Testing of the quality of solder joints through the analysis of their thermal behaviour with an interferometric laser probe

We present an optical contactless method for testing the quality of solder joints in surface mounted components by measuring their thermal dynamic behaviour. We detect surface normal displacements induced by Joule heating with a high resolution interferometnc laser probe. This probing method, based upon a homodyne Michelson interferometer, is an interesting tool for investigating the mechanisms of heat deposition and flow inside electronic devices. It allows the precise time evolution of the surface normal displacement to be measured (the laser probe has subnanometric resolution). This new approach of thermal behaviour laser testing is based upon the analysis of the diffusion of heat produced by the Joule effect in the structure (lead, solder and copper strip) from short current pulses and will influence heat diffusion. Solder joint failures (intermetallics, hidden voids, etc. ) behave as a thermal barrier. The optical test consists of measuring the dynamics of the solder joint expansion, and the variations from a standard response (good quality solder joints) will reveal defects. Important variations have been observed in solder joints that have undergone thermal cycling ageing tests. We have also investigated the thermal response of joints on IMS (insulated metallic substrate) and epoxy substrates. They show a very different time response.     

The electrochemical corrosion behaviour of Pb-free Al-Zn-Sn solders in NaCl solution

The corrosion behaviour of Pb-free X(5Al-Zn)-YSn solders, where X is 9-50 and Fis 91-50, has been investigated with regard to open-circuit potential, galvanic corrosion and potentiodynamic polarization in a 3.5% NaCl solution. The galvanic current densities for 9 (5Al-Zn)-91Sn in NaCl solution are 25, 20 and 12.5 μA/cm² with respect to Cu, Ni-Cu-P and Ni-P, respectively. The corrosion behaviour has been compared with that of 63Sn-37Pb solder. A passivation behaviour is observed for all of the investigated Al-Zn-Sn solders. The magnitudes of the passivation current densities depend on the compositions of the solders and the potentials applied. The polarization behaviour of eutectic 9 (5Al-Zn)-91Sn solder is very similar to that of 63Sn-37Pb solder. SnO₂ is formed at polarization potentials below −330 mV, while SnCl₄ forms when the potential is at 120 mV. ZnCl₂ replaces SnCl₄ on polarizing at 120 mV for an extended period of 1 h.     

Early detection of ageing in solder joints through laser probe thermal analysis of the peltier effect

We have recently developed an optical contactless method for testing the quality of solder joints during accelerated thermal cycling ageing processes.¹ The method was based upon the measurement of the dynamic thermal behaviour of the joint to short bursts of Joule heating. It has proved to be efficient in revealing the formation of cracks at the lead-solder interface. We present a method to evaluate ageing at a much earlier stage in the cycling process. We have observed in earlier work,¹ that before cracks appear, structural changes occur in the solder at the lead-solder interface. The thermal response of the solder joint is recorded over time to a Peltier heat perturbation produced by flowing a current pulse through the interface where structural changes occur. The key point in this method is to discriminate the Peltier effect from the Joule thermal response because both effects generate heat. The variation of the early Peltier response in the thermal cycling ageing tests is seen as a quantitative signature of the structural changes in the lead-solder interface.     

Temperature cycling effects between Sn/Pb solder and electroless copper plated AIN substrate

Thermal cycling effects on Sn/Pb solder and electroless Cu-plated AIN substrates are investigated. X-ray diffraction patterns reveal the existence of Cu₂O for the electroless Cu-plated AIN after thermal cycling in an environmental chamber. Moisture in the chamber results in the oxidation of electroless plated Cu and fracture takes place at the Cu₂O/Cu interface. The oxidation of Cu is also confirmed by Auger depth profile and electrical sheet resistance measurement. For the solder/Cu/AIN system, fracture occurs at the Cu/solder interface. No intermetallic compounds between solder and Cu are found after thermal cycling. Stress resulting from the thermal expansion mismatch is the major cause of loss of adhesion     

热循环条件下SnAgCu/Cu焊点金属间化合物生长及焊点失效行为

研究了热循环过程中SnAgCu/Cu焊点界面金属间化合物的生长规律及焊点疲劳失效行为。提出了热循环条件下金属间化合物生长的等效方程以及焊点界面区不均匀体模型,并用有限元模拟的方法分析了热循环条件下焊点界面区的应力应变场分布及焊点失效模式。研究结果表明:低温极限较低的热循环,对应焊点的寿命较低。焊点的失效表现为钎料与金属间化合物的界面失效,且金属间化合物厚度越大,焊点中的累加塑性功密度越大,焊点越容易失效。     

Testing of a solder replacement technology

Environmental issues have an ever-increasing influence on the selection of material and processes in electronic manufacturing. This paper discusses the use of conductive adhesives as a replacement for solder on SMT printed circuit boards. As a result of a world-wide market survey, a number of conductive adhesives has been selected. One of the two key issues of this paper has been to uncover the market for adhesive types and their composition. The other key issue has been the technical investigation of the influence of component termination and printed circuit surface types on adhesive bonding stability. Four different types of adhesives on two different metal surfaces are compared with conventional solder technology. Each adhesive has been applied to the PCBs by either screen printing or dispensing according to the manufacturer's recommendation, followed by curing. All PCBs went through thermal and humidity cycling followed by electrical measurements of resistance. Finally all variants have been adhesion tested. All adhesive variants have been microsectioned for metallurgical and microstructure examination. Energy dispersive analysis of X-ray (EDAX) of the metal particles in the adhesive has been carried out and documented. Rework of conductive joints is briefly commented on. Finally, aspects of occupational health are discussed concerning work with adhesive types. Work with epoxy based adhesives has been brought into special focus.     

Temperature and NaCl deposition dependent corrosion of SAC305 solder alloy in simulated marine atmosphere

The stable operation of electronic devices in marine atmospheric environment is affected by the corrosion deterioration of solder joints,and the effects by atmosphere temperature and chloride deposition are critical.In this work,NaCl deposition and temperature dependent corrosion of Pb-free SAC305 solder in simulated marine atmosphere has been investigated.The results indicate that higher NaCl deposition prolongs the surface wetting time and leads to the final thicker saturated electrolyte film for further corrosion.Higher temperature accelerates the evaporation and contributes to the final thinner saturated NaCl electrolyte film.Besides,the corrosion control process varies under the initially covered thicker NaCl electrolyte layer and under the final saturated much thinner NaCl electrolyte film as the evaporation proceeds.Moreover,the ready oxygen availability through the final thinner saturated NaCl electrolyte film facilitates the formation of corrosion product layer mainly of electrochemically stable SnO2,but higher temperature leads to the final corrosion product layer with smaller crystal size and large cracks.The findings clearly demonstrate the effects of NaCl deposition and temperature on corrosion evolution of SAC305 solder joints and are critical to the daily maintenance of electronic devices for longer service life in marine atmosphere.     

Life expectancies of Pb-free SAC solder interconnects in electronic hardware

The transition from lead (Pb) bearing solder to Pb-free solder has arisen in response to government restrictions on the use of lead (Pb) by the European Union. As a result, electronic manufacturers have sought a material comparable to the conventional 63Sn37Pb solder that has been traditionally used to assemble electronic hardware. Based on extensive review of various solder combination, the majority of electronic manufacturers appear to be adopting a tin–silver–copper (SAC) solder as a popular Pb-free solder replacement. Significant investments have been made by many researchers to characterize the material behavior and durability of this solder system. While the exact composition of the SAC solder is still in question, it now appears that the 96.5Sn3.0Ag0.5Cu (SAC305) solder is gaining wider acceptance as the favored Pb-free replacement, for surface mount assemblies that are going to be subjected predominantly to cyclic thermal environments. This paper presents a review of our current understanding of the life expectancy of Pb-free SAC solder interconnects for electronic hardware. To this end, the paper focuses on material characterization of SAC solder, as well as its temperature cycling and vibration fatigue reliability. From this review, SAC solder interconnects are shown to be suitable for providing adequate life expectancies for temperature cycling in electronic hardware. However, it is clear that there are differences between SAC and the conventional Sn37Pb solder, that need to be understood in order to design reliable electronic hardware.     

Surface analysis techniques and their application to materials characterisation

Surfaces of materials, the top few atomic layers, are of critical importance because it is here that a material interacts with its environment. The surface properties of materials dominate their corrosion behaviour, wettability, electrical contact resistance, frictional behaviour, adhesion properties and chemical reactivity. Over the last 25 years the vital role that surfaces play and the need to characterise them have been increasingly recognised and this has been paralleled by the development of many new surface analysis techniques. Of these, just a few have become pre-eminent for materials and devices characterisation: X-ray photoelectron spectroscopy, Auger electron spectroscopy and secondary ion mass spectrometry. The principles and capabilities of each of these techniques are briefly described. Application include the study of semiconductors, metallic and ceramic materials.     

Sn–Zn low temperature solder

Low temperature soldering is one of the key technologies before the accomplishment of total lead-free conversion in electronics industries. While Sn-Zn eutectic alloy has excellent properties as low temperature solder, it has some drawbacks. Damage by heat exposure and corrosion in humidity are two of the main concerns. Zn has an important role in chemical properties. The material physical properties, wetting, chemical stabilities and various reliabilities have been well understood on this alloy system through the numerous past works. The understanding of both materials and processing aspects enables one to manufacture sound electronic products without any serious problems. The basic properties and the current understandings on the limit of the application of this solder are reviewed in this paper.     

Creep-fatigue interaction in eutectic lead-tin solder alloy

A test protocol pivoting about stress cycling where the load waves were trapazoidal and the cycling frequency controlled by balanced time on load and off load was used to determine frequency, mean tensile stress, and compressive stress effects on the creep-fatigue behaviour of the Pb–Sn eutectic solder alloy at ambient temperature. It is consistently found that the minimum creep (or cyclic creep) rate decreases as frequency increases, that is, as hold time decreases. Both the number of cycles to failure as well as the time to failure increases as frequency increases. The cyclic creep rate increases drastically and the number of cycles to failure decreases drastically as the mean applied stress is increased. These results are consistent with anelastic strain recovery mechanism for creep-fatigue interaction. Similar results are also found in butt-joint solder junction configured specimens.     

Effect of the cooling rate on the mechanical and electrical behaviour of a 63Sn/37Pb solder bump on a metallized Si substrate

Flip-chip bonding technology, with solder bumps deposited on metal terminals on the chip, provides the highest functional densities of all the present bonding techniques. In this study, tin-lead solder bumps were screen printed onto a metallized Si substrate. Various cooling rates were employed after reflowing the solder bumps. The effect of the cooling rate on the microhardness, the adhesion strength and the electrical resistance of solder joints was studied and the mechanism explored. It was found that intermetallic compounds play important roles in both the mechanical and the electrical behaviour of the solder bumps.     

The ‘IMC rings’ growth mechanism in Sn–Ag–Cu solder ball and NiAu-plated BGA ball pad

Due to environmental pollution concerns, the law says the lead (Pb) inside electronics devices must be eliminated. Lots of lead-free materials have been introduced and been used for electronic products and Sn–Ag–Cu (SAC) is one of most popular lead-free representatives and has been used in high-volume production. The most popular IC packages, BGA packages which have higher I/O counts, and better thermal and electrical performance than lead-frame type packages, use solder balls of SAC for lead-free applications to connect with printed circuit boards. A particular phenomenon, so-called ‘IMC rings’, is only observed on BGA solder ball pad surfaces after the SAC solder balls are mounted on BGA ball pads which are plated with NiAu. It has not been found in either eutectic solder or Sn–Ag solder welding on plated NiAu pads. No significant evidence exists to show that ‘IMC rings’ degrade the strength of solder joints or cause earlier failures in mechanical tests. ‘IMC rings’ appear to be an inevitable outcome after the SAC is soldered onto a plated NiAu ball pad. This study is to find the growth mechanism of ‘IMC rings’ on the ball pad which is created between SAC ball and plated NiAu pad during solder ball temperature reflow. The design of the experiment and data have been discussed.     

Applying a physics-of-failure model to predicting surface mount solder joint reliability

To address the wide variety of solder joint configurations, the authors have developed and applied a physics-of-failure model to predicting the service life of solder joints under thermal cycling conditions. The wide variety of solder joint geometries, materials and environments makes it impractical to develop and apply empirical models to predicting the service life of solder joint interconnects. On the other hand, a physics-of-failure model that describes the failure mechanisms in solder joints can be applied to a wide range of conditions. The physics-of-failure model framework is described and a model is demonstrated for predicting the failure of a leadless surface mount solder joint under slow thermal cycling conditions.     

Experimental and numerical studies on size and constraining effects in lead-free solder joints

The durability and reliability of lead‐free solder joints depends on a large number of factors, like geometry, processing parameters, microstructure and thermomechanical loads. In this work, the nature and influence of the plastic constraints in the solder due to joining partners have been studied by parametric finite element simulation of solder joints with different dimensions. The apparent hardening due to plastic constraints has been shown to strongly depend on the solder gap to thickness ratio with an inversely proportional evolution. Due to interaction of several parameters, the macroscopic stress–strain constitutive law of lead‐free solder materials should be determined in the most realistic conditions. In order to identify the elasto‐plastic constitutive law of Sn–Ag–Cu solders, a sub‐micron resolution Digital Image Correlation technique has been developed to measure the evolution of strain in solder joints during a tensile test. Experimental results of the stress–strain response of Sn–Ag–Cu solder joints have been determined for several solder gaps. The measured load–displacement curves have been used in an inverse numerical identification procedure to determine the constitutive elasto‐plastic behaviour of the solder material. The effects of geometrical constraints in a real solder joint with heterogeneous stress and strain fields are then studied by comparing the apparent (constrained) and constitutive (non‐constrained) stress–strain relationships. Once the size dependant constraining effects have been removed from the stress–strain relationship, the scale effects can be studied separately by comparing the constitutive elasto‐plastic parameters of joints with a variable thickness. Experimental stress–strain curves (constrained and unconstrained) for Sn–4.0Ag–0.5Cu solder in joints of 0.25–2.4 mm gap are presented and the constraining and the size effects are discussed.     

The effect of pretinning on the strength of a chip carrier solder joint

Pretinning of gold-plated components is often done to avoid the formation of brittle Au/Sn intermetallic compounds, which may lower the mechanical strength of the joints. In this investigation the reliability of 32LCCs surface mounted on alumina boards is tested in internal and external temperature cycling, as well as in shear tests, and the effect of pretinning is studied. One thousand times of temperature cycling between ?55 and +125°C and 2000 h of power cycling with an upper joint temperature of +75°C did not cause any electrical opens. Shear tests made after a 168 h heat treatment at +125°C and 1000 temperature cycles showed that forces of approximately 1000 N were required to break the chip carrier joints. The microstructure of Sn62/Pb36/Ag2 solder joints containing gold was examined and the existing phases were identified using electron diffraction X-ray analysis. The joints fail at the Au/Sn intermetallics at the chip carrier metallization or at the Cu/Sn compounds formed at the substrate thick film/solder interface. No deterioration of the overall joint strength due to Au/Sn intermetallics was seen, when the gold content in the joints was approximately 4 weight per cent.     

The surface tension of tin-lead alloys in contact with fluxes

As part of an experimental investigation of the capillarity and chemical basis of wetting and spreading as it applies to joining technology, the interfacial tensions between liquid lead-tin solder alloys and several gaseous and liquid fluxes have been measured by specially adapted classical techniques. Both an organic flux and liquid zinc chloride depress the interfacial tension over the full range of solder composition, the latter showing the more remarkable effect. The relevance of these measurements to the understanding of metal wetting and spreading behaviour is discussed.     

Failure analysis of a railway copper contact strip

The present work investigated the causes of premature wear of the contact strips of a railway line working under voltage of 1.500 Vcc, current of 1120 A, normal force of 70 N and presence of graphite in the wire/strip interface. In all investigated regions (without apparent wear, moderate wear and severe wear), the presence of cracks in the hardened tribo-surface of the Cu-strip – which is caused either by work hardening or thermal cycling is usually followed by material detachment and production of hard abrasive debris. The presence of hard abrasive particles (such as SiO₂ and Al₂O₃,) and wear debris (Cu₂O and hardened Cu) promotes a regime of severe abrasion. The debris showed preferentially a flake-like morphology, being composed of graphite and highly deformed copper, suggesting the dominant action of mechanical wear mechanisms. The presence of some raindrop-like debris featuring an as-cast microstructure confirmed the occurrence of incipient fusion on the copper strip tribo-interface, possibly caused by electrical discharge (electrical induced wear). The results indicated that the wear mechanism of the Cu strip is divided into different stages. First, there is a mixed wear regime (adhesive and abrasive wear) of the graphite layer associated with lubricated adhesive wear of the Cu strip. After, there is the occurrence of a mixed wear regime between the strip/wire tribo surfaces, with simultaneous action of unlubricated adhesive wear, third body abrasive wear and electrical induced wear (local fusion). Finally, once the graphite has been completely consumed, the wear grooves are parallel to the sliding direction and the centre of the strip shows a much more severe wear rate.     

Electrochemical behaviour of molybdenum coated with flame CVD polycrystalline diamond film

Although natural diamond is a complete chemically-inert material for a wide range of aggressive environments, its comparative scarcity and problems for coating design have hampered its utility as a corrosion protective coating. The recent discovery and development of chemical vapour deposition methods for growing diamond crystals and polycrystalline diamond films has opened up a wide range of applications thanks to their excellent tribological, electronic and optical properties. Various applications are in progress for corrosion and combined wear and corrosion protection. This paper presents the first study of the corrosion behaviour of continuous polycrystalline diamond films using electrochemical impedance electroscopy. Diamond films have been deposited on molybdenum substrates by means of the acetylene flame combustion method (FCVD). Electrochemical behaviour has been studied in a 0.6 M NaCl solution, it being seen that despite the inert character and apparent continuity of the film, there are areas of the base material which are exposed to the electrolyte. This behaviour has been modelled by means of an equivalent circuit which allows for the corroboration of the proposed mechanism.     

Effect of addition of Al and Cu on the properties of Sn–20Bi solder alloy

This study investigates the effect of the composite addition of Al and Cu on the microstructure, physical properties, wettability, and corrosion properties of Sn–20Bi solder alloy. Scanning electron microscopy and X-ray diffraction were used to identify the microstructure morphology and composition. The spreading area and contact angle of the Sn–20Bi–x (x?=?0, 0.1 wt% Al, 0.5 wt% Cu, and 0.1 wt% Al–0.5 wt% Cu) alloys on Cu substrates were used to measure the wettability of solder alloys. The results indicate that the alloy with 0.1 wt% Al produces the largest dendrite and the composite addition of 0.1 wt% Al and 0.5 wt% Cu formed Cu₆Sn₅ and CuAl₂ intermetallic compounds in the alloy structure. And the electrical conductivity of Sn–20Bi–0.1Al is the best, which reaches 5.32 MS/m. The spread area of the solder alloy is reduced by the addition of 0.1 wt% Al and 0.5 wt% Cu, which is 80.7 mm². The corrosion products of Sn–20Bi–x solder alloys are mainly lamellar Sn₃O(OH)₂Cl₂ and the corrosion resistance of 0.1 wt% Al solder alloy alone is the best. The overall corrosion resistance of Sn–20Bi–0.1Al–0.5Cu is weakened and the corrosion of solder alloy is not uniform.