When people discuss what additional temperatures are needed to test packages with the new lead free finishes, the test matrix can get quite complex. The current U.S Moisture Sensitivity Classification (J-STD-020A) and Handling (J-STD-033) standards are the two specifications with the greatest impact for SMDs. Both standards are maintained under the joint standards efforts of the IPC and JEDEC. (See jedec.org for the latest standards.) The moisture sensitivity standard contains seven levels, as shown in the table. Its purpose is to identify the moisture sensitivity level at a fixed reflow temperature. This identification enables users to store and handle the SMDs to avoid subsequent thermal/mechanical damage during reflow attachment and/or repair operations. Floor Life The lower an SMD package can be classified, the longer its floor life. The goal of all package manufacturers is to achieve Level 1 with their packages-unlimited floor life. The current moisture sensitivity classification spec employs two reflow temperatures, 220°C and 235°C, to classify packages. The packages are classified generally to the lowest level for moisture sensitivity and the highest reflow temperatures possible to give OEMs the largest process window when mounting the package to board. The purpose of J-STD-033, the second standard, is to provide SMD manufacturers and users with methods for handling SMDs. These methods are provided to avoid damage from moisture absorption and exposure to reflow temperatures that can result in yield and reliability degradation.
Both standards are very important and are widely used in the dynamic semiconductor industry, and both are now undergoing the joint revision process. The primary stumbling block to completing the revision relate to determining the next testing temperature. Will it be 240, 245, 250, 255, 260°C or greater? Remember that the higher the reflow temperature, the lower the level for the moisture resistance test. It's also true that the higher the reflow temperature, the greater the cost of materials that must be used to create a reliable package. That explains simply why the lowest possible reflow temperature is being sought. Assemblers and OEMS are under the same pressure as the IC makers, but they are looking for the best process window that will allow them to produce a cost-effective final assembly. With the new lead-free alloys, sporting melting points around 220°C, the same process window would mean that packages require testing between 255-270°C. Traditional cost-effective packaging materials can not achieve these higher temperature ranges, and today's process window range will have to be decreased by at least 25% for lead-free alloys. The challenge is creating new assembly profiles at lower temperatures and tighter process windows to enable traditional (and relatively inexpensive) material to be used for packages and mounting boards in lead-free processes. The JEDEC and IPC task group is working on this problem. They have targeted the end of this year to devise a new temperature (or temperatures) to meet new lead-free processing requirements. Their task is not an easy one, and new process window requirements may obsolete a lot of the existing equipment in the assembly areas. The final outcome carries all the suspense of a real mystery thriller.
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