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Mumtaz Y. Bora, Peregrine Semiconductor, defines MSDs, relevant standards, and where the industry is headed in the handling of moisture sensitive components and circuit boards. The IPC D-35 committee is working to release a standard for moisture sensitivity control guidelines for PWBs.
Moisture sensitive devices (MSD) are electronic components, encapsulated with epoxy molding compounds, that contain other organic materials inside the package such as die attach adhesives and epoxy-glass substrates. Moisture and humidity from the ambient environment enters permeable package by diffusion. The moisture preferentially collects at dissimilar material interfaces (Figure 1).
Figure 1. Package internal interfaces.
When assemblies go through reflow, this trapped moisture expands and vaporizes. As it tries to escape, the vaporized moisture causes either cracking of the package or delamination of interfaces with the package. These defects can pass undetected through inspection and test processes and result in latent failures in the field.
The JEDEC JC.14.1 committee has jointly published the following standards with IPC:IPC/JEDEC J-STD-020D.1, “Moisture/Reflow Sensitivity Classification for Non-Hermetic Solid State Surface Mount Devices,” published in March 2008, applies to component manufacturers in conducting the moisture classification level of packages.
IPC/JEDEC J-STD-0330B.1, “Handling, Packing, Shipping and Use of Moisture/Reflow Sensitive Surface Mount Devices,” published in January 2007, applies to assembly facilities for handling, storage, and control of floor life for moisture sensitive packages.
The rise of lead-free reflow has made handling of moisture sensitive packages even more critical, as the peak reflow temperatures are much higher (245° to 260°C ) as compared to eutectic reflow (205° to 230°C). The higher reflow temperatures also affect the temperature-sensitivity of some non-IC packages, such as capacitors, inductors, LEDs, and others. J-STD-075, “Classification of Non-IC Electronic Components for Assembly Processes,” was published to address the classification of non-IC packages.
Printed wiring boards (PWB) used for assembly are also moisture sensitive commodities. Proper handling and storage of PWBs is essential to good assembly yields and minimization of delamination issues. Inadequate temperature and humidity control in storage and on the production floor can result in delamination and blistering issues during reflow. Examples of these defects are shown in Figures 2 and 3.
Figure 2. PWB delamination.Figure 3. Blistering of PWB.MSD Component Handling Guidelines
Understand the flow of your supply chain and the staging areas. Reels opened for IQC should be sealed immediately after inspection.
Ensure proper storage of MSD components in controlled environment in warehouses, staging areas, and the production floor.
Designate a log or automated system for monitoring time in/time out for all component reels. This will help to monitor a component’s floor life, or how long it can be in the ambient environment without adverse effects at reflow.
Partial reels remaining after placement operations should be properly labeled with MSL class before returning to stock.
Provide training to manufacturing personnel for monitoring floor life and conducting bake operations when floor life is exceeded. Provide training updates each time the standards are revised.
For assemblies going through double-sided reflow, maintain a time limit (4–6 hours) between side A and side B reflow.
In case of line down time or machine malfunction or a line changeover situation, all MSD packages should be sealed in moisture barrier bags and reopened when the assembly line is ready to run. The same practice should be followed at rework stations.
Conduct audits of the warehouse and production floor to ensure temperature and humidity meet the guidelines of J-STD-033B.1 (30°C/60% relative humidity — RH). If these conditions change, refer to the MSL derating chart in the appendix of the standard.
PWB Handling Guidelines
Proper guidelines for PWB material selection should be followed based on the type of reflow process. For both mid-Tg (glass transition temperature of 150°C) and high-Tg (glass transition temperature of 170°C or above) materials, the glass decomposition temperature (Td) should be taken into consideration to minimize delamination during lead free reflow. Typical guidelines for Td are 310°C or above.
PWBs should be baked dry and packaged in moisture barrier bags with desiccant prior to shipment to assembly facilities.
Adequate controls for storage and handling at the warehouse and on the production floor are essential to achieving good yields at the assembly facility.
Typical guidelines for PWB shelf life state 6 months. Storage beyond shelf life may require baking prior to reflow. Practice first-in/first-out (FIFO) inventory management to minimize the instances of PWBs exceeding shelf life.
A PCB’s surface finish should be taken into consideration for baking, as some finishes such as organic solderability preservative (OSP), immersion silver (ImmAg), and immersion tin (ImmSn) are sensitive to bake cycles. Baking can result in oxidation of the surface finish and impact solderability.
The IPC D-35 committee is working to release a standard for moisture sensitivity control guidelines for PWBs. IPC1601, “Printed Board Handling and Storage Guidelines,” is currently in a draft version.
For questions regarding handling of MSD components, access the MSD Council via the Surface Mount Technology Association (SMTA) website at www.smta.org. The goals of the MSD council are:• To expand awareness through on-line information, educational programs, webcasts and tutorials;• To advance the understanding and practice of MSD control in global supply chain;• To provide standards updates at the MSD Council Meeting at SMTA International; and • To conduct MSD webcasts on assembly needs.