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Ag-wire and Ag alloy wire relaibility and molding compound
Keywords: molding compound, Ag wire, control ion behavior
There have been many studies on replacing Au bonding wire with other bonding wire materials such as bare Cu wire, Pd coated Cu wire, now Ag wire has become another option because Ag bonding wire could be used for stress sensitive die during wire bonding process and reverse bonding for stacked die application. Hitachi Chemical is now developing molding compounds for Ag wire in accordance with the developments of the Ag wire market. The concern to use Ag wire even with Ag alloy type wires is humidity reliability test such as bias HAST. The failure mode could be corrosion and crack, which is the same as for Cu wire. In addition, another concern could be Ag migration issue. Ion impurity contained in molding compounds could accelerate these failures. Therefore, we started studying for molding compounds which are more stable and cause less corrosion and migration under bias HAST(130degC, 85RH%). We are studying with two approaches, which are actual bias HAST test and also chemical model simulation to understand failure mechanism better and solve the concerned matters. At first, we did failure analysis of failed samples. By the cross section of failed pad, we found that crack and corrosion. EDX showed Cl as well as Ag, Al and O at corroded area. We explored which ion spices could cause corrosion. We found that not only Chloride ion but also other ions could corrode Ag/Al IMCs which are created at bonding area between Ag wire and Al Pad. Secondary, we studied corrosion mechanism by chemical model simulation. We confirmed that Ag rich IMCs and Ag poor IMC could be created after bonding for both pure Ag case and Ag alloy cases. We checked the desorption energy of Chloride ion and other ions from various Ag/Al IMCs for pure Ag wire case as well as Ag/Pd/Au/Al IMCs for Ag alloy cases. These results suggested that several different ions could attack Ag rich IMCs. In case of Cu wire, Cu rich IMC was corroded by Chloride ion. However, other ions seemed not to cause corrosion of Cu/Al IMCs. Therefore, the other ions’ involvement in corrosion issue is new for Ag wires. Thirdly, we explored how to improve molding compounds resistance to corrosion and migration. Molding compounds for Ag wire are designed to delay ion mobility by optimizing resin matrix and cross linking, which should be effective method for all kinds of ions. In addition, new ion trappers development has been done to catch various ions effectively. We confirm bias HAST performance of developed Ag wire compatible molding compounds.
Aya Mizushima,
Hitachi chemical
Yuuki-shi, Ibaraki
JAPAN


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