Characterization of transfer molding effects on RF performance of power amplifier module

In this study, impacts of transfer molding material on electrical performance of a Power Amplifier (PA) module for GSM/GPRS mobile phone application were characterized. Critical areas on the module to cause output power (Pout) degradation were identified, which serves as a guidance for design compensation to minimize molding effects. Molding compound dielectric property is an important variable to the molding effects. Therefore, dielectric property measurement techniques and capabilities were established up to 10 GHz. Various molding compound materials from vendors were obtained. Dielectric constant (Dk) and loss tangent (Df) were measured up to 10 GHz. Electromagnetic (EM) simulations on the PA module US PCS band (1850 to 1910 MHz) die output matching physical structures, including Cu transmission lines and MOS capacitors and their wirebonds, bias feeding, and daisy chain coupler before and after molding, were conducted using measured dielectric property data. Simulations show that the most sensitive area to molding are MOS capacitors and their wirebonds, which caused center band frequency shifting 31 MHz lower after molding. Selective glob top encapsulate experiments were conducted on the PA module US PCS band path, and critical areas for power-out (Pout) drop and band frequency shift were identified. MOS capacitors and their wirebonds in the output matching circuit accounted for 58% of total Pout drop after molding, and GaAs die with inter-stage matching passives on and off chip accounted for 22% of total Pout drop. Small signal gain measurements show that pass band center frequency shifted lower after glob top encapsulate the MOS capacitors and their wirebonds, which significantly dropped gain at high frequency edge of 1910 MHz. Design compensation to minimize Pout drop was used to tune module band purposely higher to account for the frequency shift after molding. Next generation die and module design was compensated, and band center was purposely shifted higher.