Effects of different bonding parameters on the electrical performance and peeling strengths of ACF interconnection

The effects of different bonding parameters, such as temperature, pressure, curing time, bonding temperature ramp and post-processing, on the electrical performance and the adhesive strengths of anisotropic conductive film (ACF) interconnection are investigated. The test results show that the contact resistances change slightly, but the adhesive strengths increase with the bonding temperature increased. The curing time has great influence on the adhesive strength of ACF joints. The contact resistance and adhesive strength both are improved with the bonding pressure increased, but the adhesive strengths decrease if the bonding pressure is over 0.25 MPa. The optimum temperature, pressure, and curing time ranges for ACF bonding are concluded to be at 180–200 °C, 0.15–0.2 MPa, and 18–25 s, respectively. The effects of different Teflon thickness and post-processing on the contact resistance and adhesive strength of anisotropic conductive film (ACF) joints are studied. It is shown that the contact resistance and the adhesive strength both become deteriorated with the Teflon thickness increased. The tests of different post-processing conditions show that the specimens kept in 120 °C chamber for 30 min present the best performance of the ACF joints. The thermal cycling (−40 to 125 °C) and the high temperature/humidity (85 °C, 85% RH) aging test are conducted to evaluate the reliability of the specimens with different bonding parameters. It is shown that the high temperature/humidity is the worst condition to the ACF interconnection.     

Anisotropic conductive film flip chip joining using thin chips

In this study, flip chip interconnections were made on very flexible polyethylene naphthalate substrates using anisotropic conductive film. Two kinds of chips were used: chips of normal thickness and thin chips. The thin chips were very thin, only 50 μm thick. Due to the thinness of the chips they were flexible and the entire joint was bendable. The reliability properties of the interconnections established with these two different kinds of chips were compared. In addition, the effect of bending of the chip and joint area on the joint reliability was studied. Furthermore, part of the substrates was dried before bonding and the effect of that on the joint performance was investigated.The pitch of the test vehicles was 250 μm and the chips had 25 μm high gold bumps. For resistance analysis there were two four-point measuring positions in each test vehicle. For finding the optimal bonding conditions for the test vehicles, the bonding was done using two different bonding pressures, of which the better one was chosen for the final tests.Furthermore, the test vehicles were subjected to thermal cycling tests between −40 and +125 °C (half-an-hour cycle) and to a humidity test (85%/85 °C). Part of the test vehicles were bent during the tests. Finally, the structures of the joints were studied using scanning electron microscopy.     

Study of adhesive flip chip bonding process and failure mechanisms of ACA joints

The flip chip bonding process using anisotropic conductive adhesives (ACA) and the consequent joint reliability were studied. The substrates used were rigid FR-4 boards, which are interesting due to their low cost and wide range of applications. The problems associated with the technique are discussed in this paper from the reliability point of view. Also, some aspects concerning production are introduced.The reliability of the joints was studied by accelerated environmental tests. A temperature cycling test was performed between temperatures −40 and +125 °C. Constant humidity testing was conducted at 85 °C and RH85%. In addition, reflow aging tests were performed using a conventional Sn/Pb reflow profile. For reducing the bonding cycle time, a two-stage curing process was used, which also utilizes the reflow process.The results show that the three bonding parameters, temperature, time, and pressure, all affect joint reliability. Most detrimental, however, seems to be reflow treatment performed after bonding. Most failures occurred only very briefly during the temperature cycling at the moment the temperature changed, while the joints were still conducting at both temperature extremes. However, a different failure mechanism caused a different kind of behavior during temperature cycling. The relationship between the failure modes and the failure mechanisms was studied using a scanning electron microscopy.     

Reliability of 80 μm pitch flip chip attachment on flex

The interest toward flip chip technology has increased rapidly during last decade. Compared to the traditional packages and assembly technologies flip chip has several benefits, like less parasitics, the small package size and the weight. These properties emphasize especially when flip chip component is mounted direct to the flexible printed board. In this paper flip chip components with Kelvin four point probe and daisy chain test structure were bonded to the polyimide flex with two different types of anisotropically conductive adhesive films and one anisotropically conductive adhesive paste. The reliability of small pitch flip chip on flex interconnections (pitch 80 μm) was tested in 85°C/85% RH environmental test and −40↔+125°C thermal shock test. According to the results it is possible to achieve reliable and stable ohmic contact, even in small pitch flip chip on flex applications.     

Feasibility study of non-conductive film (NCF) for plasma display panel (PDP) application

In this study, attempts were made to develop an electrically reliable and low cost bonding process for plasma display panels (PDPs) by using non-conductive film (NCF). The adhesive joints were constructed with Ag electrodes on a glass substrate and Au finished Cu electrodes on a flexible printed circuit (FPC). To clarify the feasibility of NCF application, all experimental results of NCF were compared with those of anisotropic conductive film (ACF) which is currently used for PDP application. The reliability of adhesive interconnections using NCF and ACF was evaluated by measuring daisy resistance during high temperature and humidity storage test (85 °C, 85% RH for 1000 h). Ag electrochemical migration, which leads to the degradation of surface insulation resistance, was also investigated by measuring insulation resistance during the storage test (1000 h at 85 °C and 85% RH). The initial resistance of NCF joints was lower than that of ACF joints due to larger contact area between Ag and Au finished Cu electrodes in the NCF joints. During the reliability test, the daisy resistances of both joints increased slightly due to Z-directional swelling of the adhesives, but NCF joints showed more stable resistance than ACF joints. In addition, upon application of high voltage of 100 V (DC current), severe degradation of insulation resistance due to the Ag migration was observed in ACF joints, whereas no remarkable decrease of insulation resistance was observed in NCF joints. Conclusively, the application of NCF to PDP interconnection process has good potential from a practical point of view.     

Comparison of different flex materials in high density flip chip on flex applications

This paper presents the results from the evaluation of different types of flexible substrates for high-density flip chip application. In this work four different flexible substrates were used. The flex substrates were Espanex, Upilex and epoxy glass with 80 μm pitch and Upilex with 54 μm pitch. Two different test IC’s were used for both pitches. In test IC1 (80 μm pitch) and IC3 (54 μm pitch) the bumps were in one row and test IC2 (80 μm pitch) and IC4 (54 μm pitch) in two rows. The total amount of contacts in test IC1 was 190, in test IC2 173, in test IC3 293 and in test IC4 270. The anisotropically conductive adhesive that was used in the tests was epoxy based thermosetting adhesive film with conductive particles. The conductive particles in the adhesives were isolated soft metal-coated polymer particles. The contact resistance was measured using Kelvin four-point method and the continuity and series resistance using daisy chain structure. The reliability of the flip chip interconnections was tested in temperature cycling test and environmental test. Cross section samples were made to analyse the possible reason for failures. The results presented in this paper are from FLEXIL development project that is part of European Union IST research program.     

Board level reliability assessment of chip scale packages

A large program had been initiated to study the board level reliability of various types of chip scale package (CSP). The results on six different packages are reported here, which cover flex interposer CSP, rigid interposer CSP, wafer level assembly CSP, and lead frame CSP. The packages were assembled on FR4 PCBs of two different thicknesses. Temperature cycling tests from −40°C to +125°C with 15 min dwell time at the extremes were conducted to failure for all the package types. The failure criteria were established based on the pattern of electrical resistance change. The cycles to failure were analyzed using Weibull distribution function for each type of package. Selected packages were tested in the temperature/humidity chamber under 85°C/85%RH for 1000 h. Some assembled packages were tested in vibration condition as well. In all these tests, the electrical resistance of each package under testing was monitored continuously. Test samples were also cross-sectioned and analyzed under a Scanning Electronic Microscope (SEM). Different failure mechanisms were identified for various packages. It was noted that some packages failed at the solder joints while others failed inside the package, which was packaging design and process related.     

Interconnect resistance characteristics of several flip-chip bumping and assembly techniques

Several flip-chip interconnection methods were compared by measuring interconnect resistance before and after exposure to environments including pre-conditioning, 85°C/85% RH exposure, 150°C storage, and 0–100°C temperature cycling. The goal was to determine an acceptable low-cost, reliable method for bumping and assembling chips to flexible or rigid substrates using flip-chip assembly techniques. Alternative flip-chip bumping methods are compared to a traditional wafer solder bumping method. Flip-chip interconnection methods evaluated included high lead content solder, silver filled conductive adhesive, and gold stud bumps. Under bump metallurgies evaluated included bare aluminum, evaporated Cr/Cr–Cu/Cu, and electroless nickel plating.     

Formulation and characterization of anisotropic conductive adhesive paste for microelectronics packaging applications

There has been a steadily increasing interest in using electrically conductive adhesives as interconnecting materials in electronics manufacturing. In this paper, several anisotropic conductive adhesive (ACA) pastes were formulated, which consist of diglycidyl ether of bisphenol F or diglycidyl ether of bisphenol A as polymer matrix, imidazoles as curing agents, and different sizes of silver (Ag) powders or gold (Au)-coated polymer spheres as conductive particles. The effects of ACA resin and different curing agents, as well as different conductive particles, on flexible substrate of the flip-chip joint were studied. The results show that the size and type of different conductive particles have very limited influence on an ACA flip-chip joint. The ACA resin as well as the curing agent can affect the reliability of the joint. The same results can be applied for the failure analysis of ACA flip-chip technology.     

Compound semiconductor activation energy in humidity

Product reliability investigations typically include accelerated humidity testing. Originally, the “standard” test was a biased 85 °C/85% relative humidity (RH) lifetest for 1000 h. Recently, a substitute accelerated version of this test has been used. The accelerated version is called highly accelerated stress test (HAST). The HAST conditions are also biased, at 130 °C, 85%RH, and approximately 18 PSI overpressure. The duration of the HAST test is normally 96–100 h – to be equivalent to the 85/85 test. This study is intended to investigate thermal acceleration and show that equivalent HAST tests on compound semiconductors are more highly accelerated and could be conducted with much shorter durations.     

Characteristic study of anisotropic-conductive film for chip-on-film packaging

Chip-on-film (COF) is a new technology after tape-automated bonding (TAB) and chip-on-glass (COG) in the interconnection of liquid crystal module (LCM). The thickness of the film, which is more flexible than TAB, can be as thin as 44 μm. It has pre-test capability, while COG does not have. It possesses great potential in many product fabrication applications.In this study, we used anisotropic-conductive film (ACF) as the adhesive to bind the desired IC chip and polyimide (PI) film. The electric path was formed by connecting the bump on the IC and the electrode on the PI film via the conductive particles in the ACF. In the COF bonding process experimental-design method was applied based on the parameters, such as bonding temperature, bonding pressure and bonding time. After reliability tests of (1) 60 °C/95%RH/500 h and (2) −20 to 70 °C/500 cycles, contact resistance was measured and used as the quality inspection parameter. Correlation between the contact resistance and the three parameters was established and optimal processing condition was obtained. The COF samples analyzed were fabricated accordingly. The contact resistance of the COF samples was measured at varying temperature using the four points test method. The result helped us to realize the relationship between the contact resistance and the operation temperature of the COF technology. This yielded important information for circuit design.     

Influence of medical sterilization on ACA flip chip joints using conformal coating

Health care is a field in which electronics is increasingly used. If electronics is used in a device that is implanted, it needs to withstand a harsh environment. Moreover, before implantation the device needs to be sterilized, and this should not impair the function or reliability of the device. Since implantable applications are small in size, electronics packaging solutions need to be space saving. Conformal coating is needed to shield the electronics from conditions inside the human body, and on the other hand to protect the body from the materials used in electronics. This paper studies the effect of medical sterilization on the reliability of adhesive joints. Flip chip technology with anisotropically conductive adhesives (ACA) was used and epoxy and parylene C were used as the conformal coating materials. Differential scanning calorimetry (DSC) was used to ascertain the curing degree of the adhesive after bonding. Sterilization was performed using gamma radiation sterilization and the reliability tests were conducted in a constant humidity test, at 85 °C and 85%RH. The results clearly indicated that parylene C coating is the most reliable choice and gamma radiation does not affect the reliability of parylene C coated devices. However, the gamma radiation increased the reliability of the non-coated test lot.     

Long time reliability study of soldered flip chips on flexible substrates

Due to the requirements of new light, mobile, small and multifunctional electronic products the density of electronic packages continues to increase. Especially in medical electronics like pace makers the minimisation of the whole product size is an important factor. So flip chip technology becomes more and more attractive to reduce the height of an electronic package. At the same time the use of flexible and foldable substrates offers the possibility to create complex electronic devices with a very high density. In terms of human health the reliability of electronic products in medical applications has top priority.In this work flip chip interconnections to a flexible substrate are studied with regard to long time reliability. Test chips and substrates have been designed to give the possibility for electrical measurements. Solder was applied using conventional stencil printing method. The flip chip contacts on flexible substrates were created in a reflow process and underfilled subsequently.The assemblies have been tested according to JEDEC level 3. The focus in this paper is the long time reliability up to 10,000 h in thermal ageing at 125 °C and temperature/humidity testing at 85 °C/85% relative humidity as well as thermal cycling (0 °C/+100 °C) up to 5000 cycles. Daisy chain and four point Kelvin resistances have been measured to characterise the interconnections and monitor degradation effects.The failures have been analysed in terms of metallurgical investigations of formation and growing of intermetallic phases between underbump metallisation, solder bumps and conductor lines. CSAM was used to detect delaminations at the interfaces underfiller/chip and underfiller/substrate respectively.     

Reliability of Conductive Adhesives as a Pb-free Alternative in Flip-Chip Applications

The temperature-humidity reliability of anisotropic conductive film (ACF) and non-conductive film (NCF) interconnects is investigated by measuring the interconnect resistance during temperature-humidity testing (THT) at 85°C and 85% relative humidity. The four-point probe method was used to measure the interconnect resistance of the adhesive joints constructed with Au bumps on Si chips and Cu pads on flexible printed circuits (FPCs). The interconnect resistance of the ACF joints was markedly higher than that of the NCF joints, mainly due to the constriction of the current flow and the intrinsic resistance of the conductive particles in the ACF joints. The interconnect resistances of both interconnects decreased with increasing bonding force, and subsequently converged to about 10 mΩ and 1 mΩ at a bonding force of 70 N and 80 N, for the ACF and NCF joints, respectively. During the THT, two different conduction behaviors were observed: increased interconnect resistance and the termination of Ohmic behavior. The former was due to the decreased contact area caused by z-directional swelling of the adhesives, whereas the latter was caused by either contact opening in the adhesive joints or interface cracking.     

The effect of reflow process on the contact resistance and reliability of anisotropic conductive film interconnection for flip chip on flex applications

The work presented in this paper focuses on the effect of reflow process on the contact resistance and reliability of anisotropic conductive film (ACF) interconnection. The contact resistance of ACF interconnection increases after reflow process due to the decrease in contact area of the conducting particles between the mating I/O pads. However, the relationship between the contact resistance and bonding parameters of the ACF interconnection with reflow treatment follows the similar trend to that of the as-bonded (i.e. without reflow) ACF interconnection. The contact resistance increases as the peak temperature of reflow profile increases. Nearly 40% of the joints were found to be open after reflow with 260 °C peak temperature. During the reflow process, the entrapped (between the chip and substrate) adhesive matrix tries to expand much more than the tiny conductive particles because of the higher coefficient of thermal expansion, the induced thermal stress will try to lift the bump from the pad and decrease the contact area of the conductive path and eventually, leading to a complete loss of electrical contact. In addition, the environmental effect on contact resistance such as high temperature/humidity aging test was also investigated. Compared with the ACF interconnections with Ni/Au bump, higher thermal stress in the Z-direction is accumulated in the ACF interconnections with Au bump during the reflow process owing to the higher bump height, thus greater loss of contact area between the particles and I/O pads leads to an increase of contact resistance and poorer reliability after reflow.     

Reliability of adhesive joined thinned chips on flexible substrates under humid conditions

Thin chips are an interesting option for reducing the thickness of an electronics package. In addition to the reduced size, thinned chips are flexible and can dissipate more heat than thicker ones. Joining of the thin chips can be done using several different techniques. Of these, anisotropic conductive adhesives (ACA) are an interesting option as they have several advantages, such as low bonding temperature and capability for high density interconnections. The reliability of ACA flip chip joints under thermal cycling conditions has been found to increase when thinned chips are used. However, the effect of humidity has not been fully explored. In this study the reliability of thinned chips (50 μm) under humid conditions was investigated using thin flexible substrates. Seven test lots were assembled with thinned chips using two different ACA films and liquid crystal polymer (LCP), polyimide (PI) and thin FR-4 substrates. A high humidity and high temperature test was used to study the reliability of the interconnections. A finite element model (FEM) was used to analyse the stresses in the test samples during testing. Several failures occurred during the test and significant differences between the substrates were seen. Additionally, bonding pressure was found to be a critical factor for the reliability under the humid conditions.     

Pb-free high temperature solders for power device packaging

Reliabilities of joints for power semiconductor devices using a Bi-based high temperature solder has been studied. The Bi-based solder whose melting point is 270 °C were prepared by mixing of the CuAlMn particles and molten Bi to overcome the brittleness of Bi. Then, joined samples using the solder were fabricated and thermal cycling tests were examined. After almost 2000 test cycles of −40/200 °C test, neither intermetallic compounds nor cracks were observed for CTE (Coefficient of Thermal Expansion) matched sample with Cu interface. On the other hand, certain amount of intermetallic compound such as Bi₃Ni was found for a sample with Ni interface. In addition, higher reliability of this solder than Sn-Cu solder was obtained after −40/250 °C test. Furthermore, an example power module structure using double high temperature solder layers was proposed.     

Reliability of adhesive interconnections for application in display module

The reliability of adhesive interconnections using anisotropic conductive film (ACF) and non-conductive film (NCF) was evaluated by measuring connection resistance during 500 cycles of thermal shock testing. The four-point probe method was used to measure the connection resistance of the adhesive joints constructed with Au bump on Si chip and Cu pad on flexible printed circuit (FPC). The connection resistance of the ACF joints was markedly higher than that of the NCF joints, mainly due to the constriction of the current flow and the intrinsic resistance of the conductive particles in the ACF joints. The connection resistances of both interconnections decreased with increasing bonding force, and subsequently converged to about 10 and 1 mΩ at a bonding force of 70 and 80 N, for the ACF and NCF joints, respectively. During the thermal shock testing, two different conduction behaviors were observed: increased connection resistance and the termination of Ohmic behavior. The former was due to the decreased contact area caused by z-directional swelling of the adhesives, whereas the latter was caused by either contact opening in the adhesive joints or interface cracking.     

Flexible Chip-on-Flex (COF) and embedded Chip-in-Flex (CIF) packages by applying wafer level package (WLP) technology using anisotropic conductive films (ACFs)

Due to increasing demand for higher performance, greater flexibility, smaller size, and lighter weight in electronic devices, extensive studies on flexible electronic packages have been carried out. However, there has been little research on flexible packages by wafer level package (WLP) technology using anisotropic conductive films (ACFs) and flex substrates, an innovative packaging technology that requires fewer process steps and lower process temperature, and also provides flexible packages. This study demonstrated and evaluated the reliability of flexible packages that consisted of a flexible Chip-on-Flex (COF) assembly and embedded Chip-in-Flex (CIF) packages by applying a WLP process.The WLP process was successfully performed for the cases of void-free ACF lamination on a 50 μm thin wafer, wafer dicing without ACF delamination, and a flip-chip assembly which showed stable bump contact resistances. The fabricated COF assembly was more flexible than the conventional COF whose chip thickness is about 700 μm. To evaluate the flexibility of the COF assembly, a static bending test was performed under different bending radiuses: 35 mm, 30 mm, 25 mm, and 20 mm. Adopting optimized bonding processes of COF assembly and Flex-on-Flex (FOF) assembly, CIF packages were then successfully fabricated. The reliability of the CIF packages was evaluated via a high temperature/humidity test (85 °C/85% RH) and high temperature storage test (HTST). From the reliability test results, the CIF packages showed excellent 85 °C/85% RH reliability. Furthermore, guideline of ACF material property was suggested by Finite Element Analysis (FEA) for better HTST reliability.