Setting Up the ESS Process
ESS is product unique, since each product has its own set of potential defects and since the applied ESS stresses affect each product differently. Even though the ESS process must be set up individually for each product, there are many common features of both products and stresses which cause many ESS processes to be similar.
The stresses applied in ESS are expected to precipitate manufacturing defects. They are not necessarily those the product will see in service. The two most common ESS stresses for electronic products are temperature cycling and vibration. They may be applied sequentially or simultaneously.
It is critical that electronic equipment be monitored during ESS. This is the only way to detect failures under extreme conditions. More importantly, the stresses used in ESS can induce reversible damage not detected in tests conducted at ambient conditions. This induced damage is itself a latent defect, and the ESS process can actually cause early field failures.
Reducing or Eliminating ESS
Since ESS is an inspection step, it does not add value to the product and should be reduced or eliminated as quickly as possible. This cannot be done without proper justification, which requires relevant data.
ESS must be set up to provide data which can be used to reduce or eliminate it. The following eight steps illustrate what should be done:
1. Collect failure rate data during the ESS process.
Failure data must be collected, not just at the beginning and the end, but during the ESS process. It is not enough to know that failures occurred; their time of occurrence must be recorded. Data from all ESS attempts, whether or not there was a failure, must be collected and recorded.
2. Prepare a plot of failure rate vs time.
This is the type of plot shown in Figure 1. If the failure rate decreases with time, there is an opportunity to reduce it if proper product improvements can be made.
If the curve is constant, or if it increases with time, the ESS process cannot be effective because either there are no infant-mortality defects or the wrong stresses, or levels thereof, are being used. If this is the case, ESS should be modified or discontinued and some other means of product improvement must be implemented.
ESS may be conducted anywhere in the manufacturing process flow. Table 2 shows some examples of the types of stresses used for ESS at the component, subassembly, assembly and system levels for electronic equipment.8 Table 38 shows the types of defects which may be detected by temperature cycling and vibration.
The specific levels of ESS stresses are selected to precipitate the relevant defects in a relatively short time, and yet not consume a significant portion of the life of nondefective items. For electronic equipment, the lower end of the temperature cycling range is usually from -40(degree)C to -50(degree)C, and the upper end is from +75(degree)C to +85(degree)C.
The rate of temperature change can also be important. Figure 2 illustrates the effects of temperature rate-of-change on surface-mount transistor lifting.7 Selecting the vibration level can be quite challenging, especially if the defects are susceptible to a range of frequencies.
In general, multiaxis, repetitive shock vibration is much more effective and efficient than single-axis vibration. Simultaneous temperature cycling and vibration also are much more efficient than either separate or sequential application of the two stresses.