Qualification

Measurements in Practice

Qualifications and compliance verifications of ESD protective items are challenging to realize in practice. Most of the control measurements are related to evaluation of possible electrostatic source circuits. Therefore a resistivity and a resistance to ground are the most commonly used measurements in protection against ESD. It is often suspected that the measured resistance can be used for determining time constant of electrostatic source circuit. This assumption may also cause fundamental errors due to several uncertainty factors related to low current measurement techniques such as capacitive loading, orientation, voltage dependency, contact resistances, electrification, leakage, etc.

Standardized test methods do not necessarily cover the case specific needs of qualification or compliance verification because of the product and process related risks, which cannot be generalized. According to modern ESD control program an analysis of electrostatic behavior of the ESD protective item is required. That is why process monitoring is often focused on electrostatic field, potential, mobile and immobile charges and charge transfer. Resistances are measured after electrostatic analysis. Resistance measurements nullify possible electrostatic source circuits.

Measurement techniques of electrostatics and ESD measurements vary. Sometimes the slow response of very low current is measured in a function of time. On the other hand there are often needs to measure extremely fast phenomenon and high currents. Technically measurements are not always complicated, but an adequate analysis of the results may be challenging to perform. Random errors can be estimated statistically, but an analysis of systematic errors and avoidance of gross errors require theoretical background and experience in measurements.

ESD tests are related to sensitivity classification of components or final products. There are several standardized test models for ESD testing: Human Body Model (HBM), Human Metal Model (HMM), Charged Device Model (CDM), and Machine Model (MM). Additionally there are also non standard ESD withstand test methods for example a cable discharge, a transmission line pulse (TLP), a charged board event (CBE) etc. Withstand tests can be made in independent or internal test laboratories.

Primarily a standard test method shall be used in qualification and compliance verification. A non standard test method is chosen when standard test method is inadequate because of the specific reason. Correlation shall not be found between inadequate and adequate test method.

Measurement Conditions

There are several reasons why qualification has to be carried out after 48 h preconditioning in low humidity conditions, T = (23 ± 2) °C, RH = (12 ± 3) %. 12 % relative humidity is a compromise which is not a worst case condition as often explained. Preconditioning affects the results. Charge dissipation is slower and charge accumulation is more efficient in low humidity. Therefore electrostatic phenomena can be reproduced more accurately in low humidity. Leakage current in test setup may also be difficult to control in moist conditions. Compliance verification is typically carried out in ambient conditions by internal or external service provider depending on scope and cost structure of ESD protection.

Measurement Services

Product qualification is primarily a reliability issue but it has also impact for expenditures. Characteristics which do not exist should not be paid for. Qualification and compliance verification may be compared to the similar services such as calibration and maintenance. Measurement activities require relatively high initial costs, which can be avoided by using external services.

Organization’s internal measurements become profitable when service provider’s costs are expected to reach the own measurement costs. This is a rationale and objective character for planning measurement routines. In nutshell, it would be expensive to establish the own measurement service if the work load of measurements remains on relatively low level. From the business point of view an internal measurement service reduces a profit margin of the core business. Independent laboratory service reduces a margin of sales.

Qualification

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