Lead-Free Interconnect Technique by Using Variable Frequency Microwave

Journal of Electronic Materials, Jul 2005 by Moon, Kyoung-Sik, Li, Yi, Xu, Jianwen, Wong, C P

A novel lead-free interconnect technique using variable frequency microwave (VFM) was investigated. The lead-free solder interconnection between the component chips and the metal pads through VFM was first demonstrated. Comparison between the microstructures of the lead-free solder joints on Cu and Sn surfaces formed by a conventional thermal reflow process and VFM was conducted. The VFM heating technique successfully created the lead-free solder/Cu and Sn joints through their intermetallic compounds (IMCs), while maintaining the substrate temperature as low as 210�C.

Key words: Lead-free solder, variable frequency microwave (VFM), selective heating, low-temperature lead-free interconnects, reflow process, intermetallic compound (IMC)

INTRODUCTION

As a result of aggressive government legislation pressure against the use of solder containing lead, the electronics industry has made lead-free solder interconnect technology a priority. Many types of lead-free solder candidates have been sought and suggested by the industry leading consortiums. Among those candidates, Sn-based alloys have been the primary consideration to cost and performance. In particular, Sn-Ag-Cu or Sn-Ag alloys have been suggested by many consortiums for the reflow process. However, those candidates have 30-40�C higher melting points than the conventional eutectic Sn-Pb alloy (183�C). The higher process temperature introduces higher thermal stress, brings out reliability issues for assembled devices and forces manufactures to employ high temperature durable substrates with higher cost. As such, lowering process temperatures for the lead-free solders will lead improvement in the device reliability and save in cost.

A significant amount of research has been conducted on the metallurgical behavior of the lead-free solder joints after the reflow process or under aging in the harsh environmental conditions.1 In particular, research has focused on the intermetallic compound (IMC) formation between the lead-free solders and metal pads, and the grain size of the IMC.2-8 The grain size has been identified as a critical parameter in determining the reliability of the solder joints. The excess IMC, such as Cu-Sn, that forms between the Sn-based alloys and the Cu pad will weaken the solder joint strength and eventually cause the fatigue failure, due to the brittle nature of the IMCs and the thermal mismatch between the solder and the printed circuit board. An appropriate IMC thickness is required for extremely reliable joints. A fine grain size of IMCs is also desirable in solder joints for reliability, since the fine grain size improves fatigue resistance and introduces the possibility of superplastic behavior. The IMC layer at the joints and the grain size of the IMC can be affected by the peak temperature in the reflow process, the holding time at the temperature of above melting point of the solder, and aging under the harsh environment. A faster heating and a shorter holding time at the peak temperature in the reflow can reduce the excessive growth of the IMC laver and its grain size.

The solder paste is a mixture of prealloyed solder powders and the flux-vehicle that has a creamy, peanut butter-like consistency. The flux-vehicle portion of the paste is made of rosin or resin, activators, viscosity control additives, flux chemicals, stabilizers, and solvents. Due to the polarity of these constituents, the flux vehicle can be heated by VFM. This heat can be transferred to the prealloyed solder powders and then the solder powder can melt at the melting point.

This paper is the first report on soldering behaviors of the Sn-3.5Ag and Sn-3.8Ag-0.7Cu joints on Cu and Sn surfaces by VFM. The peak temperatures in the thermal reflow and VFM ovens were varied, while the preheat temperature, the ramping rate, and the entire reflow process time were fixed. The solder joints formed by a conventional reflow process were compared with those soldered in the VFM oven, and their morphologies and the IMC formation were discussed.

EXPERIMENTAL

Sn3.8AgO.7Cu and Sn3.5Ag pastes (Indium Co., Utica, NY) were used as lead-free solders. Hereafter, they are simply referred to SnAgCu and SnAg, respectively. Commercial 1206 resistors (Digi-Key Co., Thief River Falls, MN) were used as components for the interconnect tests. The commercial Cu/organic solderable preservative (OSP) and Sn surfaces on the FR-4 boards (Standley Circuit, Englewood, CO) were employed as solder wetting substrates for microstructure study.

The solder pastes were printed on the substrates through a stencil mask and solder joints were formed by means of a standard thermal reflow machine (BTU International Inc., MA) and a microwave oven. For microwave heating, a VFM (MicroCure 2100, Lambda Technologies Co., NC) oven was used. The center frequency of the microwave, the bandwidth, and the sweeping time were 6.425 GHz, 1.15 GHz, and 0.1 sec, respectively.

To monitor the temperature of the test boards, a built-in infrared (IR) sensor was used. This IR sensor reads the temperature of the boards in the chamber and gives the feedback to a power module, so that the power module controls the VFM power resulting in properly adjusting the temperature of the boards. The FR-4 board surface near the solder/pad joint was targeted by the IR light and the sensor read the temperature on the surface of the FR-4 board, as shown in Fig. 1. Thus, temperatures that are shown herein indicate the temperature of the FR-4 board surface near the solder pastes.