How to deal with moisture sensitive components in PCB design

writer: G April 02, 2019

Effect of external processing

Perhaps the most important factor is the growing number of contract manufacturers and large-scale customization. In the printed circuit board manufacturing industry, this has become a "highly mixed" production, with a reduction in batch size resulting in more product conversion on the assembly line, resulting in increased exposure time for the MSD. Every time an SMT line is converted to a new product, most of the components already loaded on the placement machine must be removed, causing many of the used trays and tapes to be temporarily stored for later use. The MSD thus stored is likely to exceed its critical moisture content before returning to the assembly line and the final solder reflow process. Therefore, during setup and processing, the exposure time must be increased to dry storage time.

IPC/JEDEC standard

Guidelines for the classification, handling, packaging, transportation and use of MSD are clearly defined in the industry standard J-STD-023, a joint publication of the American Electronic Industries Association (IPC) and the Welding Electronic Components Engineering Committee (JEDEC). Things. The document was issued in 1999 and has mainly reorganized and revised two previous standards: IPC-SM-786 and JEDEC-JESD22-A112 (both files are now obsolete). The new standard contains many important additions and changes that must be followed to update existing manufacturing systems and procedures.

In summary, the standard requires the MSD to be properly sorted, labeled, and packaged in a dry bag until ready for PCB assembly. Once the bag is opened, each component must be assembled and reflow soldered within a defined time frame. The standard requires that the total cumulative exposure time for each volume or each MSD should be tracked through a complete manufacturing process until all parts are placed. Appropriate material replenishment should be effective in reducing storage, stocking, and exposure time during implementation. In addition, the standard provides flexibility to increase or decrease maximum production life based on indoor environmental conditions and baking time.

Manufacturing process review

While the principle of assembling an MSD within a specified production life sounds like a straightforward requirement, the actual implementation in a production environment is always challenging. Because standards are sometimes misunderstood (and there is no simple way to do so), there is a big difference between the actual manufacturing process at the factory and the factory. For example, there are no written manufacturing programs at all to track and control the MSD. Instead, some companies have built some very cumbersome systems that consume a lot of time and energy, making it almost impossible for production operators to follow.

Between these extremes, most companies establish viable and simplified working procedures with many assumptions. However, this in turn causes the unwanted ones to be baked together when assembling the components that need to be baked. The first situation will affect the availability of materials, solderability, and waste of expensive components. Other conditions will affect the reliability of the final product. Unfortunately, in many organizations, MSD's work procedures were established many years ago and are not regularly revised. Changes in components, product mix, material supply, assembly processes, equipment, and standards are not reflected, so their effectiveness is greatly compromised.

MSD logo

The primary issue associated with MSD control is the identification of the tray and the tape. Once removed from their protective drying bag, how are these component trays and tape rolls identified? If the components are not received in a dry bag, Or if the bag is not properly labeled, it is possible to be treated as a non-moisture sensitive component. Material handlers and operators must have a convenient and reliable way to confirm part numbers and related information, including moisture sensitivity levels.

Most of the MSDs are packaged in plastic IC trays that conform to the standard JEDEC/EIAJ form factor. Unfortunately, these trays have no surface space to label. In most cases, individual trays are not directly labeled, with paper or labels placed on shelves, machine feeders, drying chambers, bags, and the like. All data must be transferred from the original label through different steps. Those who have been in the SMT production line for a certain period of time know the enormous difficulties caused by tracking the components of the tray packaging and the resulting human error.

It should be affirmed that it is relatively easy to place the identification label on a plastic reel. However, the surfaces available for the label vary widely (depending on the design of the reel). Sometimes the reel contains large openings, which are slightly more complicated for larger labels. A typical reel should have multiple tags with various bar codes and readable data required for the entire production and component shunt cycle. Because the standard format of the logo is not established, the assembler is sometimes forced to add personal tags in addition to all other tags, which makes handling such components very confusing.

Therefore, when the reels contain MSDs, they should clearly identify their level of sensitivity. Even so, even when the reel is properly labeled, this information may become unreadable when the reel is loaded on the feeder or attached to the adjacent feeder of the placement machine.

Do not track the hazard of exposure time

Perhaps the worst case scenario is that some assembly manufacturers rely on their material replenishment system (just in time [JIT] / early in and out [FIFO]) to ensure that all components will be assembled within the specified time limits. This has been tolerable in the past, but now, the constant changes in component technology and increasing production mix make this a very dangerous situation. In fact, most assembly manufacturers don't know how long the component is exposed and how often the MSD exceeds its maximum production life because the information is not tracked.

The actual level of hazard can be illustrated by a practical example: Suppose a reel contains 850 BGAs and one product requires one part per board. Like most PBGAs, the part is divided into a fourth stage with a production life of 72 hours. This means that after the reel is loaded onto the placement machine, the average speed of the production line must exceed 12 boards per hour, 24 hours a day, three pieces of uninterrupted three days throughout the deadline before all the components are placed. Then add the exposure time of the parts during the SMT production line setup (hopefully not to prepare the MSD on the feeder in advance), and other common situations, such as changes in production schedules, shortages, downtime, etc. Finally, in most production environments, there are more than one product conversion per day, resulting in multiple settings. Then the relevant exposure time will be extended because the same reel has to be taken up and down several times from the placement machine. When considering all exposure factors, it is clear that a large number of MSDs will exceed their specified production life before reflow soldering.