Page 54
www.us-tech.com Using Dry Cabinets in High-Mix/Low-Volume Assembly By Edward S. Wheeler, Director of Manufacturing Engineering, Trenton Technology Inc., Utica, NY H
igh-volume assembly provides numerous benefits to users: good process repeatability,
large stocks of parts neatly dry packed and ready for use, and every- thing set up in tape-and-reel or matrix trays directly from the factory. In addition, when using high-volume assembly, the assemblers know their jobs and have the assembly drawings memorized, and problems immediate- ly stand out like red flags. While there still are areas where things can (and will) go wrong, most aspects of the process are controllable. High-mix/low-volume, on the
other hand, is another story. Parts come in from vendors in packaging that is not factory fresh, parts are partially used and then returned to stock, and jobs start and stop based on changing customer requirements and conflicting deadlines while you try to maintain good floor practices. Sound familiar?
Reducing Moisture Problems One way to reduce the opportu-
nity for problems with moisture sen- sitivity is through the use of dry cab- inets. There are several manufactur- ers of these cabinets that provide quality units, and it’s a good idea to shop around for a dry cabinet that best fits the company’s needs. Here, we’re talking about Trenton Technology, Inc.’s 10 year experience with McDry cabinet systems, which have always provided extremely reli-
Cabinets on the production floor next to the SMT assembly line for easy access.
have seals on them similar to the seals used on a refrigerator. Mounted on one of the doors is a battery-oper- ated humidity meter and a dehumid- ifier unit is attached to the lower back wall. The company has more than a dozen of these cabinets, and
able service. These units are high- quality steel cabinets with lockable steel frame glass window doors that
the only maintenance ever needed is an annual battery change for the humidity indicator. Some places may want to send the recorder out to be calibrated, but at Trenton, we simply use its calibrated chart recorder to verify what the meter reports.
From a parts stand-
point, use of the cabinet is straightforward. Open the moisture barrier bag, re - move the parts to be used immediately and store the rest in the dry cabinet. This takes care of several con- cerns: resealing the mois- ture barrier bag (MBB), wor- rying about humidity indica- tor cards (HICs) and desic- cant bags, and ensuring that the operator sealed the MBB after being properly purged without causing any nicks and achieving a proper seal. How often have you taken resealed parts out of the stock room, opened them and had the HIC report that the parts have been exposed to moisture? This seems to
happen just before performing an important job with sensitive parts when the bake-out time required will put you behind schedule. Another less-than-ideal aspect of MBBs long- term parts storage is that the maxi- mum bag life is two years per
IPC/JEDEC J-STD 033C section 5.3.2. With a cabinet rated for 5 per- cent RH or less, the storage time is virtually unlimited. (See IPC/JEDEC J-STD 033C table 7-1.) Generally, Trenton still keeps parts inside MBBs to ensure part number mark- ing, and to keep the Faraday cage properties of the bag and to make sure that operators are not touching the parts directly when removing a
How often have you taken resealed parts out of the stock room, opened them and had the HIC report that the parts have been exposed to moisture?
batch of parts from the cabinet. The difference is that the bags are not sealed, merely folded over. This does not block the properties of the cabi- net because humidity does not require air movement to propagate.
No Popcorned Parts Since implementing the cabi-
nets, the company has had no known cases of popcorned parts. Before cab- inet installation, popcorning was rare because of good standard prac- tices. However, it did happen a few times. With the ever-increasing com- plexity of parts that come packaged on tape-and-reel, the company pur-
Continued on next page
July, 2012
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104