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By Russell McCarten, XDRY Corporation
At test and inspection, defects mean lost revenue through scrap or rework. Defects can occur due the board, component, materials, or other factors. One controllable defect-causing aspect of assembly is moisture.
It is never a pretty sight, finding defects on a finished PCB. At this point in the process, we have already added too much value to the assembly not to troubleshoot the failure. Defects are caused by a number of factors. They might be component related, bare-board related, brought on by too tight a process window, or at times even come from a marginal board layout or design.
The bottom line is clear. Defects identified once a board is assembled are costly.
There are various methods available to manufacturing and process engineers to reduce board-level defects, starting with the raw materials. We normally look at components and boards from the parts side; solder paste, stencil, and reflow profiles from the process side are areas of potential failure points.
One area we can address to reduce the potential for a board-level defect is moisture control. This area is typically overlooked.
Moisture-induced DefectsMoisture sensitivity in surface mount devices was identified as an opportunity for defects in the 1980s. IPC issued IPC-SM-786, "Recommended Procedures for Handling of Moisture Sensitive Plastic IC Packages," in 1990, specifically addressing this issue. Certain base materials such as FR-4 are dimensionally sensitive to moisture. Higher-than-normal moisture absorption in a bare board causes dimensional instability, which in turn raises issues during the application of solder paste, glue deposition, and component placement.
Vision alignment systems provide limited assistance in reducing moisture-related defects in the printing process, since the match between the PCB and stencil uses a global best-fit alignment solution.
Glue depositions fare better, using the board-level fiducials to modify the entire coordinate system, reducing the overall impact of stretch or shrink to the base materials.
Component placement takes advantage of the same vision system benefits with an additional layer of adjustment by using local fiducials placed near high-pin-count, fine-pitch devices (QFPs, TQFs, TSSOPs). Modification of the X/Y/? coordinates helps locate the placement spindle and component center, but still uses the best-fit algorithm for the pad-to-lead match. Sufficient moisture in the board continues to lead to marginal component placement. This was acceptable with standard eutectic solder paste given the inherent self-centering properties of the material during reflow. Today's lead-free products react differently where self-centering during the liquidus stage is greatly reduced or eliminated.
Heating moisture-laden SMD packages during the reflow yields cracks in the actual package and potential delamination of the device construct. As component temperatures increase during the heating process, moisture rapidly evaporates. Turning to steam in the short soak portion of the reflow profile, it causes miniature explosions inside the package. This can cause wire-bond fractures and/or micro-cracks in the substrate or leadframe.
According to IPC/JEDEC J-STD-020D.1,1 "The vapor pressure of moisture inside a non-hermetic package increases greatly when the package is exposed to the high temperature of solder reflow. Under certain conditions, this pressure can cause internal delamination of the packaging materials from the die and/or leadframe/substrate, internal cracks that do not extend to the outside of the package, bond damage, wire necking, bond lifting, die lifting, thin film cracking, or cratering beneath the bonds. In the most severe case, the stress can result in external package cracks. This is commonly referred to as the popcorn phenomenon because the internal stress causes the package to bulge and then crack with an audible pop. SMDs are more susceptible to this problem than through-hole parts because they are exposed to higher temperatures during reflow soldering."
Fortunately, the standard gives us measureable and acceptable definitions for moisture content and floor life, as well as methods of removing excessive moisture from devices.
Regardless of the actual failure, the result is an unusable assembly. These board failures due to MSD faults are preventable with some basic handling guidelines.
Faced with using commercial off the shelf (COTS) parts has left the military and other high-reliability board assemblers the responsibility of managing moisture sensitive devices (MSDs) and substrates. IPC/JEDEC has once again come to the rescue with J-STD-033B.1,2, 4-5 which details appropriate handling, packing, shipping, and use of moisture/reflow sensitive surface mount devices.
For board assemblers, J-STD-033B.1 helps identify acceptable methods of extending the shelf life of MSDs once they are removed from the moisture barrier bag (MBB). Shelf life is defined as "the minimum time that a dry-packed moisture-sensitive device can be stored in an unopened moisture barrier bag, such that a specified interior bag ambient humidity is not exceeded" by the standard.
A humidity indicator card (HIC) is included with all MBBs to identify the level of moisture of its contents. The typical HIC contains three color spots with sensitivity values of 5%, 10%, and 60% relative humidity (RH). Each spot changes color as the humidity inside the MBB reaches the appropriate RH. Instructions are pre-printed on the HIC to aid the user in understanding if any action is required to reset the floor-life clock.
One of the easiest methods identified by the standard for extending shelf life of MSDs with short duration exposure to normal room air is the use of a desiccant-based drying cabinet.
Shook and Googelle discuss the unique component handling issues faced during the assembly of PCBs using highly-moisture-sensitive devices.3 Among the points addressed in the paper is the use of dry cabinet containment as a safe and effective means for long-term storage to extend the floor-life survivability of MSDs.
All moisture sensitive components are scored based on their moisture sensitivity level (MSL) using J-STD-020D.1 by the manufacturer. This identifier helps determine amount of time required to reset or pause the floor life clock at the assembly facility.
Previously dry SMD packages, which have been exposed only to ambient conditions not exceeding 30°C/60% RH, may be adequately dried by room temperature desiccation using dry pack or a dry cabinet. If dry pack is used and the total desiccant exposure is not greater than 30 minutes, the original desiccant may be reused.
Placing SMD packages exposed to ambient conditions for greater than one hour in an ultra-low humidity dry cabinet can pause or reset the floor life clock as indicated in Table 1.
Extracting MoistureA recent study6 was performed to determine the amount of moisture extracted within an ultra-low-humidity dry cabinet. The experiment used a worst-case scenario with a superabsorbent (hydroscopic, organic) material.
A total of 2,000 grams of material was soaked in water with a 100% absorption rate and then placed in an ultra-low-humidity dry cabinet. Approximately 2% of the moisture was lost during the transition from soak to entering the cabinet. The remaining materials with 98% of the original moisture content entered the cabinet with an ambient temperature of 62°F (16.6°C) with RH at 37%.
Table 2 indicates the amount of moisture removed from the hydroscopic materials over the next 75 hours. Normal electronics manufacturing facilities expose components and boards to a significantly lower level of ambient RH than that used in the study.Ambient temperature and humidity in an average manufacturing facility can vary at different times of the year. A good example of the differences in outside ambient humidity can be seen by comparing a Long Beach location, with an average of 80% in the morning and 54% in the afternoon, with Los Angeles, where the morning humidity is at 79% yet in the afternoon it only drops to 65%. Long Beach and Los Angeles are separated by a little more than 25 miles.7
Electronics manufacturing is performed in temperature- and humidity-controlled buildings. The controlled environment strives to achieve a constant temperature in the 68°72°F range with 4060% RH.
Given the effectiveness of ultra-low-humidity dry cabinets to extract moisture from objects without subjecting them to a single heating cycle, users can achieve the desired drying times required by J-STD-033B.1. For MSL 4, 5, and 5a with floor life exposure not greater than eight hours, a minimum desiccating period of 10× the exposure time is required to dry the SMD packages enough to reset the floor life clock. This can be accomplished by dry pack or a dry cabinet that is capable of maintaining not greater than 5% RH.
At the same time, IPC/JEDEC only recognizes an active heat method of drying mounted and unmounted SMD packages with a minimum temperature of 40°C and 5% RH. Ultra-low humidity dry cabinets can be effective in circumventing use of this approved reset method by extending the effective floor life of the SMD packages.
Active Drying vs. Dry CabinetAn energy-efficient ultra-low-humidity dry cabinet uses about 56 W, versus a more costly 2.2 kW for a small drying unit. Power consumption for the entire 75-hour study was below that consumed in two hours by a small drying oven.
There are numerous aspects of ultra-low-humidity drying that can positively affect board-level quality. Active heating systems used for drying components and bare boards subject them to temperature cycles. Similar to the issues associated with a standard reflow oven, active drying has the potential to cause package cracking, delamination, aluminum metal deformation, and wire bond failure due to the different thermal coefficients of expansion (CTEs) for all the materials involved in the package.
Active heating to remove moisture requires appropriate ramp-up and cool-down profiles to minimize the thermal shock on components and boards. The time required to return the oven environment to room temperature limits removal of inventory from the chamber. In addition, this action now requires a ramp-up time to return to the drying cycle, exposing the inventory to yet another temperature cycle.
MonitoringSome of today's manufacturing execution software (MES) systems include modules for monitoring, identifying, and alerting users to the time limits placed on moisture sensitive components. Effective use of these tools can prevent potential damage from excessive moisture and allow an assembler to validate that proper storage and handling of critical devices occurred throughout the manufacturing process. Some vendors track the number of cycles an MSD goes through.
ConclusionAn often-overlooked method of reducing board-level defects involves proper handling of components and PCBs. Monitoring and correcting moisture issues can improve product quality and revenue by reducing or eliminating moisture-related defects.
MSD issues can be controlled with a small investment in mobile, ultra-low-humidity dry cabinets placed strategically around the manufacturing floor. Sensitive devices and boards can be placed in the mobile dry cabinets as they are received, moved to the stock room, and finally kitted for the production floor. The same cabinets can be moved to the assembly line with the inventory removed only when needed to reduce the exposure to ambient air, temperatures, and humidity.
REFERENCES: 1. IPC/JEDEC J-STD-020D.1, "Moisture/Reflow Sensitivity Classification for Non-hermetic Solid State Surface Mount Devices," March 2008.2. IPC/JEDEC J-STD-033B.1, "Handling, Packing, Shipping and Use of Moisture/Reflow Sensitive Surface Mount Devices," January 2007.3. "Handling of highly-moisture sensitive components-an analysis of low-humidity containment and baking schedules," Shook, R.L.; Googelle, J/P.; Electronics Packaging Manufacturing, IEEE Transactions, 23-2, Apr 2000, pages 81-86.4. J-STD-033B.1, page 8, Table 4-1: Reference Conditions for Drying Mounted or Unmounted SMD Packages.5. J-STD-033B.1, page 9, (Table 4-3 Resetting or Pausing the "Floor Life" Clock at User Site).6. XDry, Moisture removal experiment, April 2009.7. http://www.cityrating.com/relativehumidity.asp.8. Data from product specifications of the respective vendors.9. Hnatek, Eugene R.; Integrated circuit quality and reliability 2nd edition, revised and expanded, 1995 Marcel Dekker, Inc.
Russell McCarten, quality and applications manager, XDRY Corporation, 8275 South Eastern Ave, Suite 200, Las Vegas, NV 89123, may be contacted at (702) 938-0457 EXT 704; firstname.lastname@example.org.