Highly accelerated moisture tests

Highly accelerated moisture tests (HAST) were performed in a two-zone Hirayama pressure chamber in which samples were exposed to 145°C, 85% R.H. and 18V bias. Periodically, the devices were removed from the pressure chamber and the linear devices were tested for electrical performance in a Gen Rad 1731M Linear IC Test System. This instrument monitors several device parameters. All of these parameters remained within specifications until the devices failed due to open circuitry resulting from corrosion. Earlier tests were performed at 131°C, 85% R.H. and 30V bias (2). Under these conditions the test requires long times. Short test times are readily achieved at 145°C (6). 

Testing of encapsulated devices was done by thermal cycling and by accelerated moisture testing at elevated temperatures, high humidity and bias ( ). 

Figure 6 shows the performance of encapsulated devices under the highly accelerated stress test which utilizes the following conditions 145°C, 85% relative humidity, 37 psig, and electrical bias. The bias on this test is cycled specifically, the bias is initially off to allow for full moisture penetration and then turned on. These cycles are repeated throughout the test. The stable bromine CEN compound took 2500 hours to reach 50% failure rate, compared to the standard resin compound at 1900 hours. 

Industrial atmospheres usually accelerate the corrosion of zinc. When heavy mists and dews occur in these areas, they are contaminated with considerable amounts of acid substances such as sulphur dioxide, and the film of moisture covering the metal can be quite acid and can have a pH as low as 3. Under these conditions the zinc is dissolved but, as the corrosion proceeds, the pH rises, and when it has reached a sufficiently high level basic salts are once more formed and provide further protection for the metal. These are usually the basic carbonate but may sometimes be a basic sulphate. As soon as the pH of the moisture film falls again, owing to the solution of acid gases, the protective film dissolves and renewed attack on the metal occurs. Hudson and Stanners conducted tests at various locations in order to determine the effect of atmospheric pollution on the rate of corrosion of steel and zinc. Their figures for zinc are given in Table 4.34 and clearly show the effect which industrial contamination has on the corrosion rate. 

Metal specimens coated with the various blends were immersed in water in an accelerated test to determine the effect of high humidity. Cellulose acetate butyrate blends containing carboxylated polyesters prepared with hexahydroterephthalic acid [H(NPG), T50H(NPG)] were particularly susceptible to moisture and failed the adhesion test after immersion for only 0.5 hour similar blends containing T50I(NPG) extended with dianhydride Via or with PMDA passed the adhesion test after immersion for 16 hours. When coatings on cold-rolled steel of cellulose acetate butyrate (EAB-381-0.5) blends containing 1% of each of the... 

A penetrating dye-solution test was used in order to determine whether the foil packages permitted any transmission of moisture. High humidity accelerated the physical deterioration of effervescent tablets when stored in packets of poor integrity. ... 

The spray formulation appears to be nontoxic because both fish and algae grow readily in the ponds. The pH level of the water in the ponds has remained constant, which indicates that bacteria are not converting sulfur to sulfuric acid. Samples of the formulation were exposed to bacterial attack in a controlled environmental chamber maintained at a high moisture level to accelerate the rate of attack. No evidence of bacterial attack on the samples has been detected after 1 yr of exposure under aerobic conditions. Initial corrosion tests showed that samples of the formulation did not lose weight after exposure in 1 and 5% sulfuric acid solutions for 4 months. 

Some accelerated tests are surprisingly simple and intended to give only highly qualitative information, while others have been formulated into standard tests intended to yield more quantitative results. Since heat and moisture, to which adhesive joints are commonly exposed, are environmental factors known to greatly influence adhesive durability, most accelerated tests involve these two agents. 

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