|Ask the Experts|
April 2, 2021
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?
|Expert Panel Responses|
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.
J-STD-033C has a provision for repeated short term exposure(open bag) of parts in paragraphs: 126.96.36.199 and 188.8.131.52. 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.
Senior Project Engineer
Electronic Controls Design Inc
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:
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.
Senior Manufacturing Engineer
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 125° 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.
Vice President, Technical Director
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.
Engineering and Operations Management
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