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July, 2012


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Using Dry Cabinets in High-Mix/Low-Volume Assembly Continued from previous page


chased extra deep dry cabinet models with feeder racks inside. The loaded feeders are kept there when a line shuts down, if those feeders will be used the next day. While most parts have a three-day floor life and will be used before this time elapses, Trenton’s practice is to place them in the cabinet overnight. IPC/JEDEC J-STD-033C has


information on room temperature dehumidification of parts and shows that the trend reverses itself. Level 2 and 3 devices have a reset of 5X the floor exposure, and level 4 and 5 devices should be considered 10X floor exposure. Please note that the IPC specs are based on 10 percent RH cabinets, which means there may be an extra margin of safety if using a higher quality cabinet. The compa- ny’s original cabinets from 10 years


the weekend, line failure or some other unforeseen occurrence pre- vents the immediate processing of boards, these parts are now out on the floor absorbing moisture just as surely as if they were sitting on a table in an opened package. If you assemble the bottom side and then place the boards in the dry cabinet, you can be assured that the parts are dry going in (they recently came out of reflow and should certainly have been dry at that point) and are being kept dry until the top side of the board is ready to be built. Visits to other facilities have shown that once one side of a board has been processed, it is believed that the moisture issues on those parts have


been eliminated. Unfortunately, not everyone


realizes that the parts are still absorbing moisture at the same rate as an unprocessed part, and that they will see the same (roughly) reflow profile when processing the top side. (See review IPC/JEDEC J- STD 033C section 5.4.4.) Depending on technology, also


consider storing bare boards in a dry cabinet. For high-layer count, mili- tary and aerospace products, Trenton stores all bare boards in dry cabinets as soon as they arrive from the board house. The cabinets have exceptionally sturdy shelves and there have been no negative issues loading them with boards. However,


Page 55


it is best to check with the load rat- ings for the shelves based on the cab- inet manufacturer because stacks of bare boards become heavy quickly.


Smaller Doors Are Better When selecting a cabinet, con-


sider door layout. Trenton has found that with the exception of the cabi- nets that are used to keep loaded feeders, the units with six smaller doors are better than those with two large doors because there is less air exchange when opening the cabinet, which results in faster recovery. An additional advantage is that it helps for labeling job areas inside the cabi- net, and certain segments can be


Continued on page 63


Tape-and-reel devices drying out at 40°C/1 percentRH.


ago were 3 percent units; however, all of its recent purchases are 1 per- cent units.


Occasionally, components on


tape-and-reel come in either known wet or questionable condition. In these cases, per IPC J-STD-033C Table 4-1, Trenton uses a heated bake cabinet at 1 percent RH heated to 40°C to bake out tape-and-reeled parts without having to unreel and tray the components in order to with- stand the 125°C bake. This process requires days instead of hours, so in some cases you may be forced to tray up parts if you need them in a hurry.


Wet parts require a bake-out in a heated


bake cabinet before they can be used on the production line.


Given that oxidation and intermetal- lic layer growth increase greatly as temperature increases, the prefer- ence is to avoid high-temperature bake out whenever it is practical.


In-Process Storage One unanticipated but benefi-


cial use of the cabinets is in-process storage of double-sided boards. As assemblies increase in complexity for double-sided SMT assemblies, more and more devices on the boards are moisture-sensitive. If you assemble the bottom side and then immediate- ly assemble the top, there are no problems. However, if scheduling,


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