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60 The PCB Magazine • April 2016 data can then be used to extrapolate out to 100,000 hours to determine the expected elec- trical strength of the various laminate materi- als being compared. This data can be applied to the PWB design requirements by balancing the expected material degradation at 100,000 hours against the required dielectric needs through- out the service life of PWB assembly. Design ele- ments can then be adjusted (material selection, stack up, trace size and spacing, copper weight, ground planes, heat sinks, etc.) to ensure that the PWB remains within the thermal boundar- ies established by the temperature life curve. Loss of electrical strength due to thermal ex- posure must be a key consideration in the design of a PWB for the expected life of an application. Chemical changes within laminate are acceler- ated when the insulation of the PWB is exposed to elevated operating temperatures. Oxidation occurs degrading the physical and electrical properties of printed wiring board causing em- brittlement, discoloration, and delamination. The thermal-aging characteristics of a laminate can be determined by measuring the changes in its properties to a predetermined level by aging at each of several elevated temperatures. In this study, dielectric strength is used to determine the relative effects of different temperatures on the end of life for a given insulating system. It is also used to compare different insulating systems at a given temperature. It is important that the de - sign and construction of the PWB test vehicles are representative of the intended application and be consistent from laminate to laminate. Most data available on the thermal aging of laminate materials was specifically developed on the unclad laminate composite (resin and glass). To properly determine the thermal ag- ing characteristics of a PWB, temperature life testing must be performed on a manufactured PWB, not raw laminate. The internal structure of the board itself (amount of heat sinking ca- pacity, density of power generating compo- nents) and the intended use environment will also affect the operational life of a PWB. In addition, manufacturing processes can be del- eterious to the operational life of a fabricated PWB. Laminates will experience a number of chemical exposures, thermal excursions, and mechanical stresses during fabrication. Good process control is critical in eliminating con- taminants, obtaining proper bonding surfaces and good adhesion, and preventing mechani- cal and thermal damage to the laminate itself. If these processes go out of control or are poorly defined, operational life can be adversely affect- ed. These intangibles must be taken into con- sideration when analyzing data and predicting the operational life of a PWB. The times to failure in thermal aging test cannot be quantitatively related to the opera- tional life of a laminate system in actual use. However, they do provide a relative indication of a PWB's service life under the specific con- ditions of the test. Results of shorter time tests at higher temperatures can be extrapolated to longer times at lower temperatures. Material ag- ing standards such as UL 746B and IEEE STD 99 limit the degree to which material life data can be extrapolated. They indicate that material thermal aging testing should be performed for at least 25% of the desired operation life of the material. In order to obtain sufficient aging data for 100,000-hour operational life requirements, test duration must be at least 25,000 hours. Test Methodology Determining the operational life of printed circuit board laminates after thermal aging con- sisted of a three step approach following test details and calculations outlined in IEEE98 A.1 and UL746B. Testing materials with this ap- proach helps marry material capabilities with design requirements so proper trace spacing or other counter measures can be implemented to meet an intended design operational life of 100,000 hours. Guidance on the test methodol- ogy was provided by DfR Solutions. Phase I: 500 hours pre-screen at four fixed temperatures following IEEE98 A.1 and UL746B 20A to estimate the high temperature test boundary for long-term aging of PWB laminate material capabilities. Pre-screen data is used as an initial sort on best performing material. Criteria evaluated include highest dielectric strength, lowest overall degradation, lowest per- cent change in degradation, and anomalous or unexpected behavior (indicating instability). A pre-screening test lasting 500 hours was first employed using four fixed temperatures long-term thermal reliability of pCb materials

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