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.
Experts Comments
Your
HIC is only indicating the RH in the bag. You explain that the components
have exceeded their MSL floor life exposure and therefore they are in jeopardy
of damage during reflow. The clock needs to be reset, but there are
better, more modern ways than high temperature baking that do not promote
oxidation and intermetallics.
Ultra low humidity desiccant technology is
now 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 RH
cabinets utilizing such technology are thus dehumidified, even at ambient
temperature.
Increasing the temperature to 40C (the point at which most
alloys will not oxidize) while maintaining the ultra low RH can further
accelerate the drying time of components without oxidation or inter-metallic
growth, 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 less
than 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 a
drying period to "reset" the floor-life clock. The drying period is different
for the following two MSL groups: For MSL 2, 2a, 3 this drying period must be in a dry cabinet at
less than or equal to 10%, or resealed in a MMB for at least 5X the exposure
time.
For MSL 4, 5, 5a this drying period must be in a dry cabinet at
less than or equal to 5%, or resealed in a MMB for at least 10X the exposure
time.
You must wait this drying period or longer BEFORE you can re-expose
the parts.
Paul Austen Senior Project Engineer Electronic Controls Design Inc Paul been with Electronic Controls Design Inc. (ECD) in Milwaukie, Oregon for over 34 years as a Senior Project Engineer. He has seen and worked with the electronic manufacturing industry from many points of view, including: technician, designer, manufacture, and customer. His focus has been the design and application of thermal process measurement tools used to improve manufacturing processes like: mass reflow and wave soldering, bread baking, paint and powder curing, metal heat treatment and more.
Since
you specify 168 hours as the floor life, I will assume that your devices are
MSL 3. You probably do not need to re-bake, per J-STD-033, Table
4-3. From Table 4-3:
For MSLs 2 to 4, the exposure time must be less than 12 hours at
30 degrees C, 60%RH and the time in dry pack must be >5x the exposure
time to avoid re-baking
For MSL 5 and 5a, the time in the dry pack must be 10x the
exposure time or greater to avoid re-baking
Note
that the J-STD-033 specification is written assuming shop floor conditions of
30 degrees C, 60%RH, and if your shop floor is cooler and/or drier, your allowable
exposure 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.
Assuming
by the information provided I believe you are talking about an MSD
classification of 3 which amounts to 168 hours floor time. According to
IPC, once the 168 hours are up, the components will require a baking
period. However, the color of the humidity indicator card will help
determine how long you should bake such components. Please refer to
j-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.
What
needs to be defined is the moisture sensitivity levels of the components as
recommended by the manufacturers. Once
this is identified, it is recommended to review J-STD-033B, Handling,
Packing, Shipping and Use of Moisture/Reflow Sensitive Surface Mount Device.
The
Drying Requirements as defined in paragraph 3.2.1 states that "...SMD packages
classified at Levels 2a through 5a must be dried (Clause 4) prior to being
sealed in MBBs, Moisture Barrier Bags." It also states "...The period between
drying and sealing must not exceed the MET (Manufacturer's Exposure Time) less
the time allowed for distributors to open the bags and repack parts."
Table
4-1 References Conditions for Drying Mounted or Unmounted SMD Packages, and
defines the bake cycle to be used for the various levels of components based
upon the time the components are out of the bag. For example BGA package Levels
2 through 6 greater than 17mm x 17mm if they exceed floor life by more than 72
hours, have to be baked at 125C for 96 hours.
So
it is important to understand these documents, J-STD-033B and J-STD-020D and
make sure a process is in place to protect the components, be it either a dry
box or a nitrogen box to secure the components. This
is a complicated issue and must be fully understood to manufacture a reliable
product.
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 the
maximum exposure limit, regardless of the color of the desiccant or/and the
moisture indicator.
Every time the components are exposed to humidity, they will
absorb it. There is no other way for the trapped moisture to be taken out other
than baking it out.
I highly recommend you to label the components and track the
exposure time.
Once you reach the
exposure limit, bake the components in accordance with the manufacturer's
recommendation and re-seal. Make sure that the components are cooling off
before 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.
