Basic HALT Steps
- Precipitation Defects using cold and hot stepped stresses, random 6 axis vibration and combined vibration & thermal stresses.
- Detection
- Failure Analysis
- Corrective Action
- Verification of Corrective Action
- Enter data into a data base
- Analyze field failures
With proper equipment and technique, HALT & HASS can SAVE your company significant development time, warranty/repair costs, and a damaged reputation!
Basis Steps in HALT/HASS Detection Process
- Precipitation means to change a defect which is latent or undetectable to one that is patent or detectable. A poor solder joint is such an example. When latent, it is probably not detectable electrically unless it is extremely poor. The process of precipitation will transpose the flaw to one that is detectable, that is, cracked. The stresses used for the transformation may be vibration combined with thermal cycling and perhaps electrical overstress. Precipitation is usually accomplished in HALT or in a precipitation screen.
- Detection means to determine that a fault exists. After precipitation by whatever means, it may become patent, that is, detectable. Just because it is patent does not mean that it will actually be detected as it must first be put into a detectable state. Assuming that we actually put the fault into a detectable state and that the built-in test or external test setup can detect the fault, we can then proceed to the most difficult step, which is failure analysis.
- Failure analysis means to determine why the failure occurred. In the case of the solder joint, we need to determine why the joint failed. If doing HALT, the failed joint could be due to a design flaw; that is, an extreme stress at the joint due to vibration or possibly due to a poor match of thermal expansion coefficients. When doing HASS, the design is assumed to be satisfactory (which may not be true if changes have occurred) and in that case, the solder joint was probably defective. In what manner it was defective and why it was defective needs to be determined in sufficient detail to perform the next step, which is corrective action.
- Corrective action means to change the design or processes as appropriate so that the failure will not occur again. This step is absolutely essential for success. In fact, corrective action is the main purpose of performing HALT or HASS. One of the major mistakes happening in the industry is that manufacturers “do HALT” and discover weaknesses and then dismiss them as due to overstress conditions. It is true that the failures occurred sooner than they would in the field, due to the overstress conditions, but they would have eventually occurred in the field at lower stress levels.
- Verification of corrective action needs to be accomplished to determine that the product is really fixed and that the flaw that caused the problem is no longer present. The fix could be ineffective or there could be other problems causing the anomaly that are not yet fixed. Additionally, another fault could be induced by operations on the product, and this necessitates a repeat of the conditions that prompted the fault to be evident. One method of testing a fix during the HALT stage is to perform HALT again and determine that the product is at least as robust as it was before and it should be somewhat better. If in the HASS stage, performing HASS again on the product is in order. If the flaw is correctly fixed, then the same failure should not occur again.
- The last step of the six is to put the lesson learned into a database from which one can extract valuable knowledge whenever a similar event occurs again. Companies that practice correct HALT and utilize a well- kept database soon become very adept at designing and building very robust products with the commensurate high reliability. These companies usually are very accomplished at HASS and so can progress to HASA, the audit version of HASS and save substantial costs in the process.
References: Accelerated Reliability Engineering, Gregg K. Hobbs, Hobbs Engineering, 2005.