Application of Environmental Stress Screening Chamber

Description: Stress Screening is the product in the design strength limit, the use of acceleration techniques plus environmental stress, such as: burn in, temperature cycling (temperature cycling), random vibration (random vibration), open and close the cycle (power cycle)....

Through the environmental stress screening test chamber and other equipment to accelerate the stress to make the latent defects in the product emerge [potential parts material defects, design defects, manufacturing defects, process defects], and eliminate electronic or mechanical residual stress, as well as eliminating the stray capacitance of the multilayer circuit boards, the bathtub curve in the early and early death of the product in the early stage of the phase of the elimination and repair, so that the product through the appropriate screening, preservation of bathtub Curve of the normal period and the decline of the product. In order to avoid the product in the use of the process, by the environmental stress of the test and lead to failure, resulting in unnecessary losses, although the use of ESS stress screening will increase the cost and time, but for the improvement of product yields and reduce the number of return to the maintenance, there is a significant effect on the total cost will be reduced, in addition to the customer's trust will be enhanced, generally for the Electronic parts of the stress screening method of pre-burning, temperature cycling, high temperature, low temperature, PCB printed circuit board stress screening method for temperature cycling, for the cost of electronic stress screening for: power pre-burning, temperature cycling, random vibration, in addition to the stress screening itself is a process stage of the process, rather than a test, screening is 100% of the product program.

Environmental stress screening test chamber in this process is widely used in automated temperature-variable stress loading, to ensure that the screening efficiency consistency.

Thermal shock test chamber and temperature cycling, although the same means of accelerating environmental stress, but the cold and thermal shock simulation is an “instantaneous jump in the extreme temperature difference”, the temperature difference ≥ 100 ℃, the conversion time of < 1 minute, mainly to simulate the extreme conditions of rapid thermal fatigue, you can refer to: What for thermal shock test chamber ? And the role of the temperature Cycling more emphasis on the speed of temperature change and gradual process (such as ± 15 ℃ / min or less), corresponding to the material interface thermal strain accumulation failure; both can be combined with the thermal stress screening test chamber operation mode use.

Stress screening applicable product stage: R & D stage, mass production stage, before leaving the factory (screening test can be carried out in components, devices, connectors and other products or the whole system, according to the requirements of different screening stress can be different)

Stress Screening Comparison:

a. Constant high temperature burn-in (Burn in) stress screening, is currently used in the electronics IT industry to analyze the electronic component defects, but this approach is less suitable for screening parts (PCB, IC, resistors, capacitors), according to statistics in the United States the number of companies using temperature cycling screening parts than the use of constant high temperature burn-in on the number of companies screened more than 5 times the number of components. Thermal stress screening test chamber can realize rapid cycle control.

b. GJB / DZ34 that the temperature cycle and random vibration screening out the proportion of defects, temperature accounts for about 80%, vibration accounts for about 20% of a variety of products screened out defects in the sub-case.

c. The United States has 42 enterprises to investigate statistics, random vibration stress can be screened out 15 to 25% of the defects, while the temperature cycle can be screened out 75 to 85%, if the combination of the two can reach 90%.

d. The proportion of product defect types detected by temperature cycling: insufficient design margin: 5%, production workmanship errors: 33%, defective parts: 62%. Thermal shock testing is suitable for products that are sensitive to rapid thermal stress, such as spacecraft devices, transistors, wafers, etc., while temperature cycling is commonly used for power supply boards, welded structures, etc., and the combination of using a thermal stress screening tester can improve the screening efficiency.

Temperature Cycling Stress Screening Failure Indication:

Temperature cycling induces product failures due to alternating expansion and contraction of the product as the temperature cycles within the upper and lower extreme temperatures, causing thermal stresses and strains to develop in the product. These thermal stresses and strains will be more dramatic if there is a transient thermal gradient (temperature inhomogeneity) within the product, or if the coefficients of thermal expansion of neighboring materials within the product do not match each other. These stresses and strains are greatest at the defects, and this cycling causes the defects to grow and eventually become large enough to cause structural failures and produce electrical failures. For example, a cracked plated through-hole will eventually crack completely around it, causing an open circuit. Thermal cycling causes plated through holes in soldered and printed circuit boards.... Temperature cycling stress screening is especially suitable for electronic products with printed circuit board structures, and the temperature control accuracy of the TEMAK environmental stress screening tester is better than ±1°C, which meets the requirements for simulation of the above defect-inducing mechanisms.

Failure modes or effects on the product triggered by temperature loops are as follows:

a. Enlargement of various microscopic cracks in coatings, materials or threads

b. Slackening of poorly bonded joints

c. Slackening of poorly screwed or riveted joints

d. Slackening of press-fit joints with insufficient mechanical tension

e. Increase the contact resistance of poor-quality solder joints or cause open circuits.

f. Particle and chemical contamination

g. Seal failure

h. Packaging problems, such as linking of protective coatings

i. Transformers and coils shorted or broken.

j. Defective potentiometers

k. Poor soldering and fusion joint connections

l. Cold soldered joints

m.Multilayer boards open or shorted due to improper handling

n.Power transistor short circuit

o.Bad capacitors and transistors

p.Broken double-row accumulator circuits

q. Due to the destruction or improper assembly, resulting in almost short-circuited cartridges or cables

r.Material breakage, rupture, scoring due to improper handling.... etc.

s.Superior parts and materials

t. Resistors ruptured due to lack of synthetic rubber cushion coating.

u.Cracks in the metal strip grounded to the transistor.

v. Mica insulating gasket cracked, causing short circuit of transistor.

w. Irregular output due to improper fixing of the metal tabs of the tuning coil

x. Internal open circuit of bipolar vacuum tube at low temperature

y.Indirect short circuit of the coil

z.Ungrounded terminals

a1.Component parameter drift

a2.Improper installation of components

a3. Misuse of components

a4. Seal failure

Introduction of stress parameters for temperature cycling stress screening:

The stress parameters for temperature cycling stress screening are mainly the following: high and low temperature extreme value range, residence time, temperature change rate, and number of cycles.

Temperature extreme range: the larger the temperature extreme range, the fewer the number of cycles required, the lower the cost, but not more than the limit of the product can withstand, not to cause new failures as a principle, the difference between the upper and lower limits of the temperature change should not be less than 88 ° C, the typical range of change is -54 ° C to 55 ° C.

Residence time: in addition to the residence time can not be too short, otherwise it is too late for the products to be tested to produce the heat rise and cold shrinkage stress change, as for the residence time, it should not be too short, otherwise it is too late to make the products to be tested to produce heat rise and cold shrinkage stress change. As for the residence time, it varies from product to product, so please refer to the relevant specifications.

Cycle number: As for the number of cycles of temperature cycling stress screening, but also consider the product characteristics, complexity, temperature upper and lower limits as well as the screening rate in the determination of the number of screening should not be more than, otherwise it will allow the product to produce unnecessary harm, but also can not improve the screening rate, the number of temperature cycles from 1 to 10 cycles [ordinary screening, a screening] to 20 to 60 cycles [precision screening, secondary screening] are available, in order to Remove the most likely workmanship defects, about 6 to 10 cycles to be able to effectively remove, in addition to the effectiveness of the temperature cycle, mainly depends on the temperature variation rate of the product surface, rather than the temperature variation rate of the inside of the thermal stress screening test chamber. Tap the blue word can view Temperature shock test cycle number setting basis.

The seven main influencing parameters of temperature cycling are listed below:
（1）Temperature Range
（2）Number of Cycles
（3）Temperature Rate of Chang
（4）Dwell Time
（5）Airflow Velocities
（6）Uniformity of Stress
（7）Product Operating Condition

Comparison table of temperature variation rate and cycle number on screening rate:

Note: Assuming that the temperature difference between high and low (R) is fixed, the number of cycles is also fixed, the higher the temperature variation rate, the higher the screening rate will also be improved, such as the test conditions and specifications have provisions for a fixed temperature variation rate, the increase in the number of cycles can also be increased to improve the screening rate, but there is a certain limit to the screening rate, there is no way to linearly improve.

Stress Screening Fatigue Classification:

The general classification of fatigue research, such as, can be divided into High-cycle Fatigue, Low-cycle Fatigue and Fatigue Crack Growth, and in the low-cycle fatigue can be subdivided into Thermal Fatigue and Isothermal Fatigue two kinds.

Number of temperature cycle cycles:

a. MIL-STD-2164 (GJB 1302-90): In the defect rejection test, the number of temperature cycles for 10, 12 times, in the fault-free detection is 10 to 20 times or 12 to 24 times for the removal of the most likely to occur in the workmanship (workmanship) defects, about 6 to 10 cycles to be able to effectively remove, 1 to 10 cycles [ordinary screening, primary screening], 20 to 60 cycles [precision screening, secondary screening]. 1 to 10 cycles [general screening, primary screening], 20 to 60 cycles [precision screening, secondary screening].

b. DOD-HDBK-344 (GJB/DZ34) initial screening equipment and unit level using 10 to 20 cycles (usually ≧ 10), component level using 20 to 40 cycles (usually ≧ 25).

Temperature variation rate:

a.MIL-STD-2164 (GJB1032) clearly states: [Temperature change rate of temperature loop 5℃/min].

b. DOD-HDBK-344 (GJB/DZ34) component level 15℃/min, system 5℃/min

c. Temperature cycling stress screening where the rate of temperature change is not generally specified, the commonly used rate of change is usually 5°C/min.

List of Stress Screening Criteria

Stress Screening Criteria:

Temperature cycling standards:

Part pre-firing test standards:

System pre-burn criteria: