Circuits Assembly - November 2007 - (Page 42)

Cover Story An Alternative Drying Process for MSDs Charles S. Leech Jr. Components baked at 70˚C for 24 hr. in a vacuum passed IPC and EIA testing. Ed.: The complete article is at circuitsassembly.com. -STD-033B places a costly baking burden on the assembler. To mitigate this, it permits “alternative drying processes” with the following note: “Table 4-1 is based on worst case molded leadframe SMD packages. Users may reduce the actual bake time if technically justified (e.g., absorption/desorption data, etc.).” For lack of a better term, these alternative drying processes “conform” to J-STD-033B. In 1994, a major cellphone maker internally reported success in driving sufficient moisture from taped MSD packages to prevent delamination at solder reflow.4 The company baked components at 70˚C in a vacuum for 24 hr. The firm claims to have dried more than 10 million packages without experiencing package delamination using this method. If true, this process is a vast improvement over the J-STD-033B drying processes. The report made several suspect conclusions. Yet, in light of today’s more demanding requirements, we investigated a similar process. In our design of experiments, we worked closely with key members of the IPC/JEDEC subcommittee who authored J-STD-033B. We wanted to ensure specifiFigure 1. Moisture removal (courtesy Freescale Semiconductor). cation conformance. J Vendor-generated data are commonly viewed with suspicion. For this reason, we limited involvement in the experiment to creating the DoE, supplying sample materials, funding transportation and some services. Independent industry organizations performed all experimental procedures. Participants were Freescale Semiconductor, Advantek Inc., Solectron-Texas, Priority Labs, ST Microelectronics and Electro-Comp Taping Systems. ‘Proof of Process’ Experiment goals. We established the following goals: 1. The experimental process must conform to J-STD-033B criteria for an alternative package drying process. 2. The novel drying process need not fully reset the J-STD-033B specified package “floor-life clock.” However, it must remove sufficient moisture from the taped or tubed packages to prevent package delamination during Pb-free solder reflow. 3. The experimental process must not: • Significantly exceed cover tape peel strength specifications.5 • Functionally deform carrier or cover tapes. • Deform commercial high-impact polystyrene reels. Experimental samples. Tables 1 and 2 identify the experimental components and carriers. DoE. 1. All sample packages were initially inspected by a C-mode acoustical microscope (reflective mode) to ensure they were free of delamination. 2. All samples were baked in a convection oven at 125°C for 72 hr. to drive moisture from the packages. 3. The “dry-weight” of each sample was determined in grams to the fourth decimal place. circuitsassembly.com 42 Circuits Assembly NOVEMBER 2007 http://circuitsassembly.com http://circuitsassembly.com

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Circuits Assembly - November 2007 Contents Caveat Lector Industry News Market Watch Talking Heads Focus on Business Global Sourcing On the Forefront Screen Printing Better Manufacturing Pb-Free Manufacturing from a Tier III EMS Perspective ‘Living Documents, Subject to Change’ Creating Ideal Solder Joints An Alternative Drying Process for MSDs ‘Customer Satisfaction is More than a Score’ Optoelectronic Substrates: Will They Happen? Tech Tips Wave Soldering Process Doctor Test and Inspection Getting Lean Materials World Equipment Advances Product Spotlight Assembly Insider Technical Abstracts Ad Index

Circuits Assembly - November 2007

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