First, the concept
The Environmental Stress Screen (ESS) is a process that accelerates the internal potential defects into faults by applying reasonable environmental and electrical stresses to the electronic products, and discovers and eliminates faults through inspection. .
ESS is designed to motivate and eliminate early failures, bringing product reliability closer to the inherent reliability level of the design. The stress level is based on the principle of inducing defects without damaging the product. ESS is divided into routine screening and quantitative screening. At present, only routine screening is widely carried out in China.
ESS targets components and individual assembly levels.
Second, the typical screening stress
The strengths and costs of various typical screening stresses are listed in the table below.
Table 1 Typical screening stress intensity and cost comparison
Stress type stress intensity cost
Constant temperature, low temperature and low
Temperature cycle slower temperature change is higher
Rapid temperature change
High temperature shock
Vibration sine sweep is moderately moderate
Random vibration
Comprehensive temperature cycling and random vibration are very high
The following is a brief introduction to the above typical screening stresses and compare their screening effects.
Constant high temperature
Also known as high temperature aging, the product operates continuously at specified high temperatures, stimulating early failure. The screening method should be based on whether the screened product is hot or not. This screening is widely used for component screening and is not suitable for component level (printed circuit boards, cells or systems) screening. The screening effect is much lower than the temperature cycle.
2. Temperature cycling
The main parameters affecting the screening results are the temperature variation range, the temperature change rate, and the number of cycles. Usually used for component level filtering. The screening temperature time history considers both the energized and non-energized conditions of the screened product. The main failure modes that are triggered are:
a) Enlarge the various microscopic cracks on the coating, material or thread.
b) Relax the joint with poor adhesion.
c) Let the joints that are screwed or riveted loose.
d) Relax the press-fit joint with insufficient mechanical tension.
e) Increase the quality of the poor solder contact resistance or cause an open circuit.
f) Particle contamination.
3. Temperature shock
The main parameters affecting the screening results are temperature change rate, temperature conversion time and number of temperature shock cycles. Because of its high temperature change rate, the generated thermal stress is large, which is an effective method for screening components, especially integrated circuit devices. Pay attention to the additional damage that may be caused when assembling the grade. The screening effect is similar to the temperature cycle. The fault mode that is excited is similar to the temperature cycle.
4. Sine sweep
In a certain period of time, the resonance frequency of the component that needs to be screened in the product is sequentially excited to generate resonance. The screening effect is lower than random vibration. The main failure modes that are triggered are:
a) Fatigue of structural components, leads or component joints, especially if there are cracks or similar defects on the conductor.
b) Cable wear.
c) The screw joint is slack.
d) Install the improperly processed IC chip away from the socket.
e) The bus bar and the solder joint attached to the board are subject to high stresses, causing weak points in the solder joint.
f) Component leads that are bridged to the components that can move relative to each other are damaged by insufficient stress relief.
g) Cracks in brittle insulation materials that have been damaged or improperly installed.
5. Random vibration
Vibration is applied to the product over a wide frequency range, and the product is simultaneously stressed at different frequencies, causing many of the resonance points of the product to be simultaneously excited. The screening effect is greatly enhanced, and the screening duration is also reduced to 1/3-1/5 of the sine sweep. The fault mode of the excitation is the same as that of the sine sweep, but the fault mechanism that can be excited is more complicated, and the fault is triggered faster.
6. Comparison of screening effects
a) Vibration stress is more effective in exciting process defects such as slack, debris and tight fixation. In general, about 20% of the defects found are the result of a response to vibration.
b) Temperature cycling is more effective for finding component defects such as parameter drift and contamination. In general, about 80% of defects are found to be the result of temperature response.
Third, ESS for different assembly levels
According to Fig. 1, the assembly level is divided into a component level, a unit level, and a system (device) level. The advantages and disadvantages of the component level and the unit level and the main defects that can be screened are described below.
Component level
Advantages: small size, no power test, low cost; low thermal inertia, high temperature change rate; can be screened more than the temperature change of the unit level, and the screening efficiency is high.
Disadvantages: Failure to electrify leads to low efficiency in finding faults; if power-on detection is required, special equipment needs to be tested, and the cost will increase; it is impossible to screen defects introduced at the unit level and system level.
Defects: missing component defects; component and board solder joint separation; soldering defects; parameter drift, out of tolerance.
2. Unit level and above assembly level
Advantages: Because there is no need to specially design the detection system, it is easy to conduct power and detection during screening, and the detection efficiency is high; the interface between the various levels in the unit can also be screened; various types of potential defects can be screened; some design defects can be found .
Disadvantages: large thermal inertia, small temperature change rate, temperature cycle time need to be lengthened; the temperature limit of various components should be considered, the screening temperature change range is reduced, the screening rate is reduced, and the cost is high.
Defects: Component-level connection defects; wire connection defects; input/output connection defects; internal cable defects in the unit; defects introduced by previously unscreened components (such as purchased components, etc.) are used in the assembly.
Summary: In theory, each assembly level should be gradually ESS. In actual work, we must weigh the technical and cost factors, and determine the best screening solution based on the product. If the fault is mainly present at an assembly level based on product usage and other test information, the ESS can only be performed for that assembly level. A more practical method is to perform temperature cycle screening on the components and perform random vibration tests on the above units.
Fourth, the main work
ESS is an important process to improve the reliability of batch products and is a prerequisite for other reliability tests. The follow-up work is as follows.
1. Perform statistical analysis on various types of fault information of the products to be screened, and determine as much as possible the assembly level of the fault. The fault information mainly includes:
1) Fault information during the development phase, especially the distribution and quantity of fault types, and the location of the fault location.
2) Failure information in various types of tests, such as performance tests, environmental tests, reliability tests, etc.
3) Fault information during product use.
2. Collect the characteristic information of the temperature and vibration response of the screened products, including the hot spots of the important components in the product, the temperature stabilization time, the resonance point of the product, and the part with the largest vibration response in the product.
3. Collect information on the quality levels of components and other external components used in the product. It mainly includes whether to carry out ESS before the factory, ESS screening stress used in batch production, and so on.
4. Based on the information collected above, the ESS outline is preliminarily designed by weighing the ESS effect, test equipment capability and cost.
5. Appropriate adjustments to the outline in accordance with the prescribed design guidelines and adjustments in real time based on actual conditions make the ESS more cost effective.
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