PdCu Wire Research Articles

Research Progress of Palladium-Plated Copper Bonding Wire in Microelectronic Packaging.

Wire-bonding technology is the most commonly used chip interconnection technology in microelectronic packaging. Metal bonding wire is the key material for wire bonding and plays an important role in the reliability of electronic devices. In recent years, palladium-plated copper (PdCu) bonding wire has been widely used because of its low cost, good electrical and thermal conductivity, the fact that it is not easy to oxidize, and its high reliability. Therefore, it is necessary to review its research progress. In this paper, the preparation and application of palladium-plated copper bonding wire are reviewed. Firstly, the preparation methods of electroplating, electroless plating, and direct plating are introduced. Secondly, the factors affecting the distribution of Pd in free air balls and bonding interfaces, the effect of Pd on the formation and growth of intermetallic compounds in PdCu wire, stitch bond, and reliability of PdCu wire are summarized and analyzed in the application process. Finally, its development prospect is prospected. Hopefully, this review can help readers to have a comprehensive understanding of the preparation and application of palladium-plated copper bonding wires, and can accelerate the promotion of its application in more fields in the future.

Copper Ball Voids: Failure Mechanisms and Methods of Controlling at High Temperature Automotive Application

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.

High temperature storage reliability of palladium coated copper wire in different EFO current settings

In recent years, palladium-coated copper (PdCu) wire has been widely used in microelectronic packaging. The electronic flame off (EFO) current setting will affect the distribution of palladium (Pd) during the free air ball (FAB) formation of PdCu wire. This study investigates the influence of EFO current settings on Pd distribution in the FAB and the bonded ball. The distribution and concentration of Pd is observed by using an electron probe micro analyzer (EPMA). Mechanical tests are used to evaluate the bond strength of the first bond. A high temperature storage test (HTST) is performed on the packaged IC at 200°C for 500h and 1000h.The results indicate that different EFO current settings cause either complete or partial Pd coverage on FABs, which directly affects the Pd distribution at the bonded ball interface. The wire pull and ball shear tests show the bond strength decrease under three EFO current settings after HTST. This confirms that although Pd can serve as a protective layer for the bonded ball against attack from halides from within the epoxy molding compound (EMC), incomplete Pd coating and formation of the alloy may actually aggravate the corrosion on bonded ball.

Reliability Performance of Palladium Coated Copper Wire in High Temperature Bake at Extreme Durations

New automotive requirements expect plastic packages to survive higher operating temperatures with extended thermal duration. Further module integration and more stringent environmental requirements push modules and thus plastic packages closer to the heat source. As such, new mission profiles include more than 3500 total hours at 150°C, or 1000 total hours at 175°C. To satisfy new automotive requirements, plastic packages must meet AEC Grade 0 or higher. Therefore, there is a need to evaluate the wire bond interconnect reliability to 2X AEC Grade 0 to assess the interconnect robustness. Palladium coated copper wire (Pd-Cu) was selected to be the wire of choice on aluminum bond pad for high reliability. Low Profile Quad Flat Pack with Exposed Pad (LQFP-EP) packages wire bonded with 20 micron Pd-Cu wire were submitted to high temperature baking (HTB) at 150°C and 175°C for 4032 and 2016 hours, respectively. This paper will discuss the results of electrical test, package deprocessing and wire pull testing, and cross-sectioning through the ball bonds to examine the welding region. Cu voiding within the bulk of the Cu ball bond, specifically close to the capillary chamfer squeezed region on top of the squashed ball, and at the ball edge interfacing with the intermetallic compound was found predominantly at 175°C bake. A trace of Sulfur was found inside the Cu corroding region. Sulfur compounds in mold compounds were examined. EFO current settings were evaluated in order to reduce the propensity to Sulfur corrosion in the Pd-Cu bond balls. Cu voiding is considered as cosmetic defect as it did not lead to electrical failures even at 2X AEC Grade 0 durations. Pd-Cu wire on aluminum bond pad forms very robust and reliable interconnect in extended thermal aging condition, including 4032 hour HTB-150°C and 2016 hour HTB-175°C, making it suitable for more demanding automotive mission profiles.

Effect of Cu and PdCu wire bonding on bond pad splash

Cu wire bonding research has exploded exponentially in the past few years. Many studies have been carried out to understand the different behaviours of Cu wire and Au wire. One of the observations on Cu wire bonding is the excessive formation of aluminium (Al) splash on the bond pad due to a higher bond force. This leads to pad peeling and bond failure resulting in poor reliability performance of Cu and PdCu wire semiconductor devices. It is known that the Al splash is influenced by the front-end pad metal process and back-end wire bond process. Reported is the design of an experiment carried out to study a few factors that could influence the Al splash. The characterisation work is implemented to understand the bond pad structure using the focused ion beam (FIB) followed by a hardness test of bond pad metallisation. Then the mechanical cross-section is taken to measure the Al splash in three different directions. The results show that Al splash can be controlled by optimising the bond pad thickness, hardness and additive for reliable Cu and PdCu wire bonding.

Reliability Assessment and Activation Energy Study of Au and Pd-Coated Cu Wires Post High Temperature Aging in Nanoscale Semiconductor Packaging

Wearout reliability and high temperature storage life (HTSL) activation energy of Au and Pd-coated Cu (PdCu) ball bonds are useful technical information for Cu wire deployment in nanoscale semiconductor device packaging. This paper discusses the influence of wire type on the wearout reliability performance of Au and PdCu wire used in fine pitch BGA package after HTSL stress at various aging temperatures. Failure analysis has been conducted to identify the failure mechanism after HTSL wearout conditions for Au and PdCu ball bonds. Apparent activation energies (Eaa) of both wire types are investigated after HTSL test at 150 °C, 175 °C and 200 °C aging temperatures. Arrhenius plot has been plotted for each ball bond types and the calculated Eaa of PdCu ball bond is 0.85 eV and 1.10 eV for Au ball bond in 110 nm semiconductor device. Obviously Au ball bond is identified with faster IMC formation rate with IMC Kirkendall voiding while PdCu wire exhibits equivalent wearout and or better wearout reliability margin compare to conventional Au wirebond. Lognormal plots have been established and its mean to failure (t50) have been discussed in this paper.

Palladium-Coated and Bare Copper Wire Study for Ultra-Fine Pitch Wire Bonding

There is growing interest in copper (Cu) wire bonding due to its significant cost savings over gold wire. However, concerns on corrosion susceptibility and package reliability have driven the industry to develop alternative materials. Recently, palladium-coated copper (PdCu) wire has become widely used as it is believed to improve reliability. In this paper, we experimented with 0.6 mil PdCu and bare Cu wires. Palladium distribution and grain structure of the PdCu Free Air Ball (FAB) were investigated. It was observed that Electronic Flame Off (EFO) current and the cover gas type have a significant effect on palladium distribution. The Free Air Ball (FAB) hardness was measured and correlated to palladium distribution and grain structure. First bond process responses were characterized. The impact of palladium on wire bondability and wire bond intermetallic using a high temperature storage test was studied. These results for PdCu wire were compared with bare Cu wire.

Wearout Reliability and Intermetallic Compound Diffusion Kinetics of Au and PdCu Wires Used in Nanoscale Device Packaging

Wearout reliability and diffusion kinetics of Au and Pd‐coated Cu (PdCu) ball bonds are useful technical information for Cu wire deployment in nanoscale semiconductor device packaging. This paper discusses the HAST (with bias) and UHAST (unbiased HAST) wearout reliability performance of Au and PdCu wires used in fine pitch BGA packages. In‐depth failure analysis has been carried out to identify the failure mechanism under various wearout conditions. Intermetallic compound (IMC) diffusion constants and apparent activation energies (Eaa) of both wire types were investigated after high temperature storage life test (HTSL). Au bonds were identified to have faster IMC formation, compared to slower IMC growth of PdCu. PdCu wire was found to exhibit equivalent or better wearout reliability margin compared to conventional Au wire bonds. Failure mechanisms of Au, Cu ball bonds post‐HAST and UHAST tests are been proposed, and both Au and PdCu IMC diffusion kinetics and their characteristics are discussed in this paper.

Pd-coated Cu Wire Bonding Reliability Requirement for Device Design, Process Optimization and Testing

One of the biggest technology drivers in the semiconductor industry today is the fast transition from Au wire bonding to Cu wire bonding. The fast adaptation of Cu and Pd-coated Cu (PCC) wire has focused the whole packaging industry to develop understanding, equipment and processes that can produce a more reliable and robust Cu wire bonding technology. Although the fundamentals of wire bonding technology are very similar between Au and Cu wire bonding, there are a lot of new challenges in Cu wire bonding. Compared to Au wire bonding, Cu wire bonding needs different bond quality measures and metrology. Traditional ball diameter, ball height and shear measurements are not adequate to quantify a Cu wire bonding process. Some of the additional bond quality measures include pad material push out (pad splash), Al layer peel off (pad peel) and crack in the barrier and dielectric layer (pad crack). Another area that is quite different between Au and Cu is the reliability test requirement. In Au wire bonding, because of the fast intermetallic compound (IMC) growth rate, the HTS test is normally the hardest to pass. Due to the corrosion of Cu wire, the HAST test is the most challenging in Cu wire bonding. Reliability requirements still need more knowledge. In this paper, we conduct reliability tests for devices with 3 sets of wire bonding parameters. The bonded samples have IMC coverage between 94% and 97%, well above the industry level of 80%. The reliability (HAST) test passed for all samples at 96 hours. However, there are some failures in the HAST test at 192 hr. There are many factors that can influence reliability outcome including wire bonding and non-wire bonding related factors. The failure analysis identified two potential causes in our case. In one failure case, an abnormally high Chlorine level and void in molding compound were detected next to the failed bond while no Chlorine and void were detected elsewhere. In the 2nd failure case, the bonded ball seems to be off centered and results in poor bonded ball to pad interface. These two factors will be more tightly controlled in future tests to verify the reliability outcome. Intermetallic growth and phase transformation, aluminum oxide, and behavior of palladium in PdCu wire bonds are analyzed using transmission electron microscopy (TEM) of dual beam focused ion beam (FIB) thinned specimens. Results are compared to wire bonding measurement and reliability outcome.