Laura Frisk

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...

Effects of Cycling Humidity on the Performance of RFID Tags With ACA Joints

Radio frequency identification (RFID) tags with anisotropically conductive joints (ACAs) are used in different applications where the environmental conditions may impair their reliability. Thus the effects of different environmental stresses on reliability need to be investigated. The effects of high temperature and humidity may change the performance of the tags. More- over, the effects of constantly varying temperature and humidity conditions may be even more harmful. In this study, the effects of changing humidity conditions on the performance of a passive ultra high frequency (UHF) RFID tag with ACA joints were studied. The tags were tested in a humidity cycling test where humidity varied from 85%RH to 10% RH, and temperature from 85°C to 25°C. Tags with four different sets of bonding parameters were tested. Significant differences in the reliability between the tags with different bonding parameters were observed. The results were also compared with results from a corresponding constant humidity test where the humidity was 85%RH, and the temperature 85°C. The tags had different failure times, modes, and mechanisms in these two tests. Furthermore, the effects of bonding parameters on the reliability were different in these tests. According to this study, it is important to investigate the effects of changing humidity, when the reliability in different environments is investigated, but the constant humidity test cannot be replaced with the faster humidity cycling test.

Reliability of ACF Interconnections on FR-4 Substrates

The use of anisotropic conductive adhesives (ACA) in flip chip interconnection technology has become very popular because of their numerous advantages. The ACA process can be used in high-density applications and with various substrates as the bonding temperature is lower than that in the soldering process. In this paper, six test lots were assembled using two anisotropic conductive adhesive films (ACF) and four different FR-4 substrates. FR-4 was chosen as it is an interesting alternative for making low-cost high-density interconnections. Some of the chips were thinned to study the effect on reliability. To study the effect of bonding pressure, four different pressures were used in every test lot. The reliability of the assembled test samples was studied in a temperature cycling test carried out between temperatures of -40°C and 125°C for 10 000 cycles. A finite element model (FEM) was used to study the shear stresses in the interconnections during the test. Marked differences between the substrates were seen. The substrate thinning and also the chip thinning increased the reliability of the test samples. From the FEM, it was seen that both decreased the shear stress in the adhesive, which is assumed to be the reason for the increased reliability. A significant difference was seen in the reliability between the ACFs. This was probably caused by differences in the conductive particle materials and the T g values and of the ACFs. In addition, the bump material used with the ACFs varied, which most likely affected the reliability of the test samples.

Reliability of ACA bonded flip chip joints on LCP and PI substrates

Purpose – The purpose of this study is to investigate the reliability of flip chip joints made with anisotropic conductive adhesives (ACA) on flexible polyimide (PI) and liquid crystal polymer (LCP) substrates.Design/methodology/approach – Six test series using two ACAs and an LCP substrate were made with varying bonding pressure. The ACAs had the same matrix and conductive particles. To lower the CTE of one of the adhesives silica had been added to it. The reliability of the test series was studied in a temperature cycling test. The purpose of these test series was to find the optimal bonding pressure for both the adhesives used. According to the results from these initial tests, further test series were made with both LCP and PI substrates. The reliability of these test samples was studied using a temperature cycling test and a constant humidity test. The adhesion strength of the joints was studied before testing.Findings – Both substrates had excellent reliability during the temperature cycling test. H...

Accelerated testing for failures of tantalum capacitors

This study focused on the use of accelerated testing to find out why tantalum capacitors fail. Stress effects of humidity, temperature, and ripple voltage were examined in different combinations. Results show that a standard 85/85 test with combined enhanced moisture and temperature does not result in failure of tantalum capacitors in 2500 h. However, with added ripple voltage, failures may occur in a relatively short time. High relative humidity and high temperature both affect water diffusion, but apparently increased ripple voltage in 85/85 testing causes tantalum capacitor characteristics to weaken and capacitors to fail. The paper elaborates on the possible reasons.

Effect of substrate material and thickness on reliability of ACA bonded flip chip joints

Purpose – The purpose of this paper is to investigate the effect of substrate material and thickness on the thermal cycling reliability of flip chip joints assembled with anisotropic conductive adhesives (ACA).Design/methodology/approach – Four test lots are assembled using three different substrates. Two of the substrates are made of FR‐4. The thicknesses of these substrates are 600 and 100 μm. The third substrate is made of liquid crystal polymers (LCP) and is flexible. With the thicker FR‐4 substrate two test lots are assembled using both normal and two‐step bonding profiles to study how the bonding profile affects the deformation of the substrate. Four different bonding pressures are used to study the effect of pressure on reliability and the failure mechanism of the ACA joints. The reliability of the test samples is studied using a temperature cycling test.Findings – The reliability of the test lot with the LCP substrate is considerably better than that of the test lots with the FR‐4 substrates. Addi...

Effects of different test profiles of temperature cycling tests on the reliability of RFID tags

Abstract Passive UHF radio-frequency identification (RFID) tags are used for object identification in various environmental conditions, which may affect the reliability of these tags. The effects of different environmental stresses can be studied with accelerated life tests (ALT). Choosing the most suitable test may be challenging: The results are needed as fast as possible, but the failure mechanisms must replicate those occurring in the real operating environment. Here the effects of different temperature cycling profiles were studied by altering temperature ranges, extreme temperatures, soak times to extreme temperatures and transition times between extreme temperatures. Failure times clearly differed between the tests. The test with the fastest transition time and the shortest soak time seemed to have the most acceleration. It was also observed that the different temperature cycling profiles affected the failure mechanisms detected. Cracking of the antenna was observed with lower temperature extremes or shorter soak and transition times. However, with longer soak and transition times, cracks were seen in the RFID interconnections. Both cases led to changes in the impedance matching and consequently to failures. The totally different failure mechanisms clearly demonstrate the importance of carefully determining the test parameters in order to achieve the correct failure mechanism.

Reliability analysis of an ACA attached flex-on-board assembly for industrial application

Purpose – The purpose of this paper is to examine the long-term reliability of an anisotropic conductive adhesive (ACA) attached polyethylene terephthalate (PET) flex-on-board (FOB) assembly for industrial application used in harsh environments. In addition, the possibility of reducing reliability testing time was studied. Design/methodology/approach – A−40/+125°C thermal cycling test with 5- and 14-minute soak times was used to study the reliability. To study the functionality of the FOB assembly during testing, a real-time resistance measurement was used together with a 90° peel strength test. Failure analysis was performed on samples using scanning electron microscopy and cross sectioning. Findings – No failures or noticeable increase in the measured resistance values were seen during testing. The peel strength, however, decreased significantly with both soak times used. The highest drop in the mechanical strength occurred at the start of the temperature cycling tests. The time spent at the high temper...

Changes in Adhesion of Non-Conductive Adhesive Attachments During Humidity Test

Non-conductive adhesives (NCA) are used to attach flip chip components onto substrates. They have many advantages compared to solders. However, in a humid environment the use of NCA may cause some reliability problems. Long-term tests are essential when the reliability of NCA joints is determined. These tests, however, are often very time-consuming. Nevertheless, without the tests it is difficult to predict the failure times and places. One of the long-term failure mechanisms is the deterioration of the adhesion. In order to study the effects of moisture on the adhesion of NCA, strength tests were conducted after a humidity test, and the adhesion was found to decrease as a function of time. Additionally, finite element models of moisture and thermal expansion were formed, and on the basis of the models the most likely places for failures were defined to be the corner joints. Finally, the results of the models were compared to the results of adhesion measurements and the expected failure time was estimated to be 13 500 hours in test conditions. No failures were seen during 4000 hours of testing. Thus, this structure was found to be extremely reliable in a humid environment.

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.