. However, the color of the humidity indicator card will helpdetermine how long you should bake such components. Please refer toj-std-033.
Edithel Marietti
Senior Manufacturing Engineer
Northrop Grumman
Edithel is a chemical engineer with 20 year experience in manufacturing & process development for electronic contract manufacturers in US as well as some major OEM's. Involved in SMT, Reflow, Wave and other assembly operations entailing conformal coating and robotics.
Whatneeds to be defined is the moisture sensitivity levels of the components asrecommended by the manufacturers. Oncethis is identified, it is recommended to review J-STD-033B, Handling,Packing, Shipping and Use of Moisture/Reflow Sensitive Surface Mount Device.
TheDrying Requirements as defined in paragraph 3.2.1 states that "...SMD packagesclassified at Levels 2a through 5a must be dried (Clause 4) prior to beingsealed in MBBs, Moisture Barrier Bags." It also states "...The period betweendrying and sealing must not exceed the MET (Manufacturer's Exposure Time) lessthe time allowed for distributors to open the bags and repack parts."
Table4-1 References Conditions for Drying Mounted or Unmounted SMD Packages, anddefines the bake cycle to be used for the various levels of components basedupon the time the components are out of the bag. For example BGA package Levels2 through 6 greater than 17mm x 17mm if they exceed floor life by more than 72hours, have to be baked at 125C for 96 hours.
Soit is important to understand these documents, J-STD-033B and J-STD-020D andmake sure a process is in place to protect the components, be it either a drybox or a nitrogen box to secure the components. Thisis a complicated issue and must be fully understood to manufacture a reliableproduct.
Leo Lambert
Vice President, Technical Director
EPTAC Corporation
At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.
You definitely have to bake the components once they reach themaximum exposure limit, regardless of the color of the desiccant or/and themoisture indicator.
Every time the components are exposed to humidity, they willabsorb it. There is no other way for the trapped moisture to be taken out otherthan baking it out.
I highly recommend you to label the components and track theexposure time.
Once you reach theexposure limit, bake the components in accordance with the manufacturer'srecommendation and re-seal. Make sure that the components are cooling offbefore re-bagging.
Georgian Simion
Engineering and Operations Management
Independent Consultant
Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at georgiansimion@yahoo.com. |
