Using Dry Cabinets in High-Mix/Low-Volume Assembly
By Edward S. Wheeler, Director of Manufacturing Engineering, Trenton Technology Inc., Utica, NY
High-volume assembly provides numerous benefits to users: good process repeatability, large stocks of parts neatly dry packed and ready for use, and everything 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 immediately 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 opportunity for problems with moisture sensitivity is through the use of dry cabinets. There are several manufacturers 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 reliable service. These units are high-quality steel cabinets with lockable steel frame glass window doors that have seals on them similar to the seals used on a refrigerator. Mounted on one of the doors is a battery-operated humidity meter and a dehumidifier unit is attached to the lower back wall. The company has more than a dozen of these cabinets, and 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 standpoint, use of the cabinet is straightforward. Open the moisture barrier bag, remove the parts to be used immediately and store the rest in the dry cabinet. This takes care of several concerns: resealing the moisture barrier bag (MBB), worrying about humidity indicator cards (HICs) and desiccant 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 maximum bag life is two years per IPC/JEDEC J-STD 033C section 5.3.2. With a cabinet rated for 5 percent 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 marking, 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 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 cabinet because humidity does not require air movement to propagate.
No Popcorned Parts
Since implementing the cabinets, the company has had no known cases of popcorned parts. Before cabinet installation, popcorning was rare because of good standard practices. However, it did happen a few times. With the ever-increasing complexity of parts that come packaged on tape-and-reel, the company purchased 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 company's original cabinets from 10 years ago were 3 percent units; however, all of its recent purchases are 1 percent 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 withstand 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. Given that oxidation and intermetallic layer growth increase greatly as temperature increases, the preference is to avoid high-temperature bake out whenever it is practical.
One unanticipated but beneficial 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 immediately assemble the top, there are no problems. However, if scheduling, the weekend, line failure or some other unforeseen occurrence prevents 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, military 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, it is best to check with the load ratings for the shelves based on the cabinet manufacturer because stacks of bare boards become heavy quickly.
Smaller Doors Are Better
When selecting a cabinet, consider door layout. Trenton has found that with the exception of the cabinets 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 cabinet, and certain segments can be locked while other segments are left open to the operator. Also consider how you store your loaded feeders. Because these can get quite heavy, a stationary mounted rack inside the cabinet instead of the optional pull out style drawer may be the better choice. If the operator extends too much weight, it is conceivable that the cabinet could become unstable, potentially tipping. Given the safety concerns and the cost of feeders and parts, it is best to err on the side of caution here. Finally, when it comes to heated drying cabinets, Trenton has found that a small unit is all that is needed because, if parts are being purchased correctly, this cabinet only will be used for bake-out sparingly.
The company labels shelf areas with specific contract customers' names and job part numbers, allowing operators to quickly become accustomed to retrieving parts from the cabinet and replacing them at the end of the day. A simple tutorial about the effects of moisture-based damage along with a few pictures will go a long way toward proper component handling, and having cabinets locally at each assembly line makes the job that much easier. Combining proper training with the right tools provides better results and, for Trenton, one of the most important tools in the world of high-mix/low-volume is its dry cabinet storage.
Contact: Seika Machinery, Inc., 3528 Torrance Blvd., Suite 100, Torrance, CA 90503 310-540-7310 fax: 310-540-7930 E-mail: firstname.lastname@example.org Web: http://www.seikausa.com or Edward S. Wheeler, Director of Manufacturing Engineering, Trenton Technology Inc., Utica, NY E-mail: email@example.com Web site: www.trentontechnology.com