Editor's Note: This article originally appeared in the July 2012 issue of SMT Magazine.

This paper was originally presented at IPC APEX EXPO 2012.

Introduction

The quad flat no-lead (QFN) package is increasingly popular due to its small size, easy trace routing, and good thermal and electrical performance [1]. It has a large central underbelly pad, which dissipates heat from the die inside the package through a solder connection to the printed circuit board (PCB). The short standoff distance between the QFN package and the PCB also reduces inductance thus providing excellent electrical performance. An Agilent package called TOPS has features similar to a QFN, being leadless with a large underbelly pad.

Although the QFN package and the TOPS packages offer a number of benefits, to the authors’ knowledge, the evolution of morphology of the solder joint on the central underbelly pad has not been reported. In this paper, the authors report on a comprehensive study regarding the effect of Au content on the morphology of SnAgCu solder joints on the underbelly pad, assembled on PCBs with a Ni/Au surface finish. Samples were isothermally aged at the equivalent of 0, 2, 7, and 14 years’ service life. Representative solder joints were cross-sectioned and analyzed using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) to investigate the evolution of the solder joint morphology as a function of Au content and isothermal aging. 

Another issue with these leadless packages with a large underbelly pad is that excessive voiding often occurs in the solder joint on the underbelly pad. If the void area is large, thermal performance will be reduced. There are a few application notes that provide guidelines for pad pattern design, stencil design, reflow profile, and others [2-6]. To the authors’ knowledge, no paper has been published on the effect of Au content on the voiding. In this paper, the effect of Au content on voiding in the underbelly solder joint is reported.

Experiment

Component, Test Vehicle, and Assembly Process
 
The test vehicle is shown in Figure 1. The board finish is electrolytic Au over Ni. There are two different Au thicknesses: A flash Au finish with 0.08 ~ 0.38 µm Au over 5 µm Ni and a thick Au finish with 2 ~ 2.54 µm Au over 5 µm Ni. Two types of leadless packages, QFNs and TOPS, were assembled on the test vehicle. Figure 2 shows these packages. The QFNs were two different sizes: QFN5 is 5 mm x 5 mm and QFN6 is 6 mm x 6 mm. The size of the TOPS package is 10 mm x 10 mm. The pad finish on the QFN5 and QFN6 is matte Sn over Cu and that on the TOPS is electrolytic Au over Ni.

The assembly process was done using a standard surface mount assembly line in a realistic production environment. The solder paste used is Sn3.0Ag0.5Cu (SAC305) Type 3 with no-clean flux and metal content of 88% by weight. The stencil used is electroformed nickel, laser cut with a foil thickness of 0.1 mm (4 mils). The solder paste coverage of the central pad for QFN5 and QFN6 is 43% and that for TOPS is 56%. Since the volume of solder paste on the central pad was not measured, the nominal solder paste volume is calculated based on the stencil aperture size and the coverage. The calculated nominal Au content of solder joint on the central pad for QFN5, QFN6, and TOPS package is shown in Table 1. For details about the reflow profile and Au content calculation, please refer to papers published by the authors [7-9].