Copper Ball Voids: Failure Mechanisms and Methods of Controlling at High Temperature Automotive Application
Authors: Lee Chu-Chung , Tran TuAnh , Mathew Varughese , Ibrahim Rusli , Eu Poh-Leng 
Published: June 2019

Since 2008, fine gauge (≤ 35 μm diameter) copper (Cu) wire has been rapidly replacing fine gauge gold (Au) wire in consumer, commercial, and industrial products [2–4]. The first wave of Cu wire products used bare, uncoated Cu wire which is soon to known having Cu-Al IMC corrosion induced by mobile chlorine ions in the epoxy mold compound system when IC parts are subjected to moisture related package stress tests such as biased HAST (Highly Accelerated Stress Test) [1–7]. Additionally, when comparing to Au wires, the 2nd bond process window of bare Cu wire can be very narrow and becomes a concern of moving into HVM (high volume manufacturing) [8]. Thus, palladium-coated Cu (PdCu) wire was introduced to the semiconductor assembly market aiming to provide more margin of passing biased HAST and enhance 2nd bond process capability [9–13]. However, the use of Pd-Cu wire is not a panacea to all Cu wire bond problems. One unique anomaly for Pd-Cu wire is the Cu ball void [1] which is observed only with Pd-Cu and not bare Cu ball bonds during HTSL (high temperature storage life) tests. The mechanism of forming Cu ball voids was proven to be the galvanized corrosion mechanism with Pd-Cu coupling. Significant factors affecting the formation rate of Cu ball voids are found to be baking temperature, EFO current settings, bonding parameters and mold compound additives (sulfur). Both anodic and cathodic chemical reactions will be proposed for Cu voids in this paper.

Even though Cu void can be considered as a cosmetic defect for the majority of application since the peak temperature of device mission profile is always no larger than 175C. The application at extreme high temperature (for example, 190C) can actually cause electrical failure at the ball bon region due to the Cu void formation in terms of size and location at the Cu-Al IMC region. The main effect is due to the selected mold compound having high amount of metallic adhesion promoter which is sulfur-based and extensive high temperature storage test condition (190C). The FIB/SEM picture of failing ball bond due to Cu voids from this particular device will be presented in the paper.

Thus, a newly developed doped Cu wire without Pd coating has been proposed by many wire suppliers to overcome Cu ball voids. However, doped Cu wires without Pd coating have suffered the same high volume manufacturing issues observed by bare Cu wires. For example, short tail and mean time between assist (MTBA) for doped Cu wires without Pd coating are both as poor as bare Cu wires. We will present high temperature storage test results obtained by doped PdCu wires in this paper.

To balance high volume manufacturing issues and Cu void formation, doped PdCu wires are also proposed recently. Several doped PdCu wires whose extensive high temperature storage results (220C) will be presented in this paper. The worst case mold compound with high amount of sulfur based adhesion promoter has been used to test the effectiveness of these new wire types. At such harsh testing condition, there is one doped PdCu wire in our test can actually survive without electrical failed ball bond due to Cu voids. Factors of effectiveness of doped PdCu wires will be discussed in this paper.

Authors have chosen to focus on Cu voids at both 1st bond (ball bond) and 2nd bond (wedge). 20 um wire diameter has been used for all test vehicle in this study. All controlling factors of eliminating Cu voids will surely be included at the end of this paper.