How to deal with moisture sensitive components in PCB design
Situations involving the moisture sensitivity of plastic integrated circuits (IC) are getting worse and worse due to many industrial trends, including the continuing need for more reliable products to support critical communications and technology applications seek. The failure rate of moisture-sensitive device (MSD) alone is already at an intolerable level, coupled with the constant changes in packaging technology. Shorter development cycles, shrinking sizes, new materials and larger chips are leading to rapid growth in MSD numbers and higher levels of moisture / reflow sensitivity. Finally, the increase in the use of area array packages such as BGA and CSP has also had a significant impact. This is because these components tend to be packaged in tape-and-reel systems, each tape having a large number of components. The key issue when compared to pin components in IC trays is longer exposure to moisture.
Impact of out-of-process processing
Perhaps the most important factor is the growing number of contract manufacturers and mass customization. In the printed circuit board manufacturing industry, this has become a "highly mixed" production. The reduction in batch sizes has led to more product conversions on the assembly line, leading to increased exposure time for MSD. Each time the SMT production line is switched to a new product, most of the components already loaded on the placement machine must be removed, causing the pallets and tapes used in many parts to be temporarily stored for later use. Such stored MSD are likely to exceed their critical moisture content before returning to the assembly line and the final solder reflow process. Therefore, during set-up and handling, the exposure time must be increased to dry storage time.
IPC / JEDEC standard
Guidelines for the classification, handling, packaging, transportation, and use of MSD have been clearly defined in the industry standard J-STD-023, which is a joint publication of the American Electronics Industry Council (IPC) and the Welded Electronic Component Engineering Committee (JEDEC) Thing. This document was issued in 1999 and mainly unifies and revises two previous standards: IPC-SM-786 and JEDEC-JESD22-A112 (both documents are now obsolete). The new standard contains many important additions and changes that must be followed to update existing manufacturing systems and procedures.
All in all, the standard requires MSDs to be properly sorted, marked, and packaged in dry bags until they are ready for PCB assembly. Once the bag is opened, each component must be assembled and reflow soldered within a specified time frame. The standard requires that the total cumulative exposure time for each roll or reel of MSD should be tracked through a complete manufacturing process until all parts are placed. Proper material replenishment should effectively reduce the exposure time during storage, preparation, and implementation. In addition, the standard provides flexibility to increase or decrease maximum production life, based on indoor environmental conditions and baking time.
Manufacturing Process Overview
Although the principle of assembling MSD within a specified production life sounds like a straightforward requirement, practical implementation in a production environment is always challenging. Because standards are sometimes misunderstood (and there is no easy way to do it as required), there is a big difference between the actual manufacturing process at the factory and the factory. For example, there are companies that do not have a documented manufacturing process to track and control MSD. On the contrary, some companies have built some very troublesome systems that consume a lot of time and energy, and production operators are almost impossible to follow.
Between these extremes, most companies have established many workable, simplified work procedures with many assumptions. However, this also causes the unnecessary components to be baked together when assembling those components that need to be baked. The first situation will affect the availability, solderability, and waste of expensive components. Other conditions will affect the reliability of the final product. Unfortunately, in many organizations, the working procedures for MSD were established many years ago and are not regularly revised. Changes in components, product mixes, material supplies, assembly processes, equipment, and standards cannot be reflected, so their effectiveness is greatly compromised.
The first question related to MSD control is the identification of the trays and reels. Once removed from its protective drying bag, how are these component trays and reels identified? If the component is not received in a dry bag, or if the bag is not properly labeled, there is a danger that it may be handled as a non-moisture sensitive component. Materials handlers and operators must have a convenient and reliable way to verify part numbers and related information, including moisture sensitivity levels.
Most of the MSD 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 identified, and are placed on shelves, machine feeders, drying rooms, bags, etc. with paper or stickers. All data must be transferred from the original label in different steps. Those who spend some time in the SMT production line know the huge difficulties caused by tracking components of a tray package, and the human error that results.
It should be affirmed that it is easier to place identification labels on plastic reels. However, the surfaces available for labeling vary widely (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 shunt cycle. Because there is no standard format for establishing logos, assemblers are sometimes forced to add personal labels in addition to all other labels, which makes handling such components very confusing.
Therefore, when reels contain MSD, they should clearly identify their sensitivity level. Nonetheless, even when the reel is properly labeled, this information may become unreadable when the reel is loaded in a feeder or in an adjacent feeder of a mounter.
Harm of not tracking exposure time
Perhaps the worst case scenario is that some assembly manufacturers rely on their material replenishment systems (just in time [JIT] / early in and early out [FIFO]) to ensure that all components will be assembled within the prescribed time limit. This was tolerable in the past, but now the ever-changing component technology and increasing production mix make this a very dangerous situation. In fact, most assembly manufacturers don't know how long the components are exposed and how often the MSD exceeds its maximum production life, because this information is not tracked.
The actual level of danger can be illustrated with a practical example: suppose a reel contains 850 BGA and a product requires one part per board. Like most PBGA, this part is classified as a fourth stage with a 72-hour production life. This means that when the reel is mounted on the placement machine, the average running speed of the production line must exceed 12 boards per hour, 24 hours a day, and all components must be placed before the deadline reaches three full days. Then add the exposure time of the part during the SMT production line setting (hope not to prepare the MSD on the feeder in advance), and other common situations, such as changes in production plans, lack of materials, downtime, etc. Finally, in most production environments, there is more than one product switch per day, resulting in multiple settings. The relevant exposure time will be prolonged because the same reel will be moved up and down from the placement machine multiple times. When all exposures are considered, it is clear that a significant amount of MSD will exceed its stated production life before reflow soldering.