An integrated modeling approach to solder joint formation

The attachment of electronic components to printed circuit boards using solder material is a complex process. This paper presents a novel modeling methodology, which integrates the governing physics taking place. Multiphysics modeling technology, imbedded into the simulation tool—PHYSICA is used to simulate fluid flow, heat transfer, solidification, and stress evolution in an integrated manner. Results using this code are presented, detailing the mechanical response of two solder materials as they cool, solidify and then deform. The shape that a solder joint takes upon melting is predicted using the SURFACE EVOLVER code. Details are given on how these predictions can be used in the PHYSICA code to provide a modeling route by which the shape, solidification history, and resulting stress profiles can be predicted.

Optimisation modelling for flip-chip solder joint reliability

This paper details and demonstrates integrated optimisation-reliability modelling for predicting the performance of solder joints in electronic packaging. This integrated modelling approach is used to identify efficiently and quickly the most suitable design parameters for solder joint performance during thermal cycling and is demonstrated on flip-chip components using “no-flow” underfills. To implement “optimisation in reliability” approach, the finite element simulation tool – PHYSICA, is coupled with optimisation and statistical tools. This resulting framework is capable of performing design optimisation procedures in an entirely automated and systematic manner.

The effect of thermal cycle profiles on solder joint damage

The relationship between the damage caused at different thermal cycles is very important. The whole of accelerated thermal cycle testing is based on the premise that damage at one cycle is representative of damage at a different cycle. In this paper, the relative damage caused by six thermal cycle profiles are predicted using Finite Element (FE) modelling and the results validated against experiments. Both creep strain and strain energy density were used as damage indicators and creep strain was found to correlate better with experiment. The validated FE model is then used to investigate the effect of altering each of the thermal profile parameters (ramp and swell times, hot and cold temperatures). The components used for testing are surface mount resistors - 1206, 0805 and 0603. The solders investigated are eutectic SnAgCu and eutectic SnAg.

Study of Anisotropic Conductive Adhesive Joint Behaviour under 3-Point Bending

Flip chip interconnections using anisotropic conductive film (ACF) are now a very attractive technique for electronic packaging assembly. Although ACF is environmentally friendly, many factors may influence the reliability of the final ACF joint. External mechanical loading is one of these factors. Finite element analysis (FEA) was carried out to understand the effect of mechanical loading on the ACF joint. A 3-dimensional model of adhesively bonded flip chip assembly was built and simulations were performed for the 3-point bending test. The results show that the stress at its highest value at the corners, where the chip and ACF were connected together. The ACF thickness was increased at these corner regions. It was found that higher mechanical loading results in higher stress that causes a greater gap between the chip and the substrate at the corner position. Experimental work was also carried out to study the electrical reliability of the ACF joint with the applied bending load. As per the prediction from FEA, it was found that at first the corner joint failed. Successive open joints from the corner towards the middle were also noticed with the increase of the applied load.

Review of methods to predict solder joint reliability under thermo-mechanical cycling

Solder joints are often the cause of failure in electronic devices, failing due to cyclic creep induced ductile fatigue. This paper will review the modelling methods available to predict the lifetime of SnPb and SnAgCu solder joints under thermo-mechanical cycling conditions such as power cycling, accelerated thermal cycling and isothermal testing, the methods do not apply to other damage mechanisms such as vibration or drop-testing. Analytical methods such as recommended by the IPC are covered, which are simple to use but limited in capability. Finite element modelling methods are reviewed, along with the necessary constitutive laws and fatigue laws for solder, these offer the most accurate predictions at the current time. Research on state-of-the-art damage mechanics methods is also presented, although these have not undergone enough experimental validation to be recommended at present

Study of the thermal stress in a Pb-free half-bump solder joint under current stressing

The thermal stress in a Sn3.5Ag1Cu half-bump solder joint under a 3.82×108 A/m2 current stressing was analyzed using a coupled-field simulation. Substantial thermal stress accumulated around the Al-to-solder interface, especially in the Ni+(Ni,Cu)3Sn4 layer, where a maximal stress of 138 MPa was identified. The stress gradient in the Ni layer was about 1.67×1013 Pa/m, resulting in a stress migration force of 1.82×10-16 N, which is comparable to the electromigration force, 2.82×10-16 N. Dissolution of the Ni+(Ni,Cu)3Sn4 layer, void formation with cracks at the anode side, and extrusions at the cathode side were observed

Fracture mechanics analysis of cracks in solder joint intermetallic compounds

The trend towards miniaturization of electronic products leads to the need for very small sized solder joints. Therefore, there is a higher reliability risk that too large a fraction of solder joints will transform into Intermetallic Compounds (IMCs) at the solder interface. In this paper, fracture mechanics study of the IMC layer for SnPb and Pb-free solder joints was carried out using finite element numerical computer modelling method. It is assumed that only one crack is present in the IMC layer. Linear Elastic Fracture Mechanics (LEFM) approach is used for parametric study of the Stress Intensity Factors (SIF, KI and KII), at the predefined crack in the IMC layer of solder butt joint tensile sample. Contrary to intuition, it is revealed that a thicker IMC layer in fact increases the reliability of solder joint for a cracked IMC. Value of KI and KII are found to decrease with the location of the crack further away from the solder interfaces while other parameters are constant. Solder thickness and strain rate were also found to have a significant influence on the SIF values. It has been found that soft solder matrix generates non-uniform plastic deformation across the solder-IMC interface near the crack tip that is responsible to obtain higher KI and KII.

Constructive trusts, illegal purpose and locus poenitentiae

Analyses the Family Division decision in Q v Q on the competing claims of a father and son to beneficial ownership of a house in which the father sought to on a secret agreement entered into with his sons when transferring the property into their joint names, intended to subvert the inheritance tax rules on lifetime gifts by retaining the right to have the property transferred back to him, and the son relied on a later agreement with his brother to transfer the house into his sole name, in reliance on which he and his wife had acted to their detriment by paying for its upkeep and renovation.

Property in body parts and products of the human body

An intriguing question, which until recently had not been directly explored by the courts, is the extent to which English law recognises body parts and products of the human body as property capable of ownership. Although the common law currently recognises no general property in a dead body (and only limited possessory rights in respect of it), this apparent “no-property rule” provides no justification, it is submitted, for denying proprietary status to parts or products of a living human body. The recent decision of the Court of Appeal in Yearworth v. North Bristol NHS Trust ([2009] EWCA Civ 37) lends strong support to the view that genetic material (as the product of a living human body) is capable of ownership, at least in the context of a claim in the tort of negligence and bailment. This article examines the various issues by reference to both English and Commonwealth authority.

Beneficial entitlement after Stack

Reviews the approach adopted in Stack v Dowden to determination of beneficial interests in a property purchased in the joint names of cohabitees. Considers two cases which extended the Stack v Dowden approach to encompass the beneficial entitlement of family members, namely: (1) Adekunle v Ritchie on beneficial entitlement in a property jointly purchased by a mother and son, where the transfer document contained no express declaration of trust; and (2) Abbott v Abbott on the impact of a wife's indirect financial contributions in determining her beneficial entitlement in the matrimonial home.

Joint ownership and beneficial entitlement: Stack v Dowden.

Explores the issue of the share of beneficial entitlement to the family home where the legal title is jointly owned, but where there has not been an express declaration of a beneficial joint tenancy. Discusses the House of Lords judgment in Stack v Dowden which addressed this point. Explains how the judges moved the focus away from the court imposing its own sense of fairness on the parties or imputing an intention based on the circumstances to one where the concentration will be on the parties' relevant conduct. Outlines three other points of interest referred to in the judgment: (1) whether an indirect financial contribution could support a constructive trust; (2) whether proprietary estoppel and common intention constructive trusts should be assimilated; and (3) whether a mortgage liability is equivalent to a financial contribution.

Jackson v JH Watson Property Investment Ltd.

Comments on the Chancery Division decision in Jackson v JH Watson Property Investment Ltd on whether a landlord was liable in nuisance to a long leaseholder in respect of damage caused to the demised property by a building defect which pre-dated the grant of the lease or whether the principle of caveat lessee applied. Considers whether the defect amounted to "disrepair" within the meaning of the landlord's repairing covenant.

Insolvency: ordering a sale of the family home

Comments on the Chancery Division ruling in Nicholls v Lan on whether the interests of a bankrupt husband's creditors prevailed over those of the wife, despite her circumstances being exceptional within the meaning of the Insolvency Act 1986 s.335A on account of her suffering from chronic schizophrenia, where the wife was the joint owner of another property which could be realised to buy out the trustee in bankruptcy's half share in the equity of the matrimonial home.