Circuits Assembly - November 2007 - (Page 44)

Cover Story 4. All sample packages were placed in 44 mm Advantek carrier tape and cover tape. 5. Taped samples were moisture conditioned at 60°C ± 2° at a relative humidity of 60% RH ± 3% for 168 hr., to simulate 30 days exposure to 30°C/60% RH. 6. One lot of taped samples was removed from the tape and weighed to determine its “wet-weight” in grams to the fourth decimal point. 7. All taped samples were baked in the novel apparatus for 24 hr. at 70°C ±0.1°C and ≤ 5%RH. 8. Packages from one sample were removed from the tape. The total dry weight of the packages was determined in grams to the fourth decimal place, to assess the effectiveness of the 70°C novel process. 9. The remaining six samples of taped packages were placed in a temperature/humidity chamber maintained at 30°C ± 2°/60% RH ± 3%. 10. Samples were removed from chamber and subjected to Pb-free reflow per J-STD-020C, one sample per 24 hr. This spanned the six-day factory floor life of the samples. 11. After Pb-free reflow, samples were inspected by C-SAM (reflective mode) to detect any internal package delamination. 12. The longest period of 30°C/60% RH storage showed no sign of internal package delamination after Pb-free reflow was reported as the number of solder reflow safe days gained by the novel baking process. 13. Empty 44 mm tape assemblies were baked in the 70°C novel apparatus for 24 hr., then sent to Advantek for testing, analysis and review. A sample of unbaked, assembled cover and carrier tape was submitted as a control. 14. High-impact polystyrene reels were subjected to 70°C for 24 hr. of convection baking. Reel R-1 was placed vertically, supported on its rims in a 70°C oven. Reel R-2 was horizontally arrayed and axially supported, as it would be in the novel oven. Experimental findings. Temperature is the prime factor in determining the rate that moisture is extracted from a plastic molding compound. The higher the temperature, the more rapidly moisture is expelled from the material. Increased bake duration at a lower baking temperature is not effective in removing moisture from a package. For example: Reducing the temperature of the novel baking cycle to 60°C and increasing the duration to 48 hr. resulted in negligible weight reduction and 100% package delamination after Pb-free reflow. The major portion of the ingested moisture is removed from a package during the first 24 hr. of baking. Moisture loss beyond 24 hrs. of baking continues at a very slow and declining rate. Figure 1 compares the weight of ingested moisture with the weight of moisture removed by the experimental process. It is important to note that the samples were required to be “moisture conditioned” to the level expected for 30 days’ exposure to 30˚C/60% RH. Less ingested moisture would be seen if the samples had been exposed for only six days of floor-life at this atmosphere. The rate of a drying process slows as the package’s moisture content nears the relative humidity within an oven. The novel process uses a vacuum pump to reduce the experimental chamber to ≤100 Torr. Vacuum is the most economical way to lower chamber humidity. Contrary to some claims, vacuum was not found to accelerate the rate that moisture is withdrawn from a plastic package. Also, the amount of moisture removed from a package was consistent, regardless of its position within a reel. Sufficient moisture was driven from the taped packages so they could be exposed to 30°C/60% RH for three days and be Pb-free reflowed without suffering internal delamination (Figure 2). The number of safe reflow soldering days could be extended by placing dried packages in a desiccant cabinet at ≤5% RH immediately after drying with the novel process. A simple and inexpensive experiment would be required to determine the additional solder reflow safe days that the novel process adds to a specific package. Taped assemblies were processed with the novel process and submitted to Advantek to determine how the process affected their function (Figure 3). • Cover tape deformation. No functionally adverse effect resulted from the novel bake. Advantek observed: “The cover tape had taken on some minor wave conditions, visible along the edges of the sealing flanges of the carrier tape. In addition, the cover tape does not appear to be lying as flat on the carrier, as compared with the pre-bake sample. No pocket sidewall warpage was evident and no changes appeared to have occurred that prevent normal removal of a device from a pocket.”6 • Carrier tape deformation. Advantek submitted the 70°C novel baked carrier tape to AOI inspection. They reported: “Key dimensions of the post-bake tape samples were inspected and compared with the (Advantek manufacturing specification). In the case of this square pocket, pre-bake Ao and Bo dimensions are both 25.6 mm. Post-bake measurements ranged from 25.3 mm to 25.35 mm, reflectcircuitsassembly.com Figure 2. C-SAM scan of reflowed packages after 72 hrs. at 30°C/60% RH Storage (courtesy Freescale Semiconductor). Figure 3. Tape after 70°C at 24 hrs. “Thermal-Wall” baking (courtesy Advantek Inc.). Figure 4. Cover tape peel strength test results (courtesy Advantek Inc.). Figure 5. Rim-supported reel failure (Scott Simmons, IDC). Figure 6. Tooling-supported reel (Scott Simmons, IDC). 44 Circuits Assembly NOVEMBER 2007 http://circuitsassembly.com

Table of Contents Feed for the Digital Edition of Circuits Assembly - November 2007

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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