Przemyslaw Matkowski

Effects of Printed Circuit Board Materials on Lead-free Interconnect Durability

This study investigates the effects of printed circuit board (PCB) material on interconnect durability of lead free assemblies. The assemblies involve soldering various packages (array and peripheral) on to FR4, high glass transition temperature (Tg) FR4 and Polyimide (PI) printed circuit boards using Sn3Ag0.5Cu solder alloy. The glass transition temperature of these materials ranges from 130°C to 230°C. Thermomechanical properties, such as elastic modulus and thermal expansion coefficients, of the board materials vary considerably. These properties have a direct impact on the interconnect durability. In this paper, thermomechanical properties are experimentally determined and used for solder joint durability simulation. Two kinds of environmental loadings are simulated: temperature cycling and random vibration loading. The results show that PI board provides a better solder joint durability than FR4 and high TgFR4 under temperature cycling conditions. PI assembly has better durability than FR4 assembly under random vibration. The paper also presents the effect of temperature on the vibration response of the FR4 printed circuit board assemblies. The understanding of these changes can contribute to the study on interconnect durability under combined temperature cycling and vibration loading conditions.

Application of FPGA units in combined temperature cycle and vibration reliability tests of lead-free interconnections

According to the European Directive 2002/95/EC RoHS solders that contain lead have to be replaced with lead-free equivalents. Replacing well-known materials with new materials not only force technological adaptations but also force necessity of reliability investigations, especially new failure modes. In case of lead-free solders, the main problem is reduction of solder joint strength. Lower strength of lead- free solder joint in comparison with SnPb is the result of defects that occur in the joint structure. Because of local defects, the joints are much more sensitive for stresses, Stress inducted within the area of solder joint can lead to joint fractures. Despite the crack occurrence, the solder joint can still conduct electrically. Nevertheless cracked joint should be treated as failed because it does not fulfill its constructional function. Such failures can be difficult to detect in static conditions, but can be easy detected, when the tested joint is subjected to vibrations. During vibrations short opens of the joint can occur. Additional high temperature or temperature cycles loadings can make detection easier by acceleration of crack occurrence. To detect short opens of solder joints subjected to vibrations, continuous fast measurements of resistance changes during the test are required. Within the article, the essence of fast multi-channel measurements and the construction of dedicated data storage system will be described. As a summary expected results of measurements will be shortly presented.

Influence of solder joint constitution and aging process duration on reliability of lead-free solder joints under vibrations combined with thermal cycling

Mechanical resistance of a solder joint to vibrations and thermal cycling strongly depends on its structure. Defects such as cracks, empty spaces, voids, non-uniform layers of intermetallic compounds (IMCs) can decrease mechanical strength of a solder joint significantly. Constitution of IMC layers depends on joined materials. Formation of an IMC layer can go on after a soldering process. Storage or working temperature have a great impact on thickness of formed IMC layers. Stress accumulated in solder joints during reflow process or vibrations or temperature cycles used to be relieved by cracks. Solder joints crack usually along the boundary between IMC layers and a bulk of solder. Within the frame of the study several commercial solder alloys and pad coatings were evaluated. Tested solder joints were subjected to microscopic analysis (SEM) as well as to accelerated reliability tests. During the study an influence of pad coating constitution and aging process duration on durability of tested solder joints was evaluated.

Vibration response of printed circuit board in wide range of temperature. Characterization of PCB materials

Stiffness of printed circuit boards (PCB) has a great impact on reliability of solder joints. Vibrations or external stress loading lead to stress accumulation within solder joint area. Depending on intensity, inducted stress can be relieved by creep or fractures of solder joint. Intensity of inducted stress depends on loading conditions as well as properties of joined materials (specially stiffness of printed circuit boards).Crucial parameters such as stiffness and Tg depends on the producer of laminate. Therefore accurate evaluation of PCB material is a basis for improvement of equipment reliability. Nowadays, when the number of producers and the mass of FR4 laminate imported from Asia is rapidly increasing, it is specially important to have the complete knowledge about parameters of used materials. Dedicated, designed stand will be applied to determine vibration response of printed circuit boards. The novel solutions implemented will ensure high accuracy of measurements. Proposed method for vibration response characterization will be cheaper to standard method that bases on scanning laser vibrometer. The stand will be used to determine changes of FR4 stiffness in wide range of temperature (for example from − 20 to 130 °C) and Tg parameter. Within the frame of the paper, measuring stand and the results of performed evaluations are described

Vibration tests utilization in the study of reliability of connections in microelectronics

Reliability of electronic devices is a crucial scope of interest of the electronic manufacturing industry. The reliability is connected with electrical, thermal and mechanical properties of investigated products. Electrical and thermal properties are investigated during the specified tests such as thermal shocks, temperature, and humidity cycles, dry oven storage. Mechanical properties are concerned of mounted components are mainly problem of the assembly industry. At the present electronic components are mounted to the PCB mostly by surface mount technology (SMT) or flip chip processes. The lead-solders are being presently replaced with lead-free solders or electrically conductive adhesives. It is caused by the European and Japanese restrictions. The restrictions forbid using lead in consumer electronic products because of its harmful influence on beings. The new solders and assembly technology force to the following investigations of reliability of joints. This paper contains information about information tests as accelerated tests commonly used to evaluate fatigue life of interconnections in electronic assembly. In the first part of the paper short review of accelerated tests is presented. Anands model of the solder joint used in finite elements analysis (FEA) and Darveauxs crack initiation and growth model are also mentioned, The bases of FEA and DOE are beyond the scope of this paper. In the second part of the paper harmonic as well as random variation and two different constructions of shakers, their advantages, limitations and requirements are presented. Characterization of specimen and vibration setup is considered. At the end of the paper vibration test is proposed

Structure of the Thermal Interface connection made of sintered nano silver

Continuous miniaturization of semiconductor devices enables integration in packages on increasingly smaller surfaces. Increasing power and minimization of dimensions results in diametrical increase of thermal flux density. Current works are focused on high performance TIMs (Thermal Interface materials). The main role of such TIMs is efficient dissipation of heat from electronic device to the surface. Within the frame of the study thermal interconnections made of sintered nano silver pastes were evaluated in terms of the application as layers that effectively remove the heat from semiconductor structures. Structure of TIM interconnections between silicon die and ENIG (Electroless Nickel Immersion Gold) on copper/FR4 substrate was the object of the study. X-Ray CT analyses have shown that voids presence, contact surface and interconnection thickness strongly depends on nano-filled paste composition and technique of application. The results will be compared with results of further studies (i.e. electrical and thermo-mechanical measurements).

Reliability of SnAgCu solder joints during vibration in various temperature

The results of study focused on reliability of SnAgCu/ImSn (1206 and 0805 SMD) solder joints subjected to vibration in various but constant temperature (−40 and +130) will be presented. The results will be also compared with the results of authors previous reliability tests focused on reliability of SnAgCu solder joints subjected to vibration combined with temperature cycles. Time to failure will be measured using dedicated event detector. After the tests solder joints will be inspected using ultra high-resolution nanofocus® X-ray inspection system and nanoCT®.

Mechanical strength of joints based on nano-Ag sintering phenomena

Thermal Interface Materials are used in microelectronic packaging for reducing so called thermal resistance between heat source and heat sink. They are commonly used, often in form of thermally conductive adhesives, as an attaching materials for fixing silicon chips in the integrated circuits. Therefore, beside the thermal properties, a good mechanical strength of TIMs is required. Within this paper the shear strength of few different sintered nanoAg-based pastes used as TIM is examined.

Comparative thermal analysis of commercial and novel hybrid thermal greases

Consumer electronics such as portable devices (laptops, smartphones) become smaller, thinner and more powerful. In such devices a passive cooling begins to play a dominant role in heat dissipation process. In passive cooling systems thermal resistance of thermal path between semiconductor junction and radiator should be progressively reduced. It can be done by application of efficient (high thermal conductivity) thermal interface materials. The main aim of the study was to provide and to characterize developed thermal greases based on mixture of nano and micro fillers, which could compete or replace the current commercial ones.

Comparative analysis of novel thermal interface containing nano additives

Nowadays a passive cooling based on efficient reliable thermal interfaces begins to play a dominant role in modern consumer electronics. The devices become smaller, thinner and more powerful while semiconductors become the source of higher flux heat density. In order to reduce thermal resistance between a semiconductor junction and a heat spreader, semiconductor structures remain unpackaged i.e. flip-chip structures. In the case of such solution semiconductor dies have a direct contact with a heat spreader (i.e. surface of metal radiators). In order to decrease thermal resistance of the contact some Thermal Interface Material (TIM) is usually applied. A TIM should form a low thermal resistance contact and ensure a long term stable interconnection in respect of its thermo-mechanical properties. Within the frame of the study three novel sintered nano silver pastes and one commercially available thermally conductive adhesive were compared and evaluated as potential TIMs. Formed thermal interfaces between power transistors and copper substrates were assessed in respect of their structure (X-Ray computed tomography) and heat dissipation performance (IR thermography).