Kati Kokko

The effects of chip and substrate thickness on the reliability of ACA bonded flip chip joints

Purpose – The purpose of this study is to investigate the effect of chip and substrate thickness on the thermal cycling reliability of flip chip joints assembled with anisotropic conductive adhesives (ACA) on FR‐4 substrates.Design/methodology/approach – Four test lots were assembled with two substrates and two test chips. The thicknesses of the substrates were 710 and 100 μm and the thicknesses of the chips were 480 and 80 μm. To study the effect of the bonding pressure each test lot contained four test series bonded with four different bonding pressures. The reliability of the test samples was studied using a temperature cycling test.Findings – The reliability of the test lots varied widely during the test. The test lot with a thin substrate and thin chip demonstrated considerably better reliability than the other test lots. In addition, the test lots had different failure mechanisms. After the test delamination was found in every test lot except the one assembled with the thin chip and the thin substra...

Effect of RCC on the Reliability of Adhesive Flip Chip Joints

A need for higher packaging density and functionality has increased the use of new packaging technologies, which has also caused demand for higher interconnect densities on printed circuit boards (PCBs). Sequential build-up (SBU) processes can be used to meet these demands. In the SBU process, additional dielectric and conductor layers are formed on a core board, which is typically made of FR-4. Microvias are formed on these layers to achieve an electrical connection between them and the core board. Resin-coated copper foil (RCC) is the most widely used dielectric layer in the SBU process. The effect of RCC on the reliability, of flip chip joints with anisotropically conductive adhesive film (ACF) was studied. Two substrates were used. The difference between the substrates was RCC laminated on the other substrate. The reliability of the test samples was studied using a temperature cycling test and a constant humidity test. The reliability of the substrate with the RCC was found to be better in both tests. Failure mechanisms were studied after the tests, using optical and scanning electron microscopes. After the temperature cycling. several of the test samples made with two highest bonding pressures showed delamination, which has probably caused the failures. In addition, failures occurred during the changes in the test temperature. These were probably caused by warping of the flip chip package. No delamination was found in the test samples with the lowest pressure. The failures in these series were probably caused by relaxation of the adhesive matrix and by too low deformation of the conductive particles. Several cracks had formed on the FR-4 substrates without the RCC during the temperature cycling. In addition, air bubbles were found in the test samples with the FR-4 substrates without the RCC. Since RCC is a pure resin system, it has a high coefficient of thermal expansion. which may cause problems, especially when large components are attached to it. However, in this study, the RCC was found to increase the reliability of the flip chip joints made with ACF during both temperature cycling and constant humidity testing.

Composite coating structure in an implantable electronic device

Purpose – The purpose of this paper is to investigate the influence of composite coating structure on the reliability of adhesive flip chip joints. The need for conformal coating is considered, especially for medical applications, and medical sterilization is also considered.Design/methodology/approach – Two test lots were assembled and one of them was sterilized using gamma sterilization. Both test lots were coated first with epoxy and then with Parylene C, resulting in a composite coating structure. The reliability was studied using a constant humidity test and the failure analysis was performed with cross‐sections and scanning electron microscopy analysis. These results were compared to earlier research results on conformal coatings.Findings – The reliability of both test lots proved to be good. The composite coating structure shields the joints from humidity and improves the reliability compared to non‐coated test samples. When the conformal coating was compared to the pure Parylene C coated test lot,...

Effects of conformal coating on anisotropically conductive adhesive joints; a medical perspective

Purpose – The purpose of this paper is to study epoxy and parylene C‐coated samples. These coatings are used to protect the electronic devices from harsh environments. The effect of these conformal coatings on electronics reliability is considered.Design/methodology/approach – Epoxy coating is applied using dip coating and parylene C is applied with the vapour deposition polymerisation method. Test chip used is joined using flip‐chip technology and an anisotropically conductive adhesive. Reliability of the test samples is evaluated in a constant humidity test, where test conditions are 85°C and 85%RH. The test lasts 4,000 h. Failure analysis is carried out by cross‐sectioning failed samples and using scanning electron microscopy for closer analysis.Findings – The results show variation in the reliability of adhesive joints with different conformal coating materials. Failure analysis highlights explicit failure mechanisms. Adhesion testing is also carried out on the test samples after constant humidity tes...

Thermal cycling of flip chips on FR‐4 and PI substrates with parylene C coating

Purpose – The purpose of this paper is to study the effect of conformal coating on the thermal cycling reliability of anisotropically conductive adhesive film (ACF) joined flip chip components on FR‐4 and polyimide (PI) substrates.Design/methodology/approach – Test chips were joined using flip chip technology and an anisotropically conductive adhesive. The conformal coating used was parylene C and it was applied using the vapour deposition polymerisation method. The reliability of ACF joined flip chip components on FR‐4 and PI substrates was evaluated using −40/+85°C thermal cycling testing. Test lots with and without parylene C coating were studied. Additionally, one test lot with initial moisture inside the coating layer and a PI substrate was subjected to the test. The reliability results were analyzed using Weibull analysis and failure analysis was performed to study the failure mechanisms using cross sectioning and optical and scanning electron microscopy.Findings – The results show a clear differenc...

Liquid crystal polymer as a substrate material for flip chip ACA interconnections

In general higher packaging densities, thinner substrates and lighter products can be achieved using flexible printed circuit boards (FPCB) compared to traditional thicker fiber-reinforced printed circuit boards. Polyimide (PI) is popular substrate material in FPCBs due to its excellent properties. Liquid crystal polymers (LCP), on the other hand, are more rarely used in FPCBs. This is mostly due to their high cost and poor availability. However, they have an excellent combination of thermal, mechanical, chemical and electrical properties, which have caused increased interest to use them in several applications. In addition to thermoplastic PI and LCP materials, thin FR-4 laminates can be used as FPCB material. They have lower flexibility than PI and LCP, but also lower price and excellent availability. In this study the effect of substrate material to the reliability of flip chip interconnections assembled with anisotropically conductive adhesives (ACA) was investigated. Three different FPCBs made from PI, FR-4, and LCP were used. Several different tests were conducted. Results clearly showed that the reliability of ACA joints on LCP substrates was better compared to PI and FR-4 substrates under humid and corrosive environments.

Effects of failure criteria on the constant humidity test results

Accelerated life-testing is much used in the reliability study of electronics. Failure criteria need to be assessed due to the application. For some applications even small changes in a variable used as failure criterion turns out to cause catastrophic failures while in another application even great fluctuation does not disturb the normal operation of the device. When the behaviour of the variable used as failure criterion is monitored prior to failure, regularities can be seen and thereby the starting failure may be predicted. In this work the effects of failure criteria was studied in humidity tests with three different test lots. Flip chip components on FR-4 substrates were used, and two of the test lots were coated with either epoxy or parylene C. The results show differences in the behaviour of daisy chain resistance between different test lots having different coatings causing differences in the reliability results with the three selected failure criteria.

RELIABILITY ENHANCEMENT OF ACF JOINED FLIP CHIPS WITH PARYLENE C AS A PROTECTIVE COATING

Electronic devices are entering every field of life nowadays. Also medicine uses electrical instruments and devices every day, and the safety and reliability are the key factors in that sector. Also the operational aspects of devices are of course vital and make the device useful and worth wile. However, the hardware of the device plays also a key role, since without it the programs cannot function. Furthermore, the use of different packaging technologies determines the size and weight of the device. As the devices get more complicated, the packaging of electronics becomes more important. In this study the reliability of anisotropical ly conductive adhesive joined flip chip components is considered with a parylene C protective coating on top. With this coating the compatibility with human body and the reliability in medical devices are ensured. Reliability testing was made in 8585 –constant humidity test with two different test substrate layouts and test chips. These contained thin chip and substrate structure and thick chip and substrate layouts. The results show that with thin, more space saving and lighter structure, the reliability can be dramatically improved. Furthermore, parylene C proves to be an excellent protection against humid environment enhancing the long term reliability even more.

Static Bending of Flip-Chip Structures Under Humid Conditions

The miniaturization of electronics poses challenges to reliability demands on devices. In applications where both high reliability and small size are demanded, reliability testing plays a critical role. Medical applications are one such example, and in this case, not only must the durability of electronics be considered but the safety of the device to the human body is also of vital importance. This paper considers the use of thin FR-4 substrates and thin silicon chips in medical applications. The joining is made using anisotropically conductive adhesives. The assemblies are protected using a parylene C conformal coating, which has been proved to be a biocompatible material and, thus, safe for medical applications. The assemblies are bent to a fixed radius during humidity testing. Nonbent and noncoated test lots are also studied for comparison. Two different humidity testing methods are used: the 85/85 constant humidity test and humidity cycling test. Furthermore, the test structures are simulated using finite element analysis. The results show the protective nature of parylene C coating and the differences between bent and nonbent structures. Parylene C coating seems to support the structure by lowering the shear and von Mises stresses formed, resulting in better reliability. Furthermore, the results show a clear difference in the two humidity testing methods, giving value to the standard 85/85 testing.

Reliability and microstructural studies of Sn-Ag-Cu lead-free solder joints in pulse-heated reflow soldering

With the advent of miniaturization in the electronics industry, the application of flexible interconnections has become necessary and inevitable for many new designs such as flexible circuits (flex) to printed circuit boards (PCB), flex to flex, and multiwire to PCB. Pulse-heated reflow soldering is demonstrably a reliable and repeatable soldering process for manufacturing such products, especially in the attachment of flex-to-PCB in the electronics industry. This paper reports on the microstructure and reliability of flex-to-PCB solder joints. Flex-to-PCB samples were made using Pulse-heated reflow soldering and conventional reflow oven soldering. To study the reliability of the solder joints, three different environmental tests were conducted including thermal shock, thermal humidity, and thermal aging. Microstructural studies and failure analysis were performed on all samples before and after the reliability tests in order to ascertain the cause of failure in both bonding methods. Additionally, a comparison of both attachment methods comprising pulse-heated reflow soldering and reflow oven is presented demonstrating their applicability in manufacturing flex-to-PCB assemblies.