A new leadframe design solution for improved pop-corn cracking performance

The transition from through-hole packages to plastic surface mount packages has witnessed the emergence of pop-corn cracking phenomenon. Despite recent improvements in packaging materials, package designs and manufacturing technologies, the pop-corn problem is still widespread throughout the semiconductor industry. This paper reports the findings of a new leadframe design as one of the synergistic factors towards improving pop-corn performance of plastic packages. The test package was a 28×28×2.4 mm moisture sensitive 144L Quad Flat Pack (QFP) employing copper-alloy as a leadframe material. The effect of plasma cleaning on the new leadframe design was investigated for improved pop-corn performance. An atomic force microscope (AFM) and contact angle method were used to characterise surfaces of leadframe and chip backside in the uncleaned acid plasma cleaned surfaces. Finite element analysis revealed that stresses in the die-attach layer can be significantly reduced by up to 70% in the new leadframe design. Compared to standard leadframe design, package measurements showed that the warpage values were 47% lower in packages assembled with the new leadframe design. Thermal performance of package was characterised by thermal resistance (O_JA) measurements. The measured O_JA was similar for both standard and new leadframe designs at about 31 K/W for large chip size (12×12 mm²). Decreasing the chip size to 8×8 mm² has the effect of increasing O_JA by 8% to 35 K/W for standard designs and 36% to 42 K/W for new leadframe designs. Furthermore, results showed that delamination at the interface of die-pad/moulding compound and chip backside/moulding compound after temperature cycling, pressure cooker and AE3 test were substantially reduced and/or prevented. More importantly, by simply substituting the standard leadframe design with the new leadframe design, packages can achieve IPC level 3 moisture sensitive classification with or without plasma cleaning