Effects of microstructural evolution and intermetallic layer growth on shear strength of ball-grid-array Sn-Cu solder joints

Abstract

The shear strength of ball-grid-array (BGA) solder joints on Cu bond pads was studied for Sn-Cu solder containing 0, 1.5, and 2.5 wt.% Cu, focusing on the effect of the microstructural changes of the bulk solder and the growth of intermetallic (IMC) layers during soldering at 270∞C and aging at 150∞C. The Cu additions in Sn solder enhanced both the IMC layer growth and the solder/IMC interface roughness during soldering but had insignificant effects during aging. Rapid Cu dissolution from the pad during reflow soldering resulted in a fine dispersion of Cu 6 Sn 5 particles throughout the bulk solder in as-soldered joints even for the case of pure Sn solder, giving rise to a precipitation hardening of the bulk solder. The increased strength of the bulk solder caused the fracture mode of as-soldered joints to shift from the bulk solder to the solder/IMC layer as the IMC layer grew over a critical thickness about 1.2 mm for all solders. The bulk solder strength decreased rapidly as the fine Cu 6 Sn 5 precipitates coarsened during aging. As a consequence, regardless of the IMC layer thickness and the Cu content of the solders, the shear strength of BGA solder joints degraded significantly after 1 day of aging at 150∞C and the shear fracture of aged joints occurred in the bulk solder. This suggests that small additions of Cu in Sn-based solders have an insignificant effect on the shear strength of BGA solder joints, especially during system use at high temperatures.

STRUCTURE AND ELECTRICAL PROPERTIES OF Sn8Zn SOLDER ALLOY WITH Ag AND Ga ADDITIONS

“…The intermetallic compounds, which form between the SAC and copper base are similar to those found in the Sn-Pb/Cu connections. Also some studies are reported on intermetallic phases found in the Sn-Cu solder joints [3] and Sn-Ag [1,4,5]. Besides, various additions to the Sn-Ag alloy such as Ni, Cu, Al, Zn, Co, Sb, P and Au were studied in [6] and the interface layer is similar to that found in SnZn with additions.…”

“…The application of Sn-Pb alloys into copper-based connections causes the formation of the Cu 6 Sn 5 and Cu 3 Sn compounds. The increased growth of the intermetallic layers of CuSn decreases thermal endurance, tensile strength and brittle fracture of the solder joint.…”

Formation and Growth of Intermetallic Compound Cu6Sn5 at Early Stages in Lead-Free Soldering

“…[4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] These works suggest that, upon contact, Cu and liquid Sn react to form two distinct IMCs [Cu 6 Sn 5 (g phase) and Cu 3 Sn (e phase)], which precipitate at different stages of the soldering reaction. Most research seems to suggest that, at the very early stages of the soldering reaction, the g phase precipitates first.…”

“…19 Earlier works have focused on experimental characterization of the late stages of growth and coalescence of g and e phases during lead-free soldering reactions between Sn and Cu. [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] More recently, several groups have investigated the formation of the g phase at the early stages of soldering reactions. [20][21][22][23] These latter works have found that individual Cu 6 Sn 5 grains seem to appear at random positions along the (metastable) solid/liquid interface within a few milliseconds.…”

Multiphase Field Simulations of Intermetallic Compound Growth During Lead-Free Soldering

“…Understanding the solder/ substrate interactions and their microstructural evolution at the interface is important as it can provide insight into the nature of the interfacial evolution from a metallurgical point of view. [8][9][10][11][12][13] At the same time, accurate control of these reactions can have significant impact on the optimization of the soldering process. 14 Experimental investigations of intermetallic compound growth 6,7,13,[15][16][17][18] indicate that Cu 6 Sn 5 layer is formed as scallop-type grains in contact with liquid solder, while Cu 3 Sn is usually formed between the Cu substrate and the Cu 6 Sn 5 layer.…”