How do I test for OE?
To meet the requirements of OE the use of SIR, IC and PICT should be applied in the following way:
Step 1:
Obtain the OE using the SIR technique. If the circuit assembly fails the SIR test, then an investigation using IC can be used to help identify the offending chemistry and likely source.
Step 2:
Once the OE has been achieved with the SIR test, boards from the same batch can be used to establish a PICT level for this board. The PICT value for actual assembly board can be measured when the process has been transferred from the SIR test board to an actual production board. These PICT values can be used in Industry 4.0 context with regular sampling from the production line.
It is important to understand some terms when we discuss these techniques.
- Characterisation
- Characterisation is the application of a test where the data is not assessed against a standard, but the results are analysed on the principle of a comparison, or against some arbitrary metric. The test method may or not be covered by a standard.
- Validation
- Validation of objective evidence testing is an assessment that the specification captures the requirements. That data meeting the specification are evidence that the product is reliable.
- Verification
- Confirmation, through the provision of objective evidence, that specified requirements have been fulfilled.
- Qualification
- Qualification is an act or process to assure something complies with some condition, standard, or specific requirements. For example in the SIR test if the resistance is above the 10⁸Ω then the test has qualified the result to IPC-J-STD-001.
- Summary
- A characterisation study using SIR will use a reproducible method that maybe from a standard, the results are typically compared with other tests, or to library values. Validation is where which test to be used is decided, for example this could be IEC 61189-5-502 or IPC-TM-650 Method 2.6.3.7, or IPC 9202. In the validation the test methods will be reviewed according to the parameters and compared to the requirements. Using an agreed test schedule and comparing those results to a specification is a verification. The SIR test can be used to verify that the material set meet the requirements for objective evidence, for example the procedure in J Std 001 has been meet. A verification study may also meet the requirements where the resistance values are better than 10⁸Ω, and this would be a qualification. A further assessment of the product may be required to validate that a product fully meets the requirement that objective evidence has been meet.
Application of Techniques
The measurement techniques for producing OE have been mentioned numerously. The following table is useful in understanding the application aspects of these techniques. Various important defining parameters of the described tests are given on each row. These parameters are of key importance in selecting a test.
It should be noted, as shown in the table, that the ultimate arbiter of product suitability is the performance of the product in the field. If there are no field returns there can be no better affirmation as to the quality of the product. However, if the product is new, or there is some modification to the process then the identified test methods must be applied.
The 2 hour SIR test doesn't tick the box for characterisation, verification or qualification because there is no validation to this proposed length of test. Another point to consider, is that you could potentially see a characteristic in the 2 hour test that tells you a go/no go answer, but there is no peer review to validate this criteria. What has been proven is that ECM can occur after many hours during a test.
Also of note is that only the SIR test can be applied to provide all three of the process evaluation tools: characterisation, verification, and qualification. There is a caveat to SIR providing process qualification, as it is not possible to apply SIR testing to the final product. However, SIR testing can be applied to the materials and processes used in the product assembly. Indeed it is the only technique that can be used for the purpose of qualification.
It is relevant to understand how IC and FTIR can be used to characterise a product. These techniques provide key chemical information that can be used to infer that the correct materials and processes have been applied. Typically results are evaluated by comparison with library values.
The final technique that we discuss is the PICT (Process Ionic Contamination Test) test. PICT is based on the SEC test, similarly to the ROSE test. However, the test philosophy is completely different. On the ROSE test the pass/fail criteria is 1.56µg/cm², but in the PICT test the pass/fail criteria is set for each product individually using a value established by an OE study.
