Effects of Different Printed-Circuit-Board Surface Finishes on the Formation and Growth of Intermetallics at Thermomechanically Fatigued, Small Outline J Leads/Sn-Ag-Cu Interfaces

Journal of Electronic Materials, Mar 2004 by Wu, Pei-Lin, Huang, Meng-Kuang, Lee, Chiapyng, Tzan, Shyh-Rong

The effects of printed-circuit-board (PCB) surface finish and thermomechanical fatigue (TMF) on the formation and growth of intermetallic compounds (IMCs) between small outline J (SOJ) leads and Sn-3.0Ag-0.5Cu solder were investigated. The thickness of the IMC layer formed initially at the as-soldered SOJ/Sn-Ag-Cu interface over a Ni/Au PCB surface finish was about 1.7 times of that over the organic solderability preservative (OSP) PCB surface finish. The parabolic TMF-cycle dependence clearly suggests that the growth processes are controlled primarily by solid-state diffusion. The diffusion coefficient for the growth of the total IMC layer at the SOJ/Sn-Ag-Cu interface over the Ni/Au PCB surface finish is the same as that over the OSP PCB surface finish, and thus, the total IMC layer at the SOJ/Sn-Ag-Cu interface over the Ni/Au PCB surface finish is thicker than that over the OSP PCB surface finish. Using the Cu-Ni-Sn ternary isotherm, the anomalous phenomenon that the presence of Ni retards the growth of the Cu^sub 3^Sn layer while increasing the initial growth of the Cu^sub 6^Sn^sub 5^ layer can be addressed.

Key words: Lead-free solder, Sn-Ag-Cu, intermetallic compound, surface finish, Ni/Au, thermomechanical fatigue, Cu-Ni-Sn ternary isotherm

INTRODUCTION

Among Pb-free solders, Sn-Ag-Cu has been recommended for general-purpose use as a substitute for the Pb-Sn eutectic solder.1-6 Unfortunately, even for this alloy, not enough is known about its various properties. Although there have been some studies on the reaction between SnAg-based solders and Cu,7-16 more studies are needed to understand this alloy to a level close to that of the eutectic Sn-37Pb solder.

In addition, the electronics manufacturing industry is not ready to convert over to Pb-free circuitboard finishes4,15,17 and component-lead finishes. The alternate printed-circuit-board (PCB) finishes, such as immersion gold over electroless nickel (Ni/Au) and organic solderability preservative (OSP) are still in their infancy for lead-free mass production. Recent studies18'19 show that the Ni/Au coating provides enhanced solder wettability. The function of the Ni layer is to prevent rapid reaction between the solder and the Cu layer, which is under the Ni layer and is a part of the internal conducting trace. The purpose of gold is to prevent the oxidation of the underlying nickel layer, thereby enhancing solderability. Therefore, prior to converting to this new Pb-free solder technology, an evaluation of the compatibility of this solder with the PCB and component-lead terminations is necessary.

Solder joints in service conditions experience internal thermal stresses caused by coefficients of thermal expansion mismatches between the component leads, the solder, and the substrate. Thermomechanical fatigue (TMF) occurs because of thermal stresses that develop during temperature excursions encountered during service. As a result, mechanical deformation occurs in the solder joints. Although there have been studies of TMF on leadtin solder during the last decade,20-23 additional data are needed on the TMF of lead-free solders.

The objective of the present study is to examine the impact of different PCB surface finishes and TMF on the formation and growth of intermetallic compounds (IMCs) between the small outline J (SOJ) leads and the Sn-3.OAg-0.5Cu solder. In this study, the SOJ leads/Sn-Ag-Cu joint was subjected to TMF of 233-398 K, and metallographic examination was performed to investigate the evolution of the intermetallics. We also seek to better understand the fundamental aspects of the formation and growth of different intermetallics.

EXPERIMENTAL

The devices used to evaluate the integrity of the circuit-board solder joints were 44 input/output SOJ lead, integrated circuit packages. The package material was plastic. The leads were copper, with a J-lead configuration. The lead pitch and width were 1.27 mm and 0.55 mm, respectively. The lead frame was coated with Sn-0.7mass%Cu solder.

The PCB is 1.6-mm thick and is made of epoxyglass multifunctional laminate. The glass transition temperature of the laminate was 145-165°C. Two lead-free surface finishes of PCB were used for the study. One case used gold over nickel (Ni/Au) finishing and the other used OSP surface finishing applied on six-layer FR-4 test boards. The surface layer of Au, with a thickness of 0.08 ???, is for oxidation protection and is deposited by immersion. The inner Ni layer is deposited by electroplating and is about 2.5-µm thick.

The Sn-3.0mass%Ag-0.5mass%Cu solder paste containing rosin mildly activated-type flux was screen printed onto the circuit boards. The components were then placed on the PCB pads by a pick and place machine. Solder reflow of the test vehicles was performed in a reflow furnace under flowing nitrogen (200-ppm O2). In this study, the reflow process was conducted with the use of a forced-air convection oven with a reflow time of 75 sec. The peak temperature of the SOJ during the reflow soldering process was maintained at approximately 523 K. Reflow soldering was performed under N^sub 2^ atmosphere. The use of nitrogen would improve soldering performance for lead-free materials. Figure 1 shows the optical micrograph of SOJ/Sn-3.0Ag0.5Cu solder joints over different PCB surface finishes prior to the TMF test.