0 foreword
Due to its structure, materials and other factors, plastic encapsulated microcircuits were initially considered to be products prone to failure. At that time, these factors limited the use of plastic encapsulated microcircuits in high stress and high reliability environments. In particular, the reliability of plastic packaging is lower than that of air tight packaging for the following reasons:
——The coefficient of thermal expansion (CTE) of materials used in plastic encapsulated microcircuits varies greatly, which leads to temperature related problems;
——Plastic encapsulated microcircuits will absorb moisture, leading to chip corrosion and package cracking (popcorn effect).
Before the 1980s, the failure of plastic encapsulated microcircuits was a common phenomenon due to moisture intrusion, corrosion, cracking and delamination. However, by the end of 1980s, due to the great progress made in plastics, molding technology and chip processing, the early failure of plastic encapsulated microcircuits had been basically solved.
1. Test procedure
The following new and commonly used environmental tests can be used to evaluate the environmental resistance of plastic encapsulated microcircuits;
1.1 temperature evaluation
In general, high temperature testing can accelerate chemical degradation due to improper material combinations during manufacturing and contamination within the package. For military applications, the temperature range from -55 ℃ to 125 ℃ is suitable for evaluating the functionality of plastic encapsulated microcircuits. High temperature can also reduce the residual mechanical stress in the circuit metal.
1.2 temperature cycling and thermal shock
Air air conditioning medium is used for temperature circulation, and it takes several minutes to switch between the media. In terms of thermal shock, liquid liquid medium can provide a harsh temperature shock environment, and no downtime is required when switching between two temperature extremes and at room temperature.
For the above two applications, military equipment generally needs a temperature range of -65~ 150 ℃. Temperature cycling and thermal shock can accelerate the following failure mechanisms:
——Poor bonding;
——Material thermal mismatch, such as chip package interface
——Poor sealing of cover in air tight package;
——Poor plastic packaging or material vulcanization, such as epoxy resin chip bonding;
——Chip cracking or improper substrate installation;
1.3 stability baking
When the electrical non bias plastic microcircuit is subjected to ambient temperature conditions, it is necessary to bake its stability. This procedure accelerates failure mechanisms such as metallization defects, corrosion, surface instability or contamination, package defects, and plating defects. Typical baking is carried out at 125 ℃ according to the requirements of mil-std--883 method 1008 condition B.
1.4 high accelerated stress test
High accelerated stress test (HAST) is a pressurized moisture resistance test. It puts the moisture pressure into the plastic enclosure, and allows the sample to withstand static electrical bias conditions under typical working voltage and current load. According to JEDEC standard 22-a100, typical applications include the temperature range of 105~140 ℃, 85%rh, steam pressure range of 17.6~44.5psia, and duration of 25~200 hours.
High accelerated stress tests can generally identify failure mechanisms such as package defects, passivation and weak metallization. This kind of test can be carried out under both non bias and power cycle conditions.
1.5 salt spray test
Salt spray test is used to evaluate the external coating to simulate the influence of coastal atmosphere. As specified in mil-std-883e method 1009, salt spray conditioning is generally performed at a temperature of 95 ± 5 ℃, and the duration of exposure is 24 ~ 240 hours.
1.6 aging
Aging refers to the artificial aging used to improve the acceptability of electronic components and reduce the failure rate. In static aging, DC bias voltage is applied to many device junctions at high temperature. This method is helpful to identify ion contamination, inversion, channel formation, oxide defects, metallization defects and thermally activated surface defects. The objective of the dynamic aging method is the same as that of the static aging method, which is also used to identify the above defects. However, during dynamic aging, voltage pulses or sinusoidal pulses are applied to the
On the input, the output is measured according to time or instantaneous voltage. According to the requirements of mil--std-883 method 1015 conditions A-E, the aging temperature should be 125 "C. the common practice is to apply a temperature not exceeding the maximum operating ambient temperature of the device to reduce the artificial aging strength when evaluating defects.
1.7 moisture resistance test / moisture induced stress sensitivity
Moisture resistance test is to test the device in high temperature and high humidity environment. This test can identify the sensitivity of the device to the stress caused by moisture, so that the device can be properly packaged, stored and handled to avoid mechanical damage. As pointed out in JEDEC standard jesd22--a112, the typical temperature and humidity for this test are 85 ℃ and 85%rh.
1.8 pressure cooker test
Pressure cooker test (or accelerated moisture resistance test) uses harsh pressure, humidity and temperature conditions to accelerate moisture intrusion into the device to evaluate the moisture resistance of the device. As pointed out in JEDEC standard jesd22- a102-b, typical test conditions are: temperature 121 ± 1 ℃, relative humidity 100%rh, steam pressure 15 ± 1psig, and shutdown time limit 24 ~ 336 hours.
2. Conclusion
The environmental reliability test of plastic encapsulated microcircuits will continue to be an important part of the evaluation of military and aerospace applications. They also illustrate how environmental testing plays an important role in ensuring that plastic encapsulated microcircuits can be used in harsh environments. Plastic encapsulated microcircuits will continue to play an important role in these industrial sectors because of their strong structure, small size, light weight, low vulnerability and lower cost than ceramics. Moreover, its solid structure can easily withstand the influence of mechanical impact, vibration and centrifugal force. When taking advantage of the above benefits, military designers and engineers should also pay attention to potential risks, so appropriate preventive measures must be taken.