1. Basic Concepts in Reliability Design - nl3prc

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[2] Designing for ReliabilityFigure 3.20 correlates τ, described above, and the rate of Al corrosion. Al corrosion diminishes as τbecomes larger.10 550% failure at 85°C/85% BT (h)10 4 85 °C/10 385%BTPCTBT10 310 210 2 1010 2 10 3 10 4τ at 100°C (minimum)50% failure at PCT BT (h)PCT: pressure cooker testFigure 3.20Correlation between time constant of channel leak and Al corrosion athigh temperatureFigure 3.21 shows the temperature dependency of τ. The activation energy is estimated to beapproximately 0.9 eV to 1.0 eV, which conforms to the temperature dependency of bulk resistanceof resin shown in Figure 3.22. Figure 3.23 shows the degradation of bulk resistance due tomoisture absorption. It clearly indicates that bulk resistance degrades as an exponential functionof moisture absorption.From the above, it can be said that high-temperature bulk resistance of resin and bulk resistancedegradation by temperature and humidity are the important parameters in estimating resinreliability.In addition, the adhesive property between the chip surface and metal, moisture permeability andmoisture absorption are also factors with a bearing on the moisture resistance of the resin. Ingeneral, silicone resin features high moisture permeability, low moisture adsorption, goodadhesion to the chip surface and poor adhesion to the metal. Epoxy resin has properties almostopposite to that of silicone resin. The drawbacks of epoxy resin are gradually disappearing asimprovements continue to be made.(S)1000100101C Pch0.99 eVB NchC Nch1.02 eV1.01 eVB Pch0.97 eVA Nch0.98 eVA Pch0.88 eV0.12.4 2.6 2.8 3.0I/T10 3 (°K 1 )Figure 3.21 Temperature dependency of time constant τ2-34
[2] Designing for Reliability100001000r(S)10010C 1.07 eVA 1.07 eVB 0.99 eV12.4 2.6 2.8 3.0I/T10 3 (°K 1 )Figure 3.22 Temperature dependency of time constant (τ R ) for different discharge characteristics10 1410 13RB at 100°C10 12C10 11 ResinA0 0.1 0.2 0.3 0.4 (%)Figure 3.23 Absorbed moisture dependency of bulk resistanceB(2) Various Problems Caused by Mold Resin StressResin used for semiconductor encapsulation contracts due to resin polymerization, applyingconsiderable stress to the semiconductor chip in contact with the resin. Consequently, resistorvalues in the chip fluctuate due to piezo-resistance, greatly affecting device characteristics. Stressalso causes Al slide and passivation cracks. An experiment to determine the stress generated in aplastic-encapsulated silicon chip is discussed below. 23)Stress is measured on TEG devices which have resistors constructed on the silicon chip as shownin expression 3.1. A general formula for the piezo-resistance effect is:⎛ ⎞ 6δρ i = ⎜ ⎟ i = ∑i=1 πij⋅ τj3.1⎝ R ⎠where δρ i is the resistance change rate, πíj is the piezo-resistance coefficient and τj is the stress.For τj, the following applies:1= x , 2= y , 3= z4= yz , 5= zx , 6= xy3.22-35
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