Lead Frame and Wire Length Limitations to Bond Densification

Abstract

At IC package lead counts above 100, the limiting factor for bond densification in single layer packages with gold wire interconnections shifts from pad pitch to a combination of lead tip pitch and wire length. To quantify this effect, the densification capabilities of four different lead frame designs were calculated for the 164 lead plastic quad flat pack (PQFP) and compared to the pad limited (i.e., non-lead frame/wire length constrained) design. Of the four, the orthogonal layout provided maximum densification although its die size was still almost 33 percent larger than that of the pad limited design. It also had the disadvantage of a large angle of attack between the wire and the lead tip orientations at second bond. At the other extreme, the die-centered radial layout assured zero angle of attack but required a 53 percent larger die than the pad-limited layout. The other two lead frame designs resulted in die sizes which fell between these limits, slightly above the orthogonal layout value. After consideration of factors such as densification, manufacturability and die size tolerance, it was concluded that the variable center, variable radius layout, which features constant bond pad pitch and wire length, offers the best densification opportunity for wire bonded products. For maximum (i.e., pad limited) densification to optimize die yields and performance, advanced interconnect technologies such as TAB (tape-automated bonding) will be required.