Ashok: The real concern about moisture sensitivity is in plastic packaged parts. The plastic packaging used to manufacture surface mount technology devices absorb moisture from the environment. The high temperatures involved in vapor phase/reflow soldering cause the absorbed moisture to expand rapidly, causing internal stress. If too much moisture is absorbed, the resulting stress can be significant enough to cause: � Wire necking � Die cracking � Package cracking � Internal corrosion Suppliers rate and label components according to their inclination to absorb moisture. J-STD-020, which replaced IPC-SM-786, addresses moisture sensitivity issues in components. IPC will express mail it to you.
Ceramic packaged parts do not have the same concern about moisture sensitivity.
The approach you take in moisture protecting your parts depends, in part, on how long you plan to store them. Some companies buy enough material for long term support, so there is no parts obsolescence issues. Moisture sensitive parts can be stored in:
� Original, unopened packaging within the limits of the packaging as stated in the labeling. � Desiccant cabinets. Cabinets can dry or inert atmosphere. Check the archives for sources and commentary. � Moisture barrier packaging. For most applications, vacuum or gas purging of the barrier bags are not necessary. Nor is desiccant material. Desiccant indicating labels aren�t either, but they�re free and reusable, so what the hey. We have a coffee can full of them that came with original supplier packaging.
An alternative to low humidity storage is baking. Baking increases the corrosion on solderable surfaces and needs to be planned into the schedule. Additionally, some packaging will not stand baking temperatures, so thing increases parts handling. Increased parts handling leads to � and on and on.
Look at: "Handling of Highly-Moisture Sensitive Components - An Analysis of Low-Humidity Containment and Baking Schedules" http://www.ectc.net/Z5/s23p1.html
Taking a different angle, some people, some no-clean people, store all their parts in controlled environments, to reduce corrosion on solderable surfaces. So, they end-up not being concerned about moisture sensitivity, as a bonus for their paranoia about corrosion. Or maybe they�re paranoid about both corrosion and moisture sensitivity. Ooooo
Good luck
Dave F
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This massage is for clarifications on storage and handling ...
- Jan 21, 2000
by
Ashok Dhawan
Ashok: The real concern about moisture sensitivity is in p...
- Jan 27, 2000
by
davef
