Hazards of not tracking exposure time

To be sure, it is relatively easy to put the identification label on the plastic roll. However, the surfaces available for labeling vary greatly (depending on the design of the reel). Sometimes reels contain large openings, which are slightly more complicated for larger labels. A typical reel should have multiple labels, with various barcodes and readable data required for the entire production and component diversion cycle. Because there is no established standard format for identification, assemblers are sometimes forced to add individual labels in addition to all other labels, which makes handling such components very confusing.

Therefore, when volumes contain MSDS, they should clearly identify their sensitivity level. Nevertheless, even if the reel is properly identified, this information may become unreadable when the reel is loaded in the feeder or in an adjacent feeder mounted on the mounter.

Perhaps at worst, some assembly manufacturers rely on their material supply systems (just-in-time/early-in-early-out [FIFO]) to ensure that all components will be assembled within the specified time limit. This was tolerable in the past, but now, the constant changes in component technology and the increasing degree of production mixing make this a very dangerous situation. In fact, most assembly manufacturers do not know how long components are exposed and how often MSDS exceed their maximum production life because this information is not tracked.

The actual risk level can be illustrated by a practical example: suppose a reel contains 850 BGA, and a product requires one part per plate. Like most PBGA, the part is classified into a fourth stage with a production life of 72 hours. This means that when the reel is installed with the mounter, the average operating speed of the production line must exceed 12 boards per hour, 24 hours a day, without interruption for three full days to install all the components before the deadline. Then add the exposure time of the parts during the setup of the SMT line (hopefully there is no MSD prepared in advance on the feeder), and other common situations, such as changes in the production schedule, shortage of materials, downtime, etc. Finally, in most production environments, there is more than one product conversion per day, resulting in multiple Settings. Then the exposure time will be extended because the same reel will be removed from the mounter multiple times. When all exposure factors are considered, it is clear that a large number of MSDS will exceed their stated production life before reflow soldering.

Dry storage

Parts of the tray and reel removed from the mounter are usually stored in a dry environment until used again. This storage must consist of a drying chamber or a resealed drying bag with a desiccant. Many assembly manufacturers believe that the time a part is exposed while in dry storage is terminated. In fact, once the part has been exposed for a certain time (more than an hour), the absorbed moisture will remain in the package and spread to the central interface, which may cause harm. For this reason, the standard does not say to stop timing exposure times.

Recent findings clearly show that for highly moisture sensitive components (classes 4-5a), the time of dry storage is just as important as before production exposure. An example cited from a paper on the topic 1 illustrates that PLCC classified as Grade 5 (typically 48-hour production life) that is exposed for only 16 hours followed by dry storage for 70 hours actually still exceeds critical moisture levels. In any case, it is a good idea to store the components in dry storage. The drier the environment will slow down the process of moisture absorption, and if the part is left in the dry environment for enough time, the process will be reversed and the part will begin to dry again. Also, if the exposure time is limited, the entraining moisture will be removed in a relatively short time. The IPC/JEDEC standard stipulates that the exposure time can be reset to zero for parts with exposure time of less than 8 hours that last 5 times longer in a dry environment. Once again, the real problem is to provide the production operator with a workable working procedure.

Prepare just the right amount of material

Taking advantage of the principle of minimum exposure time, some assembly manufacturers have adopted the method of issuing MSDS in small quantities, preparing just enough for eight-hour assembly. If any parts remain before this limit, the parts can also be brought back to dry conditions with sufficient dry storage time. This involves detailed quantity calculations including scrap factors for each MSD. Expensive and fragile ics must be moved in and out of plastic trays by hand. In addition, the tape must be cut to the appropriate length. The latter requires a more difficult slitter operation to increase the guide tape required by the feeder and to transfer all component information from the original package to the new pallet/reel.

This operation creates a high risk of mechanical or ESD damage, with a negative impact on quality, compliance and cost. In addition, it is critical to monitor parts for no more than the prescribed eight hours and to spend more than five times that time drying before being re-issued to production.