1. Basic Concepts in Reliability Design - nl3prc

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[2] Designing for ReliabilityDiffusion penetration in the resinMoisture permeationof the interfaceResinMoisture penetration routes in a molded IC. In an epoxy resin IC, watermolecules absorbed via the resin surface penetrate the resin by diffusion. Forthis reason, the relative humidity in the vicinity of the IC is closely associatedwith the internal moisture content.Lead frameSemiconductor chipFigure 3.14 Plastic-encapsulated device(3) Dependence on applied biasAs a result of temperature and humidity tests (80°C, 90% RH for the PSG passivation TEG withbias varied at 5 V, 10 V, 15 V, 20 V and 25 V), dependence of MTF on applied voltage was obtainedas shown in Figure 3.15. It is apparent that the MTF decreases with increased bias voltage.Al corrosion due to electrochemical reaction has different modes depending on the bias polarity.That is, the failure mechanism varies with the polarity. The biased terminal with the higherpotential is called the “anode” and the terminal with the lower potential is called the “cathode.” Alcorrosion occurring on the anode and cathode sides is referred to as “anodic corrosion” and“cathodic corrosion” respectively. Cathodic corrosion is predominant in PSG passivation with Aland Al-Si metals; however, cracks or pinholes in the passivation can cause anodic corrosion due toimpurity ions (for example Cl − ).Cathodic corrosion occurs, in general, in the crystal grain boundary and appears dark whenobserved through an optical microscope. On the other hand, anodic corrosion is accompanied by asignificant expansion of Al, sometimes causing cracks in the passivation which can propagate. Insome instances, it appears as if the Al is missing when the device is observed under an opticalmicroscope; however, it sometimes remains as transparent Al 2 O 3 , and can be identified usingtechniques such as electron probe microanalysis (EPMA) or Auger electron spectroscopy (AES).10,00080°C − 90% RHMTF (hours)10001000 5 10 15 20 25 30Applied bias (V)Figure 3.15 Dependence of MTF on bias voltage in THB test2-26
[2] Designing for Reliability(4) Dependence of PSG passivation on phosphorous concentrationIt was previously described how, in order to subject the external ions to the gettering effect, PSGfilm containing phosphorous is used for top passivation. However, an excessively highconcentration of phosphorous will greatly increase the possibility of fatal Al corrosion.16) 17)Phosphorous-related corrosion is cathodic for Al or Al-Si metal and occurs as follows:First, if the PSG absorbs moisture, P 2 O 5 in the PSG is liquefied to form phosphoric acid,increasing the ionic concentration. H + ions are attracted to the surface of the Al metal on thecathode side, allowing corrosion to progress according to the following reactions:Al + 3H + →Al 3+ + 3/2H 2 ↑Al 3 + 3OH − →Al (OH) 3andAl (OH) 3 + OH − →AlO 2 + 2H 2 OFigure 3.16 shows relative lifetime values for TEG devices and LSIs versus phosphorousconcentration in PSG, which causes cathodic corrosion. It shows that lifetime shift is veryresponsive to changes in phosphorous concentration. Recently, however, this type of failure hasvirtually been eliminated because moisture resistance has been improved by using films intowhich water cannot permeate (e.g. SiN films) for top passivation.4.03.0Controlledvalue2.0Relative lifetime1.00.70.50.30.20.1LSITEG0 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8Relative phosphorous concentrationFigure 3.16 Dependence of relative lifetime on phosphorous concentration due to itsinfluence on Al metal corrosion (experimentally controlled)(5) Other unexpected eventsIn addition to the above items, factors contributing to Al corrosion include some that areattributable to the manufacturing process, such as defects and ionic contamination of toppassivation, seal leaks in hermetically sealed devices, and improper handling or soldering bycustomers. Soldering flux, which contains Cl and water, can penetrate the plastic and corrodemetal areas of the chip.2-27
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