The purpose of a cyclic THB test is to assess the ability of a product to operate reliably under condensing conditions (dew point). The rationale for more than one cycle is based on observations that repeated applications of temporary condensation events can lead to wearout type behavior ( > 1) over time. Initial condensation events “weaken” the circuit by inducing dissolution of conductor material (in this case, copper). The elevated presence of these metals eventually is sufficient to induce electrochemical migration between insulated conductors.
This white paper assesses the temperature and humidity acceleration factors both with and without DC bias. The influence of these factors on MLV lifetime will be addressed, especially as it relates to the operating environment seen by MLVs. Additionally, technical explanations of the different failure modes and failure mechanisms are provided, specifically with regards to temperature and humidity exposure. Over 50 years ago, typical life tests simulated the operational conditions of a product. However, these tests became useless due to the rapid improvement of electronic component reliability. The solution was to develop a testing methodology utilizing the same types of stress, but at higher levels than typical operating conditions. The purpose of these accelerated tests was to shorten the timeframe necessary to obtain relevant results through an aging deterioration of the device in order to induce normal failures.
While the primary focus on environmental challenges in electronics has been in regards to the requirements derived from the RoHS and REACH legislation, several other ‘greening’ trends have serious implications on the design, manufacturing, and qualification of electronic components, products, and systems. The most consequential is the concept described as Free Air Cooling or Air Economization for ubiquitous data centers that power the Internet.
The absence of humidity and the presence of a vacuum in the operating environment will provide a degree of safety above that provided in the minimum spacing requirements displayed in IPC-2221. Voltage transients of similar duration (10 nS) to the voltage spikes seen in power diode (see CALCE Report “Lifetime Assessment of Wedge Bonds in a High Current Diode”) will also provide an additional safety factor above minimum spacings recommended in IPC-2221. Operating temperatures above room temperature and processing defects will lead to a reduction in the electrical field stress necessary to induce dielectric failure.
A thorough physics-of-failure (PoF) approach is recommended to determine the integrity of a printed circuit board that violates IPC-2221. PoF is based upon documenting the assembly architecture, operational environment, and material properties, determining the relevant failure mechanisms, accurate measurement of the environmental loads (average and maximum electrical field stress), and the use of an acceleration factor to take into consideration degradation during operational lifetime. This assessment will provide a more accurate understanding of the reliability of the polyimide printed circuit board than a critical analysis of the spacing requirements defined in IPC-2221.
The electrical conductor spacing requirements defined in Table 6-1 in IPC-2221 are based upon research that was performed by Dr. Charles Jennings of Sandia National Laboratories. The results of Dr. Jennings research was published in IPC-TP-117, Electrical Properties of Printed Wiring Boards, in September 1976. IPC-TP-117 focused on dielectric breakdown, current carrying capacity and insulation resistance for 2-sided bare, coated, and encapsulated (epoxy and urethane) FR-5 (glass reinforced Fire-Resistant epoxy) printed wiring boards. Breakdown testing was conducted in air (767 Torr) and at low pressures (500 Torr).