Interconnection challenge of wire bonding — Ag alloy wire

Traditionally, gold wire has been high volume production in IC packaging industry. With soaring price of gold in recent years and the of cost-efficiency, copper wire offers an alternative for Microelectronic Assembly. But copper wire has drawbacks in process control issues such as pad crack, aluminum (Al) splash, cratering and low throughput. Those limitations of copper wire are related to pad weakness. (thickness and structures) Silver alloy wire can emerge as another alternative as the cost can compete with copper and the properties of silver are nearly identical to gold. Copper wire is not suitable for thin aluminum pad as pad cannot sustain the wire bonding force which causes pad crack, within the range of pad thickness less than 1.2um. Copper wire is not recommended for weak pad structure either, such as partial array via and Non-Via pad structure. Copper wire turns out to be a lower throughput due to slow Intermetallic (IMC) growth, especially at pad to pad bond, dropped down to 70%-80% compared to gold wire. Forward bonding has been proven no problem by copper wire, unless the exceptions mentioned previously. But pad-to-pad bond is not easy due to bond pad struck twice. Silver alloy wire can handle SSB bond well, especially thin Al pad and weak bond pad structures. All wire bonders obtained high throughput by using Ag wire compared to gold wire. In this paper, wire bonding BGA with stacked dies structure, the workability and reliability tests such as wire pull, ball shear, IMC coverage, crater and aluminum splash were conducted. Silver wire ends up with better performance than copper.