Bonding Process Window Research Articles

Impact of surface texture on ultrasonic wire bonding process

Due to the complex mechanisms, the ultrasonic (US) wire bonding process is usually optimized in the way of varying the processing parameters including normal force, US power, and processing time. In this study, a new way by creating different surface textures on substrates was used to alter the bonding process and improvements of the bonding process were detected. Three different surface textures including deposited strips, straight ditches at different angles, and elliptic ditches were designed and created on glass substrates. The results showed that the elliptic ditches hardly influence the bonding process while the deposited strips and straight ditches significantly alter the bonding process. The deposited strips help break the oxide scale and facilitate the transportation of oxides to the outside of contact. With the straight ditches, the oxide removal efficiency was significantly enhanced. Especially when the driving current exceeded 0.45 A, long chips from the ditches were clearly observed during the bonding process. The chips were aluminum and aluminum oxide which were continuously cut from the wire, accumulated in the ditches, pressed and squeezed to the outside of the contact. With a different angle of the straight ditches, the shape of the bonding footprint can be changed correspondingly. Compared to the bonding on smooth surfaces, the bonding strength on substrates with deposited strips and straight ditches was a few times higher and had a smaller deviation. The bonding process window was significantly enlarged.

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.

Gold Wirebond on Discolored Bond Pads

An integrated circuit wafer lot having some wafers with discolored bond pads and other wafers with normal bond pads was identified, and wafers with discolored pads were scrapped. The reason for scrap is the expectation of poor bondability or unreliable wirebonds on discolored pads. The cause of discoloration was unknown. We took the opportunity to run a bonding experiment and analysis as a student project, comparing a good wafer with one having the discolored bond pads. Unprobed die from each wafer were wirebonded for mechanical integrity analysis of the bonds. Bonding recipe designed experiments included different ultrasonic generator (USG) current settings within the bonding process window, two bonding forces, and two temperatures, all thermosonic binding 25um gold (Au) wire on AlCu pad metal of 0.7um thickness. 2% non-stick-on pads (NSOP) was found at a higher ultrasonic power settings on the discolored pads, indicating that the discolored pads can indeed be problematic in wirebonding. Analysis of wire pull test and bond shear test results indicate slightly less performance of bonds on the discolored pads. Chemical and physical analysis of the discolored pads reveals the nature of the unknown cause and leads to a hypothesis about what went wrong during the wafer processing.

Advancement in Thermosonic Bonding Wire

Coated copper (Cu) and alloyed silver (Ag) wires are the new developments in bonding wire sector and this paper discusses on its bonding performances. Palladium (Pd) coated Cu wire replaces rapidly with Au wire. A thin gold (Au) coating over Pd coated Cu wire revealed wider 2nd bond process window and superior 2nd bond bondability, termed as Au Flash Pd Coated Cu Wire (AFPC). The free air ball (FAB) formation, looping, 1st bond and reliability performance of the AFPC wire satisfies the basic requirements. The benefits of doped/alloyed Cu wire are also presented. Good reflectance properties of Ag wire replaces with Au wire in LED sector. Moreover, soft nature of Ag wire and its FAB near to Au, turn the focus to Ag as a potential bonding wire. Detailed examination on alloying additions, Ag electro migration, reliability, bondability, FAB formation, etc., are currently examined and discussed. During bonding, purging with forming gas is a common practice irrespective of the wires used (bare/alloyed/coated Cu or Ag wires).

Study of 6 mil Cu Wire Replacing 10–15 mil Al Wire for Maximizing Wire-Bonding Process on Power ICs

Copper (Cu) wire-bonding with its advantage in cost, mechanical enhanced characteristic, and better electrical performance is a developing alternative interconnection technology to replace gold (Au) and aluminum (Al) wires in IC packaging manufacturing. This paper discussed the experimental study of using 6-mil Cu wire on an ASM wire bonder to replace 10-15 mil Al wire in a power IC device. It encompassed wire and tool selection, wire-bonding process development, post wire-bonding integrity inspection, and bonding reliability results. The wire and tool selection included type of wire, capillary use, and bonder capability. The process development focused on two crucial stages, Free air ball (FAB) formation and bonding process window development. Design of experiment (DOE) was extensively applied in this study. The experimental studies showed that flow rate of forming gas was a key factor to form the qualified FABs and establishment of a workable process window. Wire pull and ball shear tests were conducted per JEDEC criteria for bond strength integrity. Moreover, crater test and Zygo's 3-D measurement were used to inspect any risk of underlying metal integrity in die before reliability tests. The data showed that sufficient thickness of the Al bond pad was crucial to avoid any underlying metal damage when subjected to heavy copper wire bonding forces. High-temperature baking (HTB) and pre-condition (PC) and temperature cycle (TC) were used to evaluate the samples reliability. The results of the two reliability tests showed that Cu/AL intermetallic compound (IMC) growth was slow, which indicates potential significant product life-span extension. The study concluded that with current thermosonic ball bonder, using 6-mil Cu wire could replace heavy 10-15 mil Al wire in power IC applications.

Fundamental understanding of ACF conduction establishment with emphasis on the thermal and mechanical analysis

This paper presents the thermal and mechanical contribution of anisotropic conductive films (ACFs) to the electrical conduction establishment of ACF joint. The conduction mechanism of ACF joint strongly depends on the thermal and mechanical properties of ACF. Therefore, it is important to understand the relationship of thermal and mechanical properties of ACF in a bonding process with the electrical conduction establishment. In this study, ACF flip chip process was fully designed based on the material characterization and in situ process monitoring. Moreover, the effect of degree of cure on the ACF conduction establishment was investigated in a bonding process window. The important mechanical mechanism of ACF conduction for good bonding quality is the joint clamping force due to curing and cooling-down processes of ACFs. The build-up behavior of z-axis shrinkage stress in ACF joint during curing and cooling-down processes of ACF materials was experimentally investigated with thermo-mechanical measurement of ACF. These results reveal that shrinkage stress in ACF joint developed during bonding process is the important parameter to establish the electrical conduction of interconnects using ACF material.

Optimizing the Wire-Bonding Process for Copper Ball Bonding, Using classic Experimental Designs

Two classic experimental designs were used to develop the copper ball bonding process. A two-level factorial design was used to screen five process variables simultaneously and determine whether any of them had a significant effect on the process. A four-factor central composite design was used to optimize and map the bonding process window. The efficiency of using designed experiments allowed the development of the process and the achievement of a major milestone in only 15 experiments. The experimental designs which were used have wide application for process development and optimization. The concepts can be easily taught so that the experiments can become a standard for process technicians to follow. The use and analysis of these experimental designs are described.