Search results

Studies have been made of ageing effects, both at room temperature and at 125°C, on the microstructure of 60:40 tin‐lead solders. A comparison was made of the effect of ageing on slow and rapidly cooled matrices. Precipitation of tin within lead dendrites was observed to occur very rapidly after solidification of the alloy. Subsequently the precipitates coarsened markedly over a period of a few weeks. The matrix of the alloy also coarsened at room temperature over this period. At elevated temperatures a similar sequence of events occurred, but substantially faster. The microstructural origins of the known loss in mechanical strength of solders with ageing are discussed.

A variety of lead‐free solders are now commercially available. Of those suitable for mass soldering perhaps the ones closest to a direct, drop‐in, replacement for tin‐lead are the tin‐zinc‐bismuth alloys. For most tin‐based solders it is the tin which is the active element and dominates the all‐important interfacial reactions. As a result they have many properties in common. The addition of zinc, however, radically alters this picture. Zinc oxidation products are formed at the surfaces. Zinc intermetallic compounds are also formed in preference to tin‐compounds at the substrate interfaces. The nature and implications of these changes are outlined for the common basis materials.

The purpose of this paper is to investigate the effects of reflow time, reflow peak temperature, thermal shock and thermal aging on the intermetallic compound (IMC) thickness for Sn3.0Ag0.5Cu (SAC305) soldered joints.

A four‐factor factorial design with three replications is selected in the experiment. The input variables are the peak temperature, the duration of time above solder liquidus temperature (TAL), solder alloy and thermal shock. The peak temperature has three levels, 12, 22 and 32°C above solder liquidus temperatures (or 230, 240 and 250°C for SAC305 and 195, 205, and 215°C for SnPb). The TAL has two levels, 30 and 90 s. The thermally shocked test vehicles are subjected to air‐to‐air thermal shock conditioning from −40 to 125°C with 30 min dwell times (or 1 h/cycle) for 500 cycles. Samples both from the initial time zero and after thermal shock are cross‐sectioned. The IMC thickness is measured using scanning electron microscopy. Statistical analyses are conducted to compare the difference in IMC thickness growth between SAC305 solder joints and SnPb solder joints, and the difference in IMC thickness growth between after thermal shock and after thermal aging.

The IMC thickness increases with higher reflow peak temperature and longer time above liquidus. The IMC layer of SAC305 soldered joints is statistically significantly thicker than that of SnPb soldered joints when reflowed at comparable peak temperatures above liquidus and the same time above liquidus. Thermal conditioning leads to a smoother and thicker IMC layer. Thermal shock contributes to IMC growth merely through high‐temperature conditioning. The IMC thickness increases in SAC305 soldered joints after thermal shock or thermal aging are generally in agreement with prediction models such as that proposed by Hwang.

It is still unknown which thickness of IMC layer could result in damage to the solder.

The IMC thickness of all samples is below 3 μm for both SnPb and SAC305 solder joints reflowed at the peak temperature ranging from 12 to 32°C above liquidus temperature and at times above liquidus ranging from 30 to 90 s. The IMC thickness is below 4 μm after subjecting to air‐to‐air thermal shock from −40 to 125°C with 30 min dwell time for 500 cycles or thermal aging at 125°C for 250 h.

The paper reports experimental results of IMC thickness at different thermal conditions. The application is useful for understanding the thickness growth of the IMC layer at various thermal conditions.

This paper presents the results of the investigations of LED modules soldered with the use of different soldering pastes.

The tested power LED modules are soldered using different solder pastes and soldering processes. Thermal parameters of the performed modules are tested using indirect electrical methods. The results of measurements obtained for different modules are compared and discussed.

It was shown that the soldering process visibly influences the results of measurements of optical and thermal parameters of LED modules. For example, values of thermal resistance of these modules and the efficiency of conversion of electrical energy into light differ between each other even by 15%.

The obtained results of investigations can be usable for designers of the assembly process of power LED modules.

This paper shows the investigations results in the area of effective assembly of power LEDs to the metal core printed circuit board (MCPCB) using different soldering pastes (REL22, REL61, LMPA-Q6, OM-5100, OM-338-PT, M8, OM-340, CVP-390). It was shown that the best thermal and optical properties of these modules are obtained for the OM5100 paste by Alpha Assembly.

The role of intermetallics in soldered joints is ambivalent. They are an essential part of joints to common basis materials and at low levels they have a strengthening effect on solder alloys. At higher levels, however, it is well known that they can cause joint embrittlement. In this paper three aspects of their role have been studied: the microstructure of intermetallic containing solder alloys, the effects of soldering parameters on the quantity of intermetallic formed and, finally, the rates of growth of intermetallic compounds in the solid state. The results suggest that alloys which are pre‐doped with copper tend to form slightly more interfacial intermetallic during soldering than those which are not. In the solid state the rates of growth appear to be a function of the melting point of the alloy, with the higher melting point lead‐free alloys exhibiting lower rates than lower melting point alloys such as 63Sn37Pb (183∞C) or 42Sn58Bi (138∞C).

The elimination of lead from solders for flip‐chip attachment has necessitated many new studies on the reliability of the resultant systems. There are many lead‐free solder material systems. However, most of them contain a large proportion of tin. The tin in the solder reacts with the copper layer present in some types of under bump metallisation (UBM) depleting the UBM of copper, and thereby causing loss of adhesion and a weak interface. In this work, the relative reliability of Cr/Cu/Cu and Cr/Cu/Cu/Ni UBM systems was studied. The UBM systems were deposited with an electroplated Sn‐3.5Ag lead‐free solder alloy. The results conclusively showed that the Cr/Cu/Cu/Ni UBM system is a better choice for the Sn‐3.5Ag lead‐free solder. In the Cr/Cu/Cu/Ni UBM system, the thickness of the nickel layer was found to be an important parameter. Multiple reflow and high temperature storage test results showed that serious depletion of the UBM layer could occur if the UBM layers were not optimised. The thermo‐mechanical reliability of Ni‐based UBM bumps showed promising results up to 1,500 temperature cycling.

The wetting performance and intermetallic formation of a Sn/Ag/Cu alloy on printed circuit board (PCB) surfaces and on component terminations were studied in this work. Two different PCB surface finishes, immersion gold over electroless nickel (Ni/Au) and an organic solderability preservative (OSP), were studied. Chip components with Sn/Pb coating and a gull‐wing type component with 100% Sn coating were used in these experiments. Different reflow profiles were tested, and the dependence of the wetting performance, intermetallic layer thickness and the microstructure of the solder joints on the reflow profile were investigated.It was found that reflow process conditions did not significantly influence the spreading or intermetallic formation on either of the surfaces. Neither the wetting onto the component nor the general microstructure of the solder joints varied significantly with the reflow profile. When a Sn/Pb ‐coated component was used, the content and size of Pb‐rich phases in the solder joint increased with a longer time above liquidus or a higher reflow peak temperature.

The benefits of controlling the atmosphere in an infra‐red reflow oven are evident in improved soldering yields and easier post‐soldering cleaning of the assembly. The main benefits arise from reducing the oxygen partial pressure in the atmosphere at the time when the solder is molten. The most common inerting atmosphere used is nitrogen, and to reduce the oxygen concentration to 100 ppm is relatively straightforward, but below this level the equipment and the running costs increase with decreasing oxygen requirement. This paper gives data on the effects of reducing the oxygen level on a number of parameters relevant to high quality manufacture and product reliability. The aim is to identify which aspects of the soldering process are crucially sensitive to the oxygen concentration and which are not, in order to establish a good working compromise between oxygen level and the cost of its attainment.

Electronic materials, particularly tin‐lead coated component leads, may degrade and acquire a poor solderability as a result of long‐term storage or prolonged periods at elevated temperatures (during burn‐in). This paper presents the results of studies on the surface chemistry and microstructure of such coatings together with a technique for stripping degraded coatings and replacing them with pristine finishes having excellent solderability.

An overview of the mechanical behaviour of common solder types is presented, with particular emphasis placed upon those properties relevant to the performance of soldered joints in service. The requirements for more sophisticated and complex information are highlighted in order to assist design for the increasingly arduous demands associated with miniaturisation.