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August 9, 2016

Long Term Component Storage

We need to store a variety of electronic SMD components for at least 10 years. What do you recommend for humidity and temperature settings? Would these settings be different from typical short term storage?

R.A.

Experts Comments

It has been our experience that dry cabinets that hold 25 C at 0% RH with or without nitrogen work well for long term solderability storage if they are solderable going in.
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Terry Munson
President/Senior Technical Consultant
Foresite
Mr. Munson, President and Founder of Foresite, has extensive electronics industry experience applying Ion Chromatography analytical techniques to a wide spectrum of manufacturing applications.
Most all manufactures ship their products having a Moisture Sensitivity Level (MSL) > 1 in a Moisture Barrier Bag (MMB) containing a desiccant to keep the humidity near zero and a Humidity Indicator Card (HIC) to prove it. This sealed MMB is only good for a year, unless the manufacture says otherwise.  

So the best environment to keep components, according to J-STD-033 for long periods of time is in the same basic conditions as a MMB: <5%RH. This is the best way to stop moisture absorption, contact corrosion, and prevent the need to bake the parts before use.  

The only other issue is proving that these conditions were indeed maintained for the entire 10 years! This is where a good RH and Temperature monitoring system of the storage environment is critical to prove conformance to J-STD-033. Check out the SensorWATCH system on line, a system designed to do exactly this: monitor the RH and Temperature in the manufacturing and storage areas of electronic assembly facilities. Without data to show conformance to the required RH levels in your storage system, you are forced to bake the parts before use, which can add to corrosion and possible component damage.
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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.
As an Aerospace & Defense OEM, we have the same requirements, and have implemented LTS (Long-Term Storage) protocols to deal with it. Following are the risks that require mitigation for electronic components:
  1. Oxidation of lead finishes destroys solderability
  2. IMC (Intermetallic Compound) growth through to the surface destroys solderability
  3. Moisture uptake requires pre-bake, which will incur additional risk per (1) and (2) 
Mitigation of risks (1) and (3) can be accomplished by excluding oxygen and moisture. A nitrogen-purged, sealed MBB (Moisture barrier Bag) with the proper amount of desiccant enclosed per J-STD-033 will be good for approximately three years. The time depends entirely on the quality of the bag (permeability) and the quality of the seal. Rather than re-package at intervals, storing the MBBs within a dry, nitrogen-purged cabinet will allow you to maintain the dry, oxygen-free environment within the MBBs indefinitely.

Mitigation of risk (2) is not accomplished by excluding oxygen and/or moisture. The only way to slow the growth of IMCs is to store parts at cold temperatures, but this is not recommended. Most good component finishes, when held at 20C, should not degrade to the point of losing solderability over a 10-year period.
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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.
My first question would be "why do you need to store components for 10 years". I am going to assume that this is for a user requirement but given the state of obsolescence this seems to be a long time The proper method for long term storage of devices would be as follow
  1. Make sure these devices are as moisture free as possible, dry box storage, before packaging.
  2. Utilize a properly sized MVB, Moisture Vapor Barrier bag, Do not scrimp on the side of costs. Use a top quality bag. Use the thickest bag you can get. 6.5 mil would be recommended.
  3. Use a vacuum sealer that has a N 2 purge, critical to remove as much O 2 as possible.
  4. Use a desiccant pack appropriate to the size of the total volume of the devices being packaged.
  5. Insert a moisture/humidity card to detect if a breach has occurred. Verify that the card can be read after sealing.
  6. Vacuum at the highest possible draw you can attain.
  7. Heat seal.
  8. Verify seal.
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Jerry Karp
President
JSK Associates
Based in. Northern California since 1971. Founded JSK Associates in 1979. Actively involved in soldering, cleaning, chemistries. 30 years experience in EOS/ESD control.
Preservation of solderability for such a long period of time would involve mitigating the two main reasons for deterioration of coatings. Oxidation or corrosion of the surface finish can be minimized by storing in nitrogen or, alternatively, in a sealed moisture proof bag. Diffusion and intermetallic growth between coatings and substrates can also cause solderability issues and is accelerated with temperature and time. Storage at a low temperature would therefore be an advantage. Perhaps a combination of low temperature storage within a moisture proof sealed bag would be the preferred option.
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Bryan Kerr
Principal Engineer - CMA Lab
BAE Systems
Bryan Kerr has 35 years experience in providing technical support to PEC assembly manufacturing. His experience ranges from analysis of materials and components to troubleshooting and optimizing, selecting reflow, cleaning, coating and other associated processes.
Storing of components for a long period of time is based upon the requirements of J-Std-033C.

This document specifies the bake out times especially for SMT components, which are to be used after a storage period of time.

Sect 5.3 Safe Storage, is define as meaning dry SMD packages held in a controlled humidity condition such that the floor-life clock remains at zero. Acceptable safe storage conditions for SMD packages classified as Level 2 through 5a are listed below.

5.3.1 Dry Pack
Dry packed SMD packages in intact MBBs, stored per 3.3, shall have a calculated shelf life of at least 12 months from the bag seal date shown on the caution or bar code label.

5.3.2 Shelf Life
The minimum calculated shelf life is 12 months from bag seal date. If the actual shelf life has exceeded 12 months but less than 2 years from the bag seal date and the humidity indicator card (HIC) (see 5.5.1) indicates that baking is not required, then it is safe to reflow the components per the original MSL rating. Although unanticipated, factors other than moisture sensitivity could affect the total shelf life of components. Note: An HIC (Humidity Indicator Card) that has been continuously sealed in the MBB is typically accurate for a minimum of 2 years.

5.3.3 Dry Atmosphere Cabinet
Storage cabinets which maintain low humidity by purging with dry air or nitrogen at 25 +/- 5 C. The cabinets must be capable of recovering to their stated humidity rating within one hour from routine excursions such as door opening/closing.

5.3.3.1 Dry Cabinet at 10% RH
SMD packages not sealed in a MBB may be placed in a dry atmosphere cabinet, maintained at not greater than 10% RH. These dry cabinets should not be considered a MBB. Storage of SMD packages in these dry cabinets should be limited to a maximum time per Table 7-1. If the time limit is exceeded they should be baked according to Table 4-2 to restore the floor life. 5.3.3.2 Dry Cabinet at 5% RH SMD packages not sealed in a MBB may be placed in a dry atmosphere cabinet, maintained at not greater than 5% RH. Storage in these dry cabinets may be considered equivalent to storage in a dry pack with unlimited shelf life.

Therefore, 5.3.3.2 infers that storage in an environment of less than 5% RH, may be considered to be stored in a dry pack with unlimited shelf life.

It is important to remember however that this is for moisture entrapment within the component. We must also be concern with the solderability of the component leads which may be impacted by two elements, one being the growth of the intermetallic layer due to solid state diffusion and second through exposure to contamination being deposited on the surfaces of the component leads.   Therefore it would be advantageous to store the devices below 30C [86F].
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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.
The key is to preserve solderability and keep moisture out. While removing moisture is relatively easy by doing a bake oxide, increases in oxidation are harder to deal with. Higher oxidation brought on by exposure to air for many years may later require the use of more active flux systems. In some cases using a stronger flux is not allowed. So preserving solderability and reducing oxidation is critical. Oxidation can be reduced by using vacuum packing or flushing with nitrogen before sealing in low permeation plastic bags. The plastic bags used should restrict both air and moisture.  

As for the storage environment it should be free of oxidizing agents, free of sulfur containing cardboard, be low in humidity 20% or less and at room temperature or cooler in the range of 60-70F.
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Peter Biocca
Senior Market Development Engineer
Kester
Mr. Biocca was a chemist with many years experience in soldering technologies. He presented around the world in matters relating to process optimization and assembly. He was the author of many technical papers delivered globally. Mr. Biocca was a respected mentor in the electronics industry. He passed away in November, 2014.
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