Embedded Passives in High Layer Count High Reliability Printed Wiring Boards

摘要

This paper will discuss the use of thin film buried resistors and thin core plane pairs in high layer count high reliabilityrnprinted wiring boards used in single and double sided surface mount assemblies. Very high thermal stress durability isrnrequired for assembly and component repair/replacement. The high density multilayer and sequential built designs of 14-26rnlayers use a combination of buried vias, blind vias, thin cores, several layers of buried resistors and thin core plane pairs tornmeet controlled impedance and circuit performance. The design of the resistors for BGA pull down terminations and for incircuitrnresistors contain various values and tolerances. Overall thermal performance of the board is critical to the productrnreliability requiring the buried resistors and thin core plane pairs to also survive extreme thermal exposure. Thermal stressrntesting, as required by the applicable standard, was extended to multiple times in order to determine product robustness forrnlaminate defects, plating integrity and inner plane connection durability. In addition, Current Induced Thermal CyclingrnTesting can be performed for comparative analysis. Special test coupons can be designed that duplicate the actual conditionsrnof the board and allow the buried resistor layer to be present in the microsection for evaluation during the requiredrninspections. As with conventional printed wiring technology, failures do occur, but typically only after extreme thermal stressrnor rework. Failure analysis was performed to identify the cause. Pictorial views of the component density, resistor designs,rnlayer stackup, and failure analysis activity are contained in the paper. This paper will show that embedded passive technologyrncan be implemented with success for high density, high layer count and high reliability printed wiring boards.