Handling Moisture-sensitive Devices

Estimated reading time: 13 minutes

Moisture/reflow sensitivity of plastic encapsulated components is a critical manufacturing issue that cannot be dismissed with "easy-to-follow" assembly procedures. In fact, a general lack of control exists comparable to that which prevailed with the electrostatic discharge (ESD) related problems of a decade ago. However, possible improvements in handling, tracking and controlling portend improved reliability of products in the field.

By Francois Monette

The situation involving the moisture sensitivity of plastic integrated circuits (IC) is becoming progressively worse owing to numerous industry trends, including the ongoing quest for higher reliability products to support critical communication and technology applications. The rate of failure for individual moisture-sensitive devices (MSD) is already at an intolerable level, spurred by ongoing changes in packaging technology. Shorter development cycles, shrinking size, new materials and larger chips are resulting in a rapid increase in the quantity of MSDs and higher levels of moisture/reflow sensitivity. Finally, the growing use of area array packaging for such items as ball grid arrays (BGA) and chip scale packages (CSP) has had a significant impact. This is because these devices tend to be packaged in tape-and-reel systems having a large quantity of components-per-unit packaging. When compared to leaded devices in IC trays, the crucial result is a longer exposure time.

Outsourcing ImpactPerhaps the most significant factor is the growing role of contract manufacturers (CM) and mass customization. In the printed circuit board (PCB) manufacturing industry, this translates into "high-mix" production in which the general reduction in lot size is causing more changeovers on assembly lines and leading to an increase in exposure times for MSDs. Every time an SMT line is changed for a new product, most of the components already loaded on the placement machines must be removed, causing a large number of partial trays and reels to be stored temporarily for later use. The MSDs so stored are most likely to exceed their critical moisture content before being returned to the assembly line and final solder reflow processing. Thus, exposure time must be added for dry storage to the time during set-up and handling.

IPC/JEDEC StandardThe guidelines for classification, handling, packing, shipping and use of MSDs are defined clearly in the industry standard J-STD-033, a joint publication of the IPC Association Connecting Electronics Industries and the Joint Electronic Device Engineering Council (JEDEC). This document was released in 1999 following a major consolidation and revision of two prior standards: IPC-SM-786 and JEDEC-JESD22-A112 (both now obsolete). The new standard contains many significant additions and changes that must be followed to update existing manufacturing systems and procedures.

In summary, the standard requires that MSDs be properly classified, identified and packaged in dry bags until ready for PCB assembly. Once the bags are opened, each device must be assembled and reflowed within a specific timeframe. The standard requires that total cumulative exposure time for each reel or tray of MSDs be tracked through the complete manufacturing process until all are placed before reflow. Proper material logistics should effectively minimize exposure time during storage, kitting, staging, etc. Additionally, the standard provides flexibility to increase or decrease the maximum floor life, based on ambient conditions and to minimize the duration of bake. These tables are categorized according to the sensitivity level and body thickness of each component, adding to the complexity of the manufacturing procedures.

Manufacturing Procedure OverviewAlthough the principle of assembling MSDs within a specified floor life sounds like a straightforward requirement, actual implementation in a production environment always has been challenging. Because the standard is sometimes misunderstood (and there is no simple solution to comply with the requirements), a large disparity between actual manufacturing procedures exists from one assembly plant to another. For example, there still are companies that simply have no documented manufacturing procedure in place to track and control MSDs. In contrast, some companies have developed very cumbersome systems that consume much time and energy and are nearly impossible to follow for production operators.

In between those extremes are most companies who have made numerous assumptions in the development of workable simplified procedures. However, this generally represents a compromise that results in baking components that do not need it while assembling those that should be baked. The first scenario will affect material availability, solderability and may lead to a waste of expensive components. The other option will compromise the reliability of the finished product. Unfortunately in many organizations, the MSD procedure was installed many years ago and has not been revised on a regular basis. Evaluations of its efficiency based on changing components, product mix, material logistics, assembly process, equipment and revised standards are sacrificed.

MSD IdentificationThe first major issue associated with MSD control is identification of the trays and reels containing them once they are removed from their protective dry bags. If the parts are not in dry bags upon receiving, or if the bags are not properly identified, there is a high risk that they will be treated as non-moisture sensitive. Material handlers and operators must have a convenient and reliable method to identify the part numbers and the associated information, including moisture sensitivity levels.

The majority of MSDs are packaged in plastic IC trays that comply with the standard JEDEC/EIAJ outline. Unfortunately, the trays do not have surfaces with room to apply any label. In most cases, individual trays will be identified indirectly via pieces of paper or stickers placed on shelves, machine feeders, dry cabinets, bags, etc. All data must be manually transferred from the original label through the different processes. Anyone who has spent time on an SMT line is aware of the great difficulty caused by tracking components packaged in trays and the number of human errors this creates.

To be sure, it is easier to apply identification labels on plastic reels. However, the surfaces available for labels vary widely (based on most reel designs). The reel sometimes contains large openings that complicate the task of applying a larger label. A typical reel will have multiple labels with various barcode and readable data that are required throughout the production and component distribution cycle. Because there is no established standard format for identification, the assembler sometimes is forced to add personal labels in addition to everything else, which can become extremely confusing for those handling these components.

Accordingly, when reels contain MSDs, they should be clearly identified as such together with their sensitivity levels. However, even when the reels are properly identified, this information may become impossible to read once the reels are loaded on tape feeders or when adjacent feeders are loaded on a placement machine.

Danger of Untracked Exposure TimeProbably the worst assumption made by some assemblers is to rely on their material logistics (just in time [JIT]/first in, first out [FIFO]) for ensuring that all components will be assembled within the specified time limit. This was tolerable in the past but ongoing changes in component technology and increasing product mix now make this a very dangerous situation. In fact, most assemblers are unaware of how much time components are exposed and how often MSDs exceed their maximum floor life because this information remains untracked.

The actual level of risk can be illustrated with a practical example: assume a reel containing 850 BGAs and a product that requires one component per board. The part, like the majority of plastic ball grid arrays (PBGA), is classified Level 4 with a floor life of 72 hours. This means that while the reel is loaded on a placement machine, the average run rate of the line must exceed 12 boards per hour, 24 hours a day, without interruption for three full days to place all parts before they expire. Then take into account the time the parts are exposed during the SMT line set-up (hopefully, without off-line feeder set-up for MSDs), and other common situations such as changes in production schedule, component shortages, machine down situations, etc. Finally, in most production environments there will be one or more product changeovers per day to be accounted for, resulting in a multiplication of the set-ups. The associated exposure time then will be extended since the same reel will be loaded and unloaded from the placement machine multiple times. When all exposure elements are considered, it becomes obvious that a significant number of MSDs will exceed their specified floor life before reflow soldering.

Dry StorageThe partial trays and reels removed from placement machines typically are stored in a dry environment until they are needed again. Such storage may consist of a dry cabinet or resealed dry bags containing a desiccant. Many assemblers assume that it is acceptable to "stop the clock" of exposure time while the parts are in dry storage. In reality, once the parts have been exposed for a significant period (more than one hour), the absorbed moisture will remain in the package and diffuse toward the center interface, most likely to create damage. For this reason, the standard does not provide an option to stop the clock of exposure time.

Recent findings clearly have identified that time in dry storage is just as important as the prior floor exposure for high-moisture-sensitive components (Level 4-5a). One example cited from a paper on this subject1 illustrated that a plastic leaded chip carrier (PLCC) device classified Level 5 (normally 48 hours floor life) actually will exceed the critical moisture level after having only been exposed 16 hours, followed by 70 hours of dry storage. Nevertheless, it still is good practice to place parts in dry storage. The drier environment will slow the moisture absorption process, and if the parts are left in a dry environment for a sufficient period, the process will reverse and the components will begin to dry again. Also, if exposure time is limited, the trapped moisture may be removed in a relatively brief period. The IPC/JEDEC standard specifies a minimum duration in dry storage of five times the prior exposure period for parts exposed less than eight hours to re-set the clock of exposure time back to zero. Once again, the real issue is to provide workable procedures for production operators.

Kitting Exact QuantitiesTaking advantage of the short duration exposure rule, some assemblers have taken the approach of releasing MSDs in small kits containing exact quantities for assembly in an eight-hour shift. If any parts remain before that limit, it still will be possible to bring them back to a dry condition via a sufficient storage period. This involves detailed quantity calculations, including scrap factor for each MSD and production run. Expensive and delicate ICs must be handled manually to move them in and out of plastic trays. Additionally, sections of reels must be cut to the proper length. The latter requires a difficult splicing operation to add the leader tape required to set up the machine feeder, and to transfer all component information from the original package to the new tray/reel.

This handling will result in high risk for mechanical or ESD damage with their deleterious affect on quality, yields and costs. In addition, it is critical to monitor parts so that they do not exceed the specified eight hours and that they spend more than five times the prior exposure time in dry storage before being re-released for production.

Bake EverythingAnother approach is to systematically bake all partial trays and reels left at the end of a production run. This is a simpler procedure to manage but it may create more problems than it actually prevents. It is important to note that the default conditions for baking have been significantly increased in the latest IPC/JEDEC standard. The cycle is now 48 hours at 125±C for parts packaged in high-temperature trays. Components in reels or low-temperature trays must be baked at 40±C for 68 days. In most companies, this compromise relative to material availability is simply not acceptable. The standard specifies that unless otherwise indicated, one bake cycle is allowable on a finished component. If more than one bake cycle is needed, the supplier should be consulted.

Manual Logging TimeIn many companies, the MSD procedure requires that production operators manually log the date and time when parts are initially removed from their protective dry bags. Because most MSDs are packaged in JEDEC/EIAJ trays or in tape-and-reel, the exposure time log sheet (for those in reels) can be included in an adhesive label. But the overall dimension of the label is limited by the space available on the reel. In the case of trays, there is no method to attach process data directly to the container. Because of these constraints, it is difficult to maintain the association between the components and their respective log sheets because the trays and reels are loaded/unloaded from placement machines, dry cabinets, etc.

In addition to the actual physical format of the log sheet, the dynamically changing data that it contains creates further difficulties. The information must include the part number and sensitivity level since such data are lost once the bags are opened and thrown away. To track the exposure time, the log sheet must include at least one column to enter the date and time when bags are initially opened. In order for this procedure to take into account the time spent in dry storage, the log sheet also must include columns to record the date and time the material is entered and removed from a dry cabinet or dry bag (potentially multiple times).

The greatest difficulty regarding this type of manual logging is that calculations based on date and time are not simple arithmetic operations. It takes significant time and effort even for a trained individual to perform this basic operation and still it induces a high level of human error. This time consuming task also has a direct impact on set-up time and resulting machine/line utilization.

Feeder and Placement Machine VerificationThe time logging procedure during material movement is a good start; however, it provides scarcely any visibility of the current status of parts while they are loaded on placement machines. Why is this important? The probability is that individual components eventually will exceed their maximum exposure time here, as it is where they spend the longest time.

A partial measure of control can be achieved with a periodic verification. The frequency depends greatly on sensitivity level as well as the number of product changeovers and associated feeder set-ups. In practice, it means that the production operators must perform additional date/time calculations based on previous log points and eventually remove components from the placement machines before expiration.

Because of the lack of visibility, MSDs can be left exposed for extended periods while loaded on feeders that are stored on machines, feeder racks, feeder banks, etc. Great care should be taken for production lines that operate with fixed feeder set-ups or where unused trays and reels are not removed from the placement machines.

In addition to these concerns, there are various other difficulties associated with related processes. These include bake-out, resealing in dry bags, relabeling, repair and rework, device programming, retaping, double-side reflow, derating for ambient conditions, and more.

ConclusionThere are many significant obstacles that prevent assemblers from properly controlling damage to MSDs. In many cases, adequate written procedures are in place but they soon become humanly impossible to follow. This creates an unacceptable level of defect escapes. PCB assembly operations should re-evaluate their MSD procedures based on the most recent IPC/JEDEC standard. Although the control of moisture damage is just as important as that for electrostatic damage, it does not receive near the same level of attention. New systems and methods are required to provide workable and reliable solutions to this problem on the production floor.

REFERENCES

• R. L. Shook and J.P. Goodelle, "Handling of Highly Moisture-sensitive Components: An Analysis of Low-humidity Containment and Baking Schedules," Lucent Technologies, 1999 ECTC.
• R. Rowland, "Moisture-sensitive Components," SMT Magazine, October 2000.
• S. Davis, "Reliability: First Step to Availability," RTC Magazine, April 2000.
• J-STD-033, IPC/JEDEC, April 1999.

FRANCOIS MONETTE may be contacted at Cogiscan Inc., 50 De Gaspe, Suite A5, Bromont, Quebec, Canada J2L2N8; (450) 534-2644; Fax: (450) 534-0092; Email: fmonette@cogiscan.com; Web site: www.cogiscan.com.