RF and Microwave Power Amplifiers assembly – Interaction between materials, design and process on reliability

Abstract

The demand for data anywhere and at anytime (mobile internet) is putting increasing pressure on network operators in their deployment of wireless communication infrastructure. This has pushed upwards across the supply chain, from suppliers to customers. At the core of the infrastucture are RF power amplifiers (PA) with very costly design, hence important to get it right first time. Thermo-mechanical compliance are key attributes to consistent product performance and reliability amidst stringent mission profiles. Most transistor component manufacturers focus on their core competence – component design. Network operators on the other hand focus on the PA design. In most cases, the assembly of the PA module is an after-thought and a key enabler. These transistors are usually high power devices (> 100 watts), hence demanding excellent thermal management. The packages used have a metal flange in addition to RF/DC leads which are soldered in the PA module. These act as a heat spreader and electrical interconnect which affects the product performance. Since these interconnects are in different planes, assembly requires heatsinks with cavities (flange soldering). The main challenge being tackled in this study is the response of the materials being soldered in a different plane – leads to board and flange to heatsink interface. This is why proper soldering of the transitors (flange & leads) is an essential part in PA assembly due to the impact on module performance & reliability. To achieve the desired thermal and reliability requirements, a variety of factors are critical including the cavity design, solder and reflow soldering process. In this paper, these factors are evaluated via both thermo-mechanical simulations and experimental work. Since there are lots of materials with different properties involved in the assembly, it is vital to evaluate its behavior prior to the actual process evaluation as it has impact on cost and timing effectiveness of the study. These simulations are used as initial indication for trends which are later validated by experimental evaluations. The thermal impedance (Rth) in combination to acoustic microscopy / x-rays is used to assess the soldered flange during assembly & reliability.