1. Basic Concepts in Reliability Design - nl3prc

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[2] Designing for ReliabilityThe following is a discussion of the reliability of surface-mount packages in relation to moistureabsorption and moisture removal characteristics, and package cracks due to soldering heat.(a) Package Moisture Absorption and Moisture RemovalResin used for plastic-encapsulated devices is basically porous and has some moisturepermeability. In the case of SMDs, in areas where the mold resin is especially thin moisturecan pose a significant reliability problem. This can occur during soldering when moisture,absorbed into the package, evaporates with a sudden rise in temperature, causing thepackage itself to expand or gaps to form between the lead frame and resin. There is thereforea close relationship between the amount of moisture absorbed by the SMD package and itsreliability after soldering. The following discusses moisture absorption and moisture removalin an SMD.• Moisture AbsorptionFigure 3.26 shows the moisture absorption characteristics of an 80-pin QFP in a shelfenvironment (with constant temperature and varying relative humidity). The horizontalaxis indicates the time that the device remains on the shelf, and the vertical axis indicatesthe rate of change in the amount of moisture absorbed. The rate of change in moistureabsorption is a percentage value given by dividing the amount of moisture absorbed by theweight of the package at the start of the shelf test. This diagram reveals that, when thetemperature is low, more time is required for the saturation region to be reached; and whenthe relative humidity is low, the amount of moisture absorbed at saturation is smaller.Figure 3.27 shows comparative data for the moisture absorption characteristics of an 80-pin QFP (with a mold resin thickness of 2.7 mm) and a 20-pin SOP (with a mold resinthickness of 1.5 mm) in a shelf environment (at a temperature of 85°C and relativehumidity of 85%RH). As evident from this diagram, the time to saturated moistureabsorption varies with the package size. When real products are being tested for the effectsof soldering heat, the products under test are forced to absorb moisture prior to testing, sothat each package can be evaluated with different moisture absorption times.Rate of change ofmoisture absorption (wt%)1.00.185°C/85%RH85°C/60%RH85°C/40%RH30°C/60%RH0.011 10 100Time on shelf (h)1000Figure 3.26 Moisture absorption characteristics of 80-pin QFPs (2.7 mm thick) in variousenvironments2-38
[2] Designing for ReliabilityRate of change of moistureabsorption (wt%)1.00.1QFP80 (2.7mm)SOP20 (1.5mm)0.011 10 100Time on shelf (h)1000Figure 3.27 Comparison of moisture absorption characteristics of packages at 85°C, 85% RH• Moisture RemovalDevices with a large package or large chip size, or those housed in a thin package, aresubject to limitations with respect to their soldering mount methods. This is because if thepackage contains moisture, soldering heat during mounting damages the package asdescribed above. To prevent this problem, the device must be baked to remove internalmoisture before it is soldered to the board.Figure 3.28 shows moisture removal characteristics for an 80-pin QFP (with mold resinthickness 2.7 mm) in a shelf environment. The horizontal axis is the time the device is lefton the shelf, and the vertical axis is the rate of change of residual moisture content in thepackage, expressed as wt%. The diagram shows that even a package that has becomesaturated with absorbed moisture can have almost all moisture removed by baking at125°C for 20 hours. Moisture removal characteristics are similar to those of moistureabsorption in that the higher the temperature, the less time required for moisture removal,with residual moisture after baking approaching 0%. However, note that when removingmoisture from devices, the baking temperature is subject to limitations depending on thethermal resistance of the tray and the device terminals’ soldering properties. Before baking,refer to the instructions on the product packaging material or contact the manufacturer.Note that heat-resistant trays are marked “HEAT PROOF”, meaning that they cannormally resist temperatures of up to 125°C.2-39
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