What's Environmental Stress Screening ?

Environmental Stress Screening (ESS) refers to a series of tests conducted under relatively severe environmental stresses—such as fatigue vibration, mechanical shock, thermal shock, and temperature cycling—to reveal defects and eliminate early failures. ESS aims to detect and remove faulty components, parts, and process defects in products, thereby preventing early-stage failures.

For electronic products, the primary stresses selected for ESS are temperature (high and low) cycling and random vibration. The combination of these two stresses is highly effective in exposing most faults at various assembly levels. Research indicates that the effectiveness of high-low temperature cycling depends on four factors: the set values of high and low temperatures, the dwell time at each temperature, the rate of temperature change, and the number of cycles. Increasing the temperature change rate generally improves effectiveness. Additionally, a greater number of cycles is recommended for devices with a higher quantity of components.

Random vibration screening is more effective than swept-frequency sinusoidal vibration. Within limits that avoid product damage, a stronger vibration stress yields better results. ESS for components is a critical part of component screening. Corresponding Chinese military standards specify the screening requirements and methods for different types of components.

Common Screening Stress Ratios

For different screening stresses, their effectiveness must be studied and designed in correspondence with the targeted defects to be exposed. In any screening program, correct application is essential to activate potential failures. Identified failures require analysis to determine which latent defects caused them. For these specific latent defects, the most suitable screening method should then be determined through testing.

Temperature cycling involves periodic temperature variations between predetermined temperature extremes. All temperature cycling parameters are product-dependent. The temperature extremes must be lower than the product's limit values but sufficiently wide to permit an appropriate rate of temperature change, and the cycling should enable the product to fully expand and contract.

The rate of temperature change in the product depends on its thermal characteristics, the direction of heat transfer between the product and the air, and the air velocity.

The primary failure modes or effects on products induced by temperature cycling are as follows:
a. Expansion of micro-cracks in coatings, materials, or wire leads;
b. Loosening of poorly bonded connections;
c. Loosening of improperly screwed or riveted joints;
d. Loosening of press-fitted joints with insufficient mechanical tension;
e. Increased contact resistance or open circuits in poor-quality solder joints;
f. Particulate contamination;
g. Seal failures.

According to GJB-Z-34 Guidelines for Quantitative Environmental Stress Screening of Electronic Products, the screening effectiveness of temperature cycling is defined as follows:

where:
R is the temperature variation range (Tu − TL), in ℃;
V is the temperature change rate, in ℃/min;
N is the number of cycles;
E is the base of the natural logarithm.

Random vibration is considered the most effective among the three types of vibration. It refers to subjecting a product to vibration across a predetermined frequency range, typically 20 Hz to 2000 Hz. Random vibration applies excitation over a broad frequency spectrum, simultaneously subjecting the product to stress at multiple frequencies and exciting all its resonant frequencies at once. Many manufacturers use this method to further enhance product quality.

Random vibration screening generally requires less time than other screening methods and is more effective at revealing structural defects such as loose solder joints, poor bonding, and printed circuit board short circuits. Its main drawbacks are the cost of equipment and the operational challenges associated with conducting the screening.

It is precisely due to the broadband characteristic of random vibration that its failure modes are similar to those of sinusoidal vibration, but the failure mechanisms are more complex. The speed of fault detection is significantly faster than with swept sinusoidal vibration, which is also a result of simultaneously exciting multiple resonance points. The primary failure modes induced and their effects on products are as follows:
a. Fatigue in structural components, leads, or component connections, especially when micro-cracks or similar defects exist on conductors;
b. Chafing of cables, such as when sharp edges are present near loosely bundled cables;
c. Loosening of improperly manufactured screw connections;
d. Dislodgement of poorly installed integrated circuit chips from sockets;
e. High stress on bus bars and soldered joints on circuit boards leading to failure at weak solder points;
f. Damage to component leads with inadequate stress relief in bridge-like connections to relatively moving parts;
g. Cracking of brittle insulating materials that are already damaged or improperly installed.

According to GJB/Z 34 Guidelines for Quantitative Environmental Stress Screening of Electronic Products, the screening effectiveness of random vibration is defined as follows:

where:
Grms is the root mean square acceleration, in g;
t is the vibration duration, in minutes.

The defect failure rate for random vibration screening is consistent with that of constant high-temperature screening.

High-temperature aging is typically a static process during which products are subjected to elevated temperatures for a predetermined duration. The theoretical basis of this screening method stems from the observation that continuous operation of products tends to reveal early-life failures. 

Research indicates that screening efficiency is more closely associated with temperature variations during heating and cooling phases rather than during periods of constant temperature. Steady-state stress screening under constant temperature conditions generally exhibits relatively low efficiency. However, effectiveness can be significantly improved by combining dynamic temperature changes with applied electrical stress.

The failure modes that can be triggered by constant high temperature are described in MIL-HDBK-344, Environmental Stress Screening Methods for Electronic Equipment, primarily as follows:

a. Oxidation of unprotected technical surfaces, leading to poor contact or mechanical blockage (jamming). Such unprotected conditions may result from improper torque application during screw fastening or from pinholes and cracks in protective coatings.
b. Acceleration of intermetallic diffusion, such as between base metals and plated layers, brazing solder and components, or semiconductor materials with weak barrier layers and deposited metals.
c. Drying out of liquids, such as caused by leakage in electrolytic capacitors or batteries.
d. Softening of thermoplastics, which may induce product creep if these components are subjected to excessive mechanical stress.
e. Increased chemical reaction rates, accelerating processes involving internal contaminants.
f. Insulation breakdown at partially damaged insulation areas.

According to GJB/Z 34 Guidelines for Quantitative Environmental Stress Screening of Electronic Products, the screening effectiveness of constant high temperature is defined as follows:

where:
R is the temperature variation range (R = Tu - 25℃), in ℃;
t is the duration of constant high temperature, in hours.

The failure rate of defects during constant high temperature screening can be calculated according to the following formula:

where:
λD is the failure rate, in failures per hour;
SS is the screening effectiveness;
t is the duration, in hours.

Electrical stress refers to the process of manipulating a circuit or simulating the temperature of semiconductor devices. There are two fundamental types:
Power cycling, which involves turning the product on and off at specified intervals;
Voltage margining, which involves varying the power input above and below the product's rated power requirements.

Studies have shown that electrical stress is less effective than temperature cycling or vibration in exposing defects and is therefore considered relatively inefficient. However, it can be combined with other screening methods at relatively low cost to improve overall screening efficiency. For power products, electrical stress cycling can achieve effects similar to temperature cycling and is also a relatively straightforward approach.

Electrical stress is typically not applied alone but is used in combination with temperature cycling or high-temperature stress.

The following stresses, while generally offering limited screening effectiveness, can be highly effective for specific types of product defects. Targeted application can yield better results. A brief introduction is provided below.

Sine, fixed-frequency vibration: A form of vibration generated at a fixed sinusoidal or single operating frequency. This method requires a mechanical vibration table with a frequency range of up to 60 Hz. Although less costly and easier to control compared to random vibration, fixed-frequency vibration screening is generally considered unable to provide an effective stress level.

Low-temperature screening: Similar to high-temperature screening, it primarily relies on thermal exchange between the product and a low-temperature environment to trigger latent defects and uncover early-life failures in the product.

Swept-frequency vibration: A form of swept sinusoidal or multi-frequency vibration. It typically requires a frequency range extending to 500 Hz. In terms of overall effectiveness, it is considered to have comparable screening results to sine vibration.