Automatic wire bonding is a highly mature, cost-efficient and broadly available back-endprocess, intended to create electrical interconnections in semiconductor chip packaging. Modern production wi ...

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Unconventional applications of wire bonding create opportunities for microsystem integration

Chapter 1 gives an introduction to Cu wire bonding technology. The advantages of Cu over Au, such as lower cost, higher mechanical strength, and higher electrical and thermal conductivity, are discussed. The chapter describes the adoption of Cu wire bonding in the semiconductor industry, as well as future projections for its usage.

Copper Wire Bonding

Novel structure design and shielding mechanism of various shielding materials are critically reviewed. Measurement methods of far-field and near-field shielding are presented. Challenges and future perspectives for shielding materials are discussed.

Electromagnetic interference shielding materials: recent progress, structure design, and future perspective

Wire bonding is the predominant mode of interconnection in microelectronic packaging. Gold wire bonding has been refined again and again to retain control of interconnect technology due to its ease of workability and years of reliability data. Copper (Cu) wire bonding is well known for its advantages such as cost-effectiveness and better electrical conductivity in microelectronic packaging. However, extended reliabilities of Cu wire bonding are still unknown as of now. Extended reliabilities of Au and Pd-coated Cu (Cu) ball bonds are useful technical information for Au and Cu wire deployment in microelectronic packaging. This paper discusses the influence of wire type and mold compound effect on the package reliability and after several component reliability stress tests. Failure analysis has been conducted to identify its associated failure mechanisms after the package conditions for Au and Cu ball bonds. Extended reliabilities of both wire types are investigated after unbiased HAST (UHAST), temperature cycling (TC), and high-temperature storage life test (HTSL) at 150, 175, and 200 °C aging temperatures. Weibull plots have been plotted for each reliability stress. Obviously, Au ball bond is found with longer time to failure in unbiased HAST stress compared to Cu ball bonds for both mold compounds. Cu wire exhibits equivalent package and or better reliability margin compared to Au ball bonds in TC and HTSL tests. Failure mechanisms of UHAST and TC have been proposed, and its mean time to failure (t
 50), characteristic life (t
 63.2, η), and shape parameter (s) have been discussed in this paper. Feasibility of silver (Ag) wire bonding deployment in microelectronic packaging is discussed at the last section in this paper.

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Extended reliability of gold and copper ball bonds in microelectronic packaging

The package technology has matured significantly over the past several years, shifting from conventional components and direct board level assembly to chip or package level system integration. Two major commonly used approaches are System-on-Chip (SoC) and System-in-Package (SiP). Package-on-Package (PoP) that integrates logic die in the bottom package and memory die in the top package into a single 3D package is one of the promising SiP solutions.

Warpage reduction of package-on-package (PoP) module by material selection & process optimization

In semiconductor packaging, wire bonding is the main technology for electrical connections between chip and leadframe or substrate. Gold wire bonding has the advantages of a fast bonding process, excellent electrical property and stable chemical property. It has been widely used in various electronic packages. Gold prices have been raised significantly over the last few years. Many manufactures have been investigating ways to replace the conventional gold wire with various new materials. Copper wire bonding is an alternative interconnection technology. Cu wire has superior electrical and thermal conductivities as well as higher tensile strength, elongation and better “ball neck” strength. On the other hand, the higher hardness of Cu wire requires higher ultrasonic power and bonding force, which lead to high risk of cratering for ball bonding and tearing for wedge bonding. These will cause some package limitation and wire bonder machine downtime or low units per hour (UPH). Ag alloy wire has low Young's modulus and hardness property. It is a low cost wire bonding solution other than gold wire. In this study, Ag alloy wire is proposed as an alternative to Au bonding wire. Emphasis is placed on the wire bonding workability and reliability of using Ag-Au-Pd alloy wire for TSOP package. Also, wire bonding parameter such as electronic flame off (EFO), bond force, ultrasonic power, heat block temperature and time for ball and wedge bonding are optimized. Furthermore, the response for parameter optimization is determined by the Dage bond tester. Package reliability is determined through environmental tests that include pressure cooker test (PCT), temperature cycle test (TCT) and high temperature storage life test (HTSL). The tested samples were studied by focused ion beam (FIB), scanning electron microscopy (SEM) and energy dispersive spectrometer analyses (EDS). Intermetallic compound growth behavior during reliability test is characterized and compared to Al-Au and Al-Cu systems. Ag-Al didn't have excessive volume variation and void occurrence to get better bonding performance during various reliability tests.

Silver alloy wire bonding

In recent years, miniaturization, lightening, high performance, high reliability and low cost have been demanded intensely for electronic products, especially in the rapid growth of portable cell phone domain. Furthermore, multiple functional demand induces advanced package developments, such as system-on-chip (SoC) and system-in-package (SiP). System-on-chip (SoC) is an ideal package to integrate multiple functionalities in the chip level. However, the design and testing are very difficult, high cost and low manufacturing yield, these reasons drives multiple functional integration technology toward system-in-package (SiP) development gradually. The package-on-package (PoP) stacking assembly is a member of the SiP family. It is constructed by individual fabricated and tested packages from the same or different supplier provided in a stacking structure through solder joints. It can reduce the placement and routing areas on board and also allow one damaged component to be replaced which is opposite to stacked Chip Scale Package (SCSP). This paper aims to investigate warpage and stress characteristic analyses for PoP configuration by using finite element method (FEM). The structure consists of bottom PBGA package and top TFBGA package stacking through solder joints. While top TFBGA is mounted on bottom PBGA, controlling component warpage is a very important issue. The excessive warpage could induce failure on stacking process. In this study, firstly, we focus on bottom PBGA warpage investigation for package and die size, die and substrate core thickness, compound and substrate core material properties effects in order to find out optimized BOM and dimension guidelines. Furthermore, employing suitable BOM and dimension leads PBGA package assembly to achieve warpage less than 4 mil. The experimental result agrees above target very well finally. Secondly, PoP module warpage and ball stress characteristics are further to analysis as the next topic. For fixed bottom PBGA package from above-mentioned finding design and combination, module warpage and ball stress trends are investigated to take as PoP design guidelines with top TFBGA die size and stacked die number effects. For summarized results, the top TFBGA with 7 times 9 mm2 die size can reduce 24% module warpage compared to 11 times 13 mm2. The TFBGA with single die can reduce 29% module warpage than 4 stacked dice. Shrinking chip size and decreasing die stacked number can be recommended to reduce module warpage and board level ball stresses effectively including TFBGA and PBGA balls. Finally, we will investigate top TFBGA mold thickness effect on PoP structure to minimize module warpage and ball stresses.

Warpage and Stress Characteristic Analyses on Package-on-Package (PoP) Structure

As the semiconductor technology advancing, the component size is gradual miniaturizing, so either busless design or electroless plating process should be used to meet the requirement of high routing density. Between these two solutions, electroless plating process has another advantage-lower cost. In electroless plating process, ENIG is popular in the industries, but it easily causes solder joint problems due to potential black pad issue. Therefore, a new surface finish- E'less nickel E'less Palladium immersion Gold (ENEPIG) is developed to overcome this problem by introducing a Pd layer between Ni and Au. In this paper, we studied the solder joint reliability for ENEPIG after multiple reflow times ( 1x, 3x, 6x ) by using conventional ball shear test, cold-ball pull test. High speed ball shear test was also applied to simulate high strain rate loading, such as drop test. Failure mode and micro analysis were carried out by the analytical tools, including SEM and EDX. From the SEM imagines, it is founded that the IMC thickness of electrolytic Ni/Au was thicker than ENEPIG under identical reflow times. From the ball shear test results, it seems no obvious difference between different reflow times for Electrolytic Ni/Au and ENEPIG. The failure mode of Electrolytic Ni/Au and ENEPIG were both broken at solder. Cold-ball pull test showed Electrolytic Ni/Au and ENEPIG have same failure mode distribution. In the high speed ball shear test, the behavior of broken interface was quite different between Electrolytic Ni/Au and ENEPIG. The broken interface of electrolytic Ni/Au ususlly happened at IMC phase, but the ENEPIG usually happened at solder phase. It could be deduced that ENEPIG has better solder joint quality.

Investigation of IMC growth and solder joint reliability on new surface finish-ENEPIG

The growing concern of environmental issues is impacting the latest development trends for packaging materials in the semiconductor industry. Several types of brominated flame retardants are used in the electronics industry including PBB (Polybrominated biphenyl), poly brominated diphenyl ether (PBDE) and tetra bromo bisphenol-A (TBBPA). Although PBB and PBDE do not use in mold compound as flame retardant, but bromine-containing flame-retardants and antimony oxide flame retardant synergists are commonly used in molding compounds formulations and to be potentially unsafe if released into the environment. Therefore, the halogen free mold compound is further environmental issue that is cirtical thems for developing future packaging materials as that HP company to eliminate brominated flame retardants from all New HP brand products. LGA (Land grid array) package is a thinner and lighter assembly that design standard BGA without solder balls. It is suitable for applications to require high electrical performance. The halogen free mold compound is evaluated for ultra-thin LGA package in this study. Including choosing new resin structure, higher filler content, new flame retardant system to instead of the current antimony trioxide and brominated epoxy resin combination. In general, because IR reflow temperature is from 220degC to 260degC for lead free condition, it needs to consider the stress issue in package to pass lead free condition. Thus the glass transfer temperature (Tg) of new resin system will be reduced in advance halogen free mold compound technology. In this study, the halogen free mold compound is applied in ultra-thin LGA package; one key challenge for ultra-thin LGA package is the warpage and thermo mechanical stress that the CTE (Coefficient of thermal expansion) mismatch between the thinner mold compound and thinner organic substrate. However, the lower CTE of mold compound has warpage impact for ultra-thin LGA package with thinner substrate. Therefore, material property of halogen free mold compound is controlled to balance warpage performance in thinner package. On the other hand, the shrinkage of mold compound was found to affect the warpage of ultra-thin LGA package in our study. The mold shrinkage will be discussed in this paper. The reliability result such as moisture sensitivity level -JEDEC Level 3, 30degC/ 60%RH/ 192 hours at reflow temperature 260degC, TCT, PCT, HAST and HTSL are also shown in this paper.

Halogen-Free Mold Compound Development for Ultra-Thin Packages

In IC, wire bonding is the main technology for electrical connections between chip and leadframe or substrate. Gold wire bonding has the advantages of a fast bonding process, excellent electrical property, and stable chemical property. But, gold prices have risen significantly over the last few years. Many manufactures have been investigating ways to replace the conventional gold wire various new material. Copper wire bonding is an alternative interconnection technology. Compared with gold wire, Cu wire is better than gold with respect to electrical conductivity. The inherent stiffness of the copper wire also makes long wire with small diameters more resistant to wire sweep during molding. There are also some problems with Cu bonding process: (1) Copper easily oxidizes in air. The application of copper wire coated with palladium is a solution to prevent copper oxidation during the bonding process. (2) The higher hardness of wire generally requires higher ultrasonic power and bond force to bond on metal. It also lead to high risk of cratering for ball bonding and tearing for wedge bond. This paper reports a study on the influence of surface finish type, deposition thickness and bonding machine parameter for wedge bonding. In this study, 0.7mil Pd coated Cu wire was bonded on three kinds of surface finish, as electro-plating Nickel and Gold, electro-plating Nickel, palladium & gold, Electroless Nickel, Electroless Palladium and Immersion Gold (ENEPIG). The noncontact optical profiler, microhardness tester and scanning electron microscope (SEM) were used to examine surface finish characteristics. Bonder machine alarm rate, pull strength and failure mode after wire pull test were performed to confirm the quality for Pd coated Cu wire bonded on three kinds of surface finish. Then, wedge bonding mechanism of Pd coated Cu wire was discussed in this paper.

Yu Po Wang

Papers

Silver alloy wire bonding

Warpage and Stress Characteristic Analyses on Package-on-Package (PoP) Structure

Investigation of IMC growth and solder joint reliability on new surface finish-ENEPIG

Halogen-Free Mold Compound Development for Ultra-Thin Packages

Investigation of ultrasonic Pd coated Cu wire wedge bonding on different surface finish