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

Design and optimization of thermo-mechanical reliability in wafer level packaging

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

Wafer Level Packaging (WLP) has the highest potential for future single chip packages because the WLP is intrinsically a chip size package. The package is completed directly on the wafer then singulated by dicing for the assembly. All packaging and testing operations of the dice are replaced by whole wafer fabrication and wafer level testing. Therefore, it becomes more cost-effective with decreasing die size or increasing wafer size. However, due to the intrinsic mismatch of the coefficient of thermal expansion (CTE) between silicon chip and plastic PCB material, solder ball reliability subject to temperature cycling becomes the weakest point of the technology. In this paper some fundamental principles in designing WLP structure to achieve the robust reliability are demonstrated through a comprehensive study of a variety of WLP technologies. The first principle is the 'structural flexibility' principle. The more flexible a WLP structure is, the less the stresses that are applied on the solder balls will be. Ball on polymer WLP, Cu post WLP, polymer core solder balls are such examples to achieve better flexibility of overall WLP structure. The second principle is the 'local enhancement' at the interface region of solder balls where fatigue failures occur. Polymer collar WLP, and increasing solder opening size are examples to reduce the local stress level. In this paper, the reliability improvements are discussed through various existing and tested WLP technologies at silicon level and ball level, respectively. The fan-out wafer level packaging is introduced, which is expected to extend the standard WLP to the next stage with unlimited potential applications in future.

Design and Reliability in Wafer Level Packaging

A package apparatus includes a first package module, a second package module and multiple conductive elements. The first package module includes a first molding compound layer, a first conductive pillar layer disposed in the first molding compound layer, a first internal component, and a first protection layer. The first internal component electrically connects to the first conductive pillar layer and disposed in the first molding compound layer. The first protection layer is disposed on the first molding compound layer and the first conductive pillar layer. The second package module includes a second molding compound layer, a second conductive pillar layer disposed in the second molding compound layer, and a second internal component. The second internal component electrically connects to the second conductive pillar layer and disposed in the second molding compound layer. The conductive elements are disposed between the first and the second conductive pillar layers.

Package apparatus and manufacturing method thereof

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 this paper, we studied the wire bonding ability and the solder joint reliability for electrolytic Ni/Au and ENEPIG. For studying wire bonding ability, 4N wire with 20 mum in diameter was used. Pull strength of Au wire and failure mode after each pull test were both the criteria of wire bonding. After wire pull test, the pull strength and failure mode of Electrolytic Ni/Au and ENEPIG were similar. Therefore, it could be supposed the wire bonding ability of ENEPIG is similar with electrolytic Ni/Au. For solder joint reliability, different types of solder joint test were conducted- conventional ball shear test, cold-ball pull test. High speed ball shear test was also applied to simulate high strain rate loading, similar as drop test. Failure mode and micro analysis were carried out by the analytical tools, including scanning electron microscope (SEM) and energy dispersive spectrometer (EDX).

Evaluation of new substrate surface finish: Electroless nickel/electroless palladium/immersion gold (ENEPIG)

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 reliability life of solder joints is a well-known key topic for the electronics industry. Board level temperature cycling test is most common test method and provide an approximate means of relating the results from these performance tests to the reliability solder attachments for the use environments and conditions of electronic assemblies. Recently, due to environmental debates, SnPb solder has replaced by lead free materials. The two most common compositions are Sn3.0Ag0.5Cu and Sn4.0Ag0.5Cu, respectively. Since small and thin package is the future tendency, the structure of shorter interconnection of chip scale package (CSP) with ball grid array (BGA) is customarily occupied for handheld products. In virtue of recognizing package quality for actual status, board level drop impact is a great concern to simulate mishandling during usage. As well known, the common Sn3 ~ 4Ag0.5Cu of lead free solder is poor drop performance due to higher Young's modulus by high volume of Ag3Sn. Lower silver content solder was introduced to replace Sn3 ~ 4Ag0.5Cu solder to satisfy trend requirement. But, the drop performance is not still enough to satisfy the end user demand. In this study, two kinds of the solder ball were investigated, as the pure solder ball - SnlAg0.5Cu and polymer cored solder ball. There are three major layers for polymer cored solder ball. The cored material is polymer. Then, 1 um thickness of Cu was coated on the surface of polymer cored. The outer layer is 10 um thickness of plating SnlAg0.5Cu. A TFBGA 9times8 with 300 mm of ball diameter was used a test vehicle. The surface finish of substrate is Ni/Au. The same lead free flux (water soluble) and ball attach reflow was used for two type solder ball. The peak temperature of reflow profile was 240degC with 50 seconds time above 217degC. Ball shear and cold ball pull test were used to measure the solder joint performance after ball attached. OM and SEM was done the appearance inspection. The non-clean type of Sn3Ag0.5Cu solder paste was used to mount package on the PCB. The reflow profile parameter was the same as the ball attached reflow. After SMT procedure, use X-ray to check the solder joint quality. Subsequently, board level drop and temperature cycle test were performed as per JESD22-B111 test method and IPC-9701 standard, respectively. The PCB is 1/6/1 stack-up with OSP (organic solderability preservative) surface finish. For board level drop test, there must be 15 daisy chained integrated circuits spaced equally on PCB. The PCB must experience a peak acceleration of 1500 G's on each drop for duration of 0.5 milliseconds. For board level temperature cycle test, the test condition is TC 3 (- 40~125degC) with 33 pes daisy chained packages. Weibull distribution analysis was used to plot the curve between of cycle and failure or drop time and failure. Some documents show board Level TCT belong lower strain test, compared with drop test. The characteristic life depends on the standoff height, solder ball composition, package size, I/O number and surface finish. According to the cross-sectional SEM images, the polymer cored solder ball has higher standoff height than SnlAg0.5Cu. The standoff height of polymer-cored solder is 250um. But, normal solder ball is only 180um. That's why polymer cored solder ball has better characteristic life (44802cycles) than SnlAg0.5Cu (3036 cycles). The drop test results showed polymer cored solder ball also has better performance than SnlAg0.5Cu. The characteristic life of SnlAg0.5Cu and polymer cored solder ball are 362 and 467 times, respectively. It indicated that soft polymer cored could reduce the impact during drop test. The failure mode of polymer-cored solder ball was different from SnlAg0.5Cu solder ball. The major crack of polymer-cored solder went into the bulk solder initially and along the copper layer.

High drop performance interconnection: Polymer cored solder ball

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.

Liang Yi Hung

Papers

Silver alloy wire bonding

Evaluation of new substrate surface finish: Electroless nickel/electroless palladium/immersion gold (ENEPIG)

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

High drop performance interconnection: Polymer cored solder ball

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