Your interpretations in bold are correct. As to the question of opening
the cabinet door, this does not pose a problem unless the door is opened
and closed so frequently that the humidity remains above the 10% level
for extended periods.
The objective is to remove moisture to a safe level, generally accepted
as being below 10%. Realize that it takes a long time period to absorb
moisture, just as it takes a long time to remove it. Periodic short
exposures to levels above 10% are relatively harmless, as long as the
rest of the time the cabinet keeps the parts at less than 10%. Most
dessicators (McDry cabinets, etc.) hold the humidity levels to 2-3%.
Nitrogen boxes typically hold moisture levels to 8-12%.
The cabinets only protect from further moisture absorption. They cannot
be depended on to fully remove moisture over extended periods, although
I have proven that they do using a leased precision scale. Moisture
removal should be done with a bake oven. The bake oven temperatures do
not need to go above about 110 deg. C. Even a vacuum bake at 85 C does
an excellent job without reducing the solderability significantly. I do
not ever bake above 105C, as this can harm some components. If you raise
the component temperature above the boiling temperature of water, you
will drive out the moisture without vacuum, but you can bake at a lower
temperature for a shorter time using a vacuum oven (typically 1/2
atmosphere).
To prove the process, you need to establish your own optimum bake-out
processes using a precision scale and averaging weights of fully
saturated parts (soaked in a humidity chamber set at 60% RH min.), then
use your desired baking process and weigh the components initially, and
then every 2 hours for a 24 hour period or until the moisture content
drops below 10% of saturated weight. Moisture content below 10% is
considered small enough to preclude component damage during reflow
events.
The hardest components to bake-out and the most sensitive to moisture
damage are silicon chips. This was surprising to me. Some flipchips
require a 72-hour bake. The next most susceptible appear to be the very
thin TBGA packages.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Kane, Amol (349)
Sent: Friday, April 21, 2006 8:07 AM
To: [log in to unmask]
Subject: [TN] Questions/Clarifications on J-STD-033B
Dear Savants,
Below is my interpretation of this std. Can someone second it and please
answer a couple of questions I have? (My interpretation is in bold) I
struggled with the std, when trying to justify the need for a desiccant
cabinet. according to my interpretation:
1. the components are baked at 125 or 150 dec C within 24 hrs of
manufacture (this is MET) and sealed in the MBBs at the component
manufacturer before shipping to the customer. the shelf life of the
components in the sealed MBB is at least 12 months
2. when the customer opens the MBB, the floor life clock of the
component starts. the floor life varies according to the MSL rating.
For MSL Level 2, 2a and 3
If exposure time <= 12 hrs,
The FLOOR LIFE can be RESET by placing in a desiccating cabinet capable
of maintaining RH <=10% for 5X the exposure time
If cumulative exposure time < the MSL max specified exposure time, the
floor life can be STOPPED/PAUSED by storing in a desiccating cabinet
capable of maintaining RH <=10%
(THUS AFTER GREATER THAN 12 HRS EXPOSURE, THE FLOOR LIFE CANNOT BE RESET
WITHOUT BAKING, MERELY STOPPED OR PAUSED BY DESICCANT CABINETS)
If cumulative exposure time exceeds FLOOR LIFE by <=72 hrs OR, If
cumulative exposure time exceeding FLOOR LIFE by > 72 hrs, bake
according to table 4-1 to RESET the floor life. at this point the shelf
life can also be reset if sealed again in a MBB
For MSL Level 4, 5 and 5a
If exposure time <= 8 hrs,
The FLOOR LIFE can be RESET by placing in a desiccating cabinet capable
of maintaining RH <=5% for 10X the exposure time
(THUS AFTER GREATER THAN 8 HRS EXPOSURE, THE FLOOR LIFE CANNOT BE RESET
WITHOUT BAKING, MERELY STOPPED OR PAUSED BY DESICCANT CABINETS)
If cumulative exposure time exceeds FLOOR LIFE by <=72 hrs
OR, If cumulative exposure time exceeding FLOOR LIFE by > 72 hrs bake
according to table 4-1 to RESET the floor life. at this point the shelf
life can also be reset if sealed again in a MBB
J-STD-033B DOES say that a desiccant cabinet capable of maintaining RH
level below 5% is equivalent to a MBB. I am not clear on how opening the
cabinet doors would affect the components though (anyone who is patient
enough to read till this point hazard a guess on this???)
Also, the moisture becomes a problem if the moisture absorbed increases
the component wt by 0.1%; if the dry wt of the component is known, then
can this be used to decide whether or not the component needs to be
baked prior to assembly?? Haven't yet read the J-STD-020 yet, plan to do
that today
Thanks in advance,
Amol Kane
M.S (Industrial Eng.)
Process Engineer
Harvard Custom Manufacturing
941 Route 38 Owego, NY 13827
Phone: (607) 687-7669 x349
[log in to unmask]
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