Bump Height at Low Temperature Analysis

Flip chip technology has grown by leaps and bounds and is getting even smaller in size. Optimization of process parameters in manufacturing is eminent due to reliability issues. This paper reports the parameters that affect the quality of the bump height in electroless nickel immersion gold (ENIG) and their relationships between each other. A total of four different combinations of parameters have been carried out for this investigation using the design of experiment (DOE) approach. It can be concluded that higher temperature of electroless nickel permits an increase of bump height where as the increment in immersion gold temperature does not nessasarily affect the value of bump height. All four samples recorded a higher value of bump height than the controlled bump height value. This implies reliability of the solder joint and assembly process robustness can be improved with an increase of bump height by increasing the time.

This paper reports the factors that affect the bump height in electroless nickel immersion gold (ENIG) and their interrelation between each other. Bump height is a critical issue that needs to be investigated because a certain quality and requirements of bump height needs to be achieved prior to reflow oven soldering process. A total of four controllable process variables, with 16 sets of experiments were studied using a systematically designed design of experiment (DOE). The result suggests that the electroless nickel bath time has the most significant effect on the formation on bump height and consequently provide larger area for conductivity.

Solder joint reliability is dependent on both thickness and morphology of the intermetallics that form and grow at the solder joint interface during soldering and subsequent thermal ageing and examining the morphology of these intermetallics is of great importance. The focus of this paper is to present experimental results of a comprehensive study of the interfacial reactions during soldering of Sn-Ag-Cu lead-free solders on copper (Cu), immersion silver (ImAg), electroless nickel/ immersion gold (ENIG) and electroless nickel/ electroless palladium/ immersion gold (ENEPIG) surface finishes. Using scanning electron microscopy detailed a study of the 3-D morphology and grain size of the intermetallics was conducted. The results showed that when soldering on ENIG and ENEPIG finishes several morphologies of intermetallics with different grain sizes form at the solder joint interface compared to a single intermetallic morphology that forms when soldering on copper and immersion silver. An attempt was made to discuss the effect of several factors that may have an influence on the type of morphology the intermetallics may grow into. The results obtained in the present investigation also revealed that the technique of removing the solder by deep etching to examine the morphology of intermetallics is a convenient and efficient method to investigate the intermetallics formed at the solder joints.

In our experiment, the possibility of forming copper wire bonds in Chip-on-Board (COB) applications was investigated. In the experiment, such surface finishes were studied and compared which are common in printed circuit board based COB applications and can be bonded with wire bonding technologies. These finishes were immersion silver (ImAg), electroless nickel / immersion gold (ENIG) and electroless nickel / electroless palladium / immersion gold (ENEPIG). On each type of surface finish, copper wires with two diameters (33 and 300 μm) were bonded with a TPT Ultrasonic bonder. The wire bonds then characterized by optical inspections and mechanical measurements which are detailed in the paper.

The technology of surface finishes for printed circuit boards is seeing a dramatic shift from the Hot Air Solder Levelling (HASL) towards alternative finishes like electroless Nickel - immersion gold (ENIG), immersion tin, immersion silver and organic solder preservatives (OSP's). This trend is mainly caused by the worldwide environmental pressure to ban the use of lead for electronic assemblies as well as the demands of modern assembly technology, which require a higher coplanarity of the surface finish for surface mount assembly. One of the main benefits of HASL is the extremely good resistance of the surface against ageing under high temperature conditions. Alternative finishes, due to their limited thickness, offer excellent coplanarity, but inferior surface protection. A detailed understanding of the ageing processes is thus a prerequisite for the implementation of alternative surface finishes in high yield PCB production. In this paper the ageing characteristics of immersion tin surface finishes are investigated and correlated with the reliability during soldering operations. Ageing of tin surface finishes takes place by intermetallic compound (IMC) formation at the Sn/Cu interface due to solid state diffusion. Hence the kinetics of the process is assumed to follow a simple law. The kinetics of IMC formation was investigated experimentally and correlated with data from literature. The results show that the simple kinetic model leads to an overestimation of the kinetics in the temperature regime below 60°C. Changes in the reaction mechanism with temperature as well as with time must be taken into account for a conclusive understanding of the ageing characteristics. The only parameter with relevance for practical solderability is the thickness of the immersion tin layer.

Electroless Nickel Immersion Gold (ENIG) is one of the important final finish techniques that is used in the printed circuit board industry. Despite its relatively expensive cost compared to other final finishing processes such as OSP and immersion Tin etc., ENIG has many advantages. The low reactivity of Au can protect the underlying Ni and Cu surfaces from oxidation and keeps the board suitable for long storage time. It provides excellent surface planarity suitable for soldering, especially for tiny parts such as BGA and FlipChip, and the surface remains solderable even after multiple reflow cycles. It is useful for contact surfaces such as membrane switches and contact points and may also be used for wire bonding. The Immersion Gold (IG) bath provides the protective Au layer after the Electroless Ni (EN) process following the below equation: 2Au+ + Ni _ 2Au + Ni <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> . As an electrochemical process, the deposition rate (or Au thickness per unit of time) is highly dependent on the redox potential of Au <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> and electropotential of the Ni surface. The former can be considered as a fixed value in a Au plating solution while the latter will vary depending on the design of boards (pad size, connection of pad to the inner-layer Cu, etc.). The differences of surface potential, or so-called galvanic effect, makes the deposition of Au non-uniform, which may influence the properties of the board in many aspects. For example, chromatic aberration of the Au surface appears and some of the extremely thick Au pads may appear as red color; solderability may become uneven across all pads; and Au may be wasted due to the poor gold thickness distribution. More seriously, the galvanic effect may cause Au skip-plating, or black Ni because of Ni over-dissolution on specific pads, resulting in the solderability problem. Our new immersion gold technology can effectively reduce the galvanic effect by introducing a new chemical system. The electropotential differences were minimized among pads with various surface areas or connected to different inner-layer copper areas thus providing a uniform gold deposition. Meanwhile, the plated Au thickness can be steadily increased from less than 1 microinch to 3 microinches and metallic lustre of the Au surface remained normal, from the pale yellow of thinner Au to the lemon yellow of thicker Au. Ni corrosion resistance remained good for a range of Au thickness from 1 microinch to 3 microinches. Modified HNO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> porosity test yielded good results. Solderability test using SAC305 solder showed good reliability after 0×, 3× reflow and baking (150 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> C, 4 hours) conditions in ball shear and ball pull test.

This study reports a number of experiments that were designed to characterize aluminum bond pad surfaces prior to electroless nickel immersion gold (ENIG). In the ENIG process, aluminum bond pads need special treatment to achieve successful nickel deposition and provide reliable interconnection of under bump metallurgy in advanced packaging. During this treatment process, the aluminum pad was cleaned, activated and then coated with a layer of zinc. Systematic study was carried out to determine the best parameters, through multiple and various exposure times of the zincation process and zincation solution concentration effect on the Ni/Au surface roughness and aluminum dissolution rate on the bond pad during multiple zincation process. The ball shear strength was evaluated between eutectic 37Pb/63Sn solder ball and under bump metallurgy (UBM) interfaces across multiple zincation process. Scanning Electron Microscope (SEM), Energy Dispersive X-Ray (EDX), Atomic Force Microscopy (AFM), Focused Ion Beam (FIB) and ball shear tester were used as analytical tools. The results suggest that the multiple zincation process consistently produces a smoother surface of ENIG UBM and consequently provides a better shear strength.

High-throughput assembly technologies to form Copper (Cu) interconnections without solders at below 200°C, and pitch below 40µm has been a major challenge in the semiconductor industry. A unique solution has been demonstrated by Georgia Institute of Technology to overcome this grand challenge. This technology utilizes thermocompression bonding to form copper interconnections with process tolerances to accommodate non-coplanarities of bumps and warpage of the substrate, without solders. The bonding pressure applied for thermocompression was 365MPa, to enable Cu bump collapse by 3µm. As thermocompression bonders are generally force-limited to 400N, such high bonding pressures may hinder scalability of this technology to fine pitches with higher I/O densities. This paper addresses this manufacturability challenge with the novel Electroless Palladium Autocatalytic Gold (EPAG) surface finish instead of the standard Electroless Nickel Immersion Gold (ENIG) or Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG) finish, previously used to prevent Cu oxidation for bonding load reduction down to 120MPa.

In this paper, the performance of a complete halogen-free (HF) board assembled with five ball grid array components using daisy chains on an 8-layer high density interconnection printed circuit board (PCB) was examined using bending tests. The solder joint was composed of the SAC405 solder ball melted with the SAC105 solder paste on the Cu pad surface of either organic solderable preservative (OSP) or electroless nickel immersion gold (ENIG), and was tested to examine the extent of intermetallic compound (IMC) formed. A board-level cyclic bending test was executed to the as-reflowed assemblies according to the JESD22-B113 standard to evaluate the reliability of various surface finished HF PCB assemblies under a recommended load condition. By Weibull analyzes the characteristic lives of the assemblies were 99098 and 130290 cycles on OSP soldering system and ENIG system respectively, indicating that the ENIG had a higher bending fatigue endurance as compared to the OSP. The dominant failure mode was the fracture of Cu trace within the micro-via caused by the crack of the resin coated copper layer. At the interface of the solder joint, the IMC formed on both OSP and ENIG was with various Ni contents of 3.9 and 20.1 at%, and different thicknesses of 5.6 and 1.1 , respectively. Numerical simulations showed that both composition and thickness of IMC influenced the stress concentrations and characteristic life of the HF test vehicle as they could slightly increase localized solder joint rigidity and result in more serious pull and drag behaviors at the corner joint. Although the on the ENIG was stiffer than that on the OSP, the thicker IMC layer instead increased the stress on the OSP system due to size effect, and resulted in a lower characteristic life.

During the reflow process of In-3Ag solder ball grid array (BGA) packages with electroless nickel immersion gold (ENIG) and immersion silver (ImAg) surface finishes, continuous (Au0.9Ni0.1)In2 and scallop-shaped (Ag0.9Cu0.1)In2 intermetallic layers form at the interfaces of In-3Ag solder with Au/Ni/Cu and Ag/Cu pads, respectively. The (Au0.9Ni0.1)In2 layer breaks into clusters with increases in the aging time and temperature. Aging at 115 °C results in the formation of an additional continuous Ni10In27 layer on the Ni/Cu pads and the migration of (Au0.9Ni0.1)In2 intermetallic clusters into the solder matrix. In contrast, the (Ag0.9Cu0.1)In2 scallops grow into a continuous layer after aging treatment. Accompanying the interfacial reactions, AgIn2 precipitates in the interior of In-3Ag solder balls and coarsens during aging, causing the ball shear strengths of reflown ENIG (1.18 N) and ImAg (1.11 N)-surface-finished solder joints to decrease gradually. However, the migration of (Au0.9Ni0.1)In2 clusters into the solder matrix of ENIG-surface-finished In-3Ag packages leads to an increase in their ball shear strengths after aging at 115 °C over 300 h. Both the ENIG- and ImAg-surface-finished In-3Ag solder joints, after ball shear tests, have fractured across the solder balls with ductile characteristics.

Electroless nickel immersion gold (ENIG) process is one of the most used selective finishing in PCB production. It involves two different electroless deposition mechanisms: (1) NiP autocatalytic deposition and (2) gold galvanic plating where coating is obtained by a displacement reaction. Because during the ENIG process, NiP is dissolved from the electrode surface into the solution, it can be considered as a controlled corrosion process. For this reason, selection of the complexing agents in solution is crucial. Since they are the most used complexing agents, in this study EDTA and citric acid have been selected. In order to understand their effects on the NiP surface during the immersion plating, open circuit potential and potentiodynamic detection measurements have been performed. Furthermore, to establish which complexes are adsorbed on the electrode surface during the plating process, in-situ Raman spectroscopy was performed. Electrochemical and spectroscopic analyses reveal that EDTA is the dominant complexing agent, while citric acid is mainly a buffering agent.

We present a novel chemical VOCs sensor based on a CMOS-MEMS gold plated micro-resonator functionalized with a self-assembled monolayer (SAM) of 1- octadecanethiol (ODT) via solution-casting being, in contrast to common techniques, fully compatible with batch production without additional lithographic steps requirement. The device operated at 2-MHz being fully integrated with an on-chip Pierce oscillator circuit for self- sustained operation. The MEMS resonator was fabricated using a commercial 0.35-μm CMOS technology and coated through electroless nickel immersion gold (ENIG) process providing thiol-based extended functionalization compatibility over standard CMOS BEOL aluminum layers. This CMOS-MEMS sensor platform opens several possibilities to multiple functional SAMs-based organic surfaces by synthetizing functionalized thiols and build a sensing matrix for breath pattern recognition, depicting tremendous potential low-cost non-invasive disease diagnosis applications.

Printed circuit board integrated wearable ion-selective electrode with potential treatment for highly repeatable sweat monitoring

In this work, two surface fimishes- Electroless Nickel Immersion Gold (ENIG) and Galvanic Gold were compared. Solder joints were tested with respect to used materials (ENIG and Galvanic Goldfimishes, lead-free solders SAC305 M8, SAC305 NC257-2), used technologies (heating factor of 218 $s^{\circ}C$ and 2295 $s^{\circ}C$). The resistance of soldered joints was measured on prepared samples. The measurement results showed the influence of the heating factor on the resistance of solderedjoints. With a heating factor of 218 $s^{\mathrm{o}}C$, the resistance of the joint is higher than that of a soldered joint realized using a higher heatingfactor, except for samples with a galvanic coating. A destructive shear strength test was also conducted. The results showed that heating factor may affect reliability of the solderedjoint as the shear strength is influenced. In the case of SAC305 M8 solder, the shear strength values of the solderedjoints with heating factor of 2295 $s^{\circ}C$ are approximately 21% higher, with the exception of the sample with half thickness of the standard thickness of ENIG surface fimish. The SAC305 NC257-2 solder does not have big differences in shear strength at lower and higher heating factors during reflow.

Eutectic (63Sn-37Pb) solder balls were doped at varying levels of copper up to 2.0 wt% to determine the effect on the strength of the ball to substrate bond. The test vehicles were fully assembled packages with the Cu-doped solder balls reflowed on electroless nickel immersion gold (ENIG) substrates. Focus is placed on the analysis of the various intermetallic compounds (IMC). SEM and EDX were used to determine the IMC growth rates and compositional analysis as devices were aged up to 1000 hours at 150/spl deg/C. Ball shear and ball pull tests were performed to determine the effects of the copper dopants as compared to the standard eutectic solder at the component level. 4-point flex testing was employed to determine the dopant's effectiveness in improving ENIG brittle fracture failures.