Solder Column Research Articles

Reliable Solder Columns to Replace Solder Balls in Large 2.5D Heterogeneous Packages

Solder balls are well suited for interconnecting normal sized ball grid array (BGA) components to printed circuit boards (PCB). However, solder balls are prone to cracking (failure) caused by excessive stresses inherently found in advanced packaging such as large-sized heterogeneous 2.5D packages where multiple dies are placed side-by-side. The growth trend is to make larger and larger BGA processors exceeding 60x60mm in size for AI and data centers. A plethora of new processor substrate and interposer designs are coming on line such as Chip-on-Wafer-on-Substrates (CoWoS). For that reason, more instances of solder ball delamination and cracking on large-sized BGA packages are probable. Solder columns were produced more than 50 years ago to reduce cracking in large ceramic packages, especially for use in harsh environments found in aerospace and defense applications. However, such legacy solder column technology is limited to Tin-Lead (SnPb) alloys. The challenge is to make Lead Free solder columns that satisfy demand for RoHS compliance, while still providing adequate stress relief (compliancy) for extending the operational life of large sized heterogeneous 2.5D processors for mission critical applications. This paper describes our ongoing work to innovate non-collapsible, compliant Lead-Free Braided Copper Solder Columns to reduce strain caused by the Coefficient of Thermal Expansion (CTE) mismatch between large BGA packages and FR4 PCBs. A braid, consisting of 16 fine copper alloy wires, serves as an exoskeleton sleeve around a solder column to provide mechanical support as well as compliancy. This novel solder column technology supports market trends to scale up the size of chip packaging while maintaining reliability. Our focus was to quickly characterize Lead-Free solder column core materials and braiding alloys by employing “Fail-Fast” destructive accelerated mechanical bend testing in our lab. Since there is an ongoing interest in Tin-Lead alloys, this paper also discusses data on Tin-Lead alloys for comparative purposes.

A numerical investigation on the influence of full vs perimeter arrays on the mechanical reliability of electronic assemblies under vibration

PurposeThis paper aims to compare and evaluate the influence of package designs and characteristics on the mechanical reliability of electronic assemblies when subjected to harmonic vibrations.Design/methodology/approachUsing finite element analysis (FEA), the effect of package design-related parameters, including the interconnect array configuration, i.e. full vs perimeter, and package size, on solder mechanical stresses are fully addressed.FindingsThe results of FEA simulations revealed that the number of solder rows or columns available in the array, could significantly affect solder stresses. In addition, smaller packages result in lower solder stresses and differing distributions.Originality/valueIn literature, there are no papers that discuss the effect of solder array layout on electronic packages vibration reliability. In addition, general rules for designing electronic assemblies subjected to harmonic vibration loadings are proposed in this paper.

Study on the Influence of Vacuum Vapor Phase Soldering Process Technology on SiP Component

Vapor phase soldering (VPS) is used in the assembly of highly reliable electronic products for aerospace, military, and industrial area for the advantages of heat transfer coefficient, good temperature uniformity, and controllable maximum soldering temperature. VPS is usually used for PCB (Printed Circuit Board) and complex components. However, little research has been done on the application of VPS process technology on the SiP (System in Package) component in the process of solder ball or solder column package. In this work, we investigated the influence of the vacuum VPS process on the SiP component. The advantages of VPS and vacuum process were discussed and the maximum shear strength of solder joints in the vacuum process was calculated. The vacuum process has no impact on the internal structure of the SiP component.

Coupling effects of mechanical vibrations and thermal cycling on reliability of CCGA solder joints

The microstructures and crack propagation behavior of CCGA (ceramic column grid array) solder joints after sinusoidal vibration loading, random vibration loading, and thermal cycling test have been discussed in this study. The failure mechanism of solder joints was analyzed using an experimental method and finite element analysis. It was found that the failed solder joints mainly distributed at the peripheral area in the solder column arrays and the crack initiation was mainly caused by mechanical vibrations. The deformation of PCB (printed circuit board) introduced by mechanical vibrations brought the outermost solder columns in CCGA devices with significant stress concentration and induced the initiation of cracks. Furthermore, cracks propagated during the process of mechanical vibrations and thermal cycling. The cracks propagated rapidly and the solder joints finally failed. The structure of the PCB holder was improved to relieve the vibration response from the peripheral joints. No visible crack was found in the solder joints after the same mechanical vibrations and thermal cycling test. The reliability of solder joints have been greatly improved with the new PCB holder.

A reliable solderless connection technique for high I/O counts ceramic land grid array package for space applications

Area Array microelectronic package with small pitch and large I/O counts is now widely used in the ceramic packaging of microelectronics Integrated Circuit (IC). This kind of Land Grid Array (LGA) device cannot be directly soldered on Printed Circuit Board (PCB). In order to mount them on PCB the LGA device back end is mostly terminated with solder columns (CGA device) or with solder balls (BGA device). For space application CGA and BGA ceramic devices are prone to early failure due to harsh environment. Indeed vibration and shock during launch as well as thermal cycling in orbit induces a high level of stress in the solder joints between board and device. Such high level of stress can ultimately cause the failure of one of the solder joints causing permanent functional electrical failure. As the trend for next generation of ceramic IC device is to increase their sizes and their density of I/O, susceptibility to mechanical and thermal loading grows even more. This statement is in contradiction with the high level of reliability required for space application. To overcome this situation CNES has taken the lead with space users and space connector manufacturers to develop for space application a reliable solderless solution that will enable to easily mount on board the actual and the next generation of ceramic IC package. This solderless solution consists in using a thin connector interposer between the ceramic LGA device and the board. A clamping system is used to hold and keep aligned on the board the stacking composed of the ceramic device and the interposer connector. This solderless solution has been evaluated according to the European space standard and has demonstrated a high robustness to harsh environment making this solution compliant for space application.

Prognostication Based on Resistance-Spectroscopy and Phase-Sensitive Detection for Electronics Subjected to Shock-Impact

Leading indicators of failure have been developed based on high-frequency characteristics, and system-transfer function derived from resistance spectroscopy measurements during shock and vibration. The technique is intended for condition-monitoring in high-reliability applications where the knowledge of impending failure is critical and the risks in terms of loss-of-functionality are too high to bear. Previously, resistance spectroscopy measurements have been used during thermal cycling tests to monitor damage progression due to thermomechanical stresses. The development of resistance spectroscopy based damage precursors for prognostication under shock and vibration is new. In this paper, the high-frequency characteristics and system-transfer function based on resistance spectroscopy measurements have been correlated to the damage progression in electronics during shock and vibration. Packages being examined include ceramic area-array packages. Second level interconnect technologies examined include copper-reinforced solder column, SAC305 solder ball, and 90Pb10Sn high-lead solder ball. Assemblies have been subjected to 1500 g, 0.5 ms pulse (JESD-B2111). Continuity has been monitored in situ during the shock test for identification of part-failure. Resistance spectroscopy based damage precursors have been correlated to the optically measured transient strain based feature vectors. High speed cameras have been used to capture the transient strain histories during shock-impact. Statistical pattern recognition techniques have been used to identify damage initiation and progression and determine the statistical significance in variance between healthy and damaged assemblies. Models for healthy and damaged packages have been developed based on package characteristics. Data presented show that high-frequency characteristics and system-transfer characteristics based on resistance spectroscopy measurements can be used for condition-monitoring, damage initiation, and progression in electronic systems. A positive prognostic distance has been demonstrated for each of the interconnect technologies tested.

Reliability of an 1657CCGA (Ceramic Column Grid Array) Package With 95.5SN3.9AG0.6CU Lead-Free Solder Paste on PCBS (Printed Circuit Boards)

In many applications such as computers and telecommunications, the IC chip sizes are very big, the on-chip frequency and power dissipation are very high, and the number of chip I/Os is very large. The CCGA (ceramic column grid array) package developed by IBM is one of the best candidates for housing these kinds of chips. There are two parts in this study. One is to show that the two-parameter Weibull life distribution is adequate for modeling the thermal-fatigue life of lead-free solder joints. This is demonstrated by comparing the two-parameter and three-parameter Weibull distributions with life test data of an 1657-pin CCGA package with the 95.5 wt %Sn3.9 wt %Ag0.6 wt %Cu lead-free solder paste on lead-free printed circuit boards under thermal cycling conditions. The other part of this study is to determine the time-history creep strain energy density of the 1657-pin CCGA solder column with two different solder paste materials, namely, 95.5 wt %Sn3.9 wt %Ag0.6 wt %Cu and 63 wt %Sn37 wt %Pb and under three different thermal cycling profiles, namely, 25↔75°C, 0↔100°C, and −25↔125°C. The effects of these solder pastes and temperature conditions on the thermal-fatigue life of the high-lead (10 wt %Sn90 wt %Pb) solder columns of the CCGA package are provided and discussed.

Mechanical properties of intermetallic compounds associated with Pb-free solder joints using nanoindentation

Mechanical properties of intermetallic compound (IMC) phases in Pb-free solder joints were obtained using nanoindentation testing (NIT). The elastic modulus and hardness were determined for IMC phases associated with insitu FeSn particle reinforced and mechanically added, Cu particle-reinforced, composite solder joints. The IMC layers that formed around Cu particle reinforcement and at the Cu substrate/solder matrix interface were probed with NIT. Moduli and hardness values obtained by NIT revealed were noticeably higher for Cu-rich Cu3Sn than those of Cu6Sn5. The Ag3Sn platelets that formed during reflow were also examined for eutectic Sn-Ag solder column joints. The indentation modulus of Ag3Sn platelets was significantly lower than that of FeSn, SnCuNi, and CuSn IMCs. Indentation creep properties were assessed in localized microstructure regions of the as-cast, eutectic Sn-Ag solder. The stress exponent, n, associated with secondary creep differed widely depending on the microstructure feature probed by the indenter tip.

An Elastoplastic Beam Model for Column-Grid-Array Solder Interconnects

An analytical model is developed and implemented to analyze the deformation of solder columns in column-grid-array(CGA) assemblies. Each solder column is modeled as a prismatic beam of circular cross section subjected to end shearing deflections caused by thermal mismatch between the module and the circuit board. The solder is idealized as an elastic-perfectly plastic material whose yielding is governed by the von Mises criterion. Since the columns are relatively short by beam standards, transverse shear deformation has been incorporated into the beam model. The results generated with the model indicate the following: 1. Yielding is governed by bending for slenderness ratios (height-to-diameter) of h/d ≥ 1/3. 2. The nonlinear stiffness relationship for a sheared column, presented in dimensionless form, reduces to a single curve, which is valid for arbitrary values of slenderness ratio (≥1/3) and material parameters. 3. The normalized relationship between maximum shear strain (in the Tresca sense) and the relative end deflection depends on Poisson’s ratio but is independent of the other material parameters and the slenderness ratio. The peak shear strain can easily be decomposed into elastic and plastic portions. 4. The maximum value of the total or the plastic shear strain in the solder column can be expressed in terms of a correction factor to be applied to the nominal shear strain (shear displacement divided by the column height). The correction factors are quite sensitive to the column slenderness and the load level. Since correction factors less than unity are possible, one should not interpret the nominal shear strain as the “average” shear strain in the column. Regardless of the load level, the nominal strain underestimates the peak strain by the greatest amount at a slenderness ratio (height/diameter) of 1.7. The nonlinear stiffness results presented in the paper may be used to create more efficient finite element models of entire assemblies by replacing each column with a single nonlinear spring element. When used in conjunction with an appropriate Coffin-Manson relationship, the maximum shear strain results presented herein may be utilized to increase column fatigue life.

Deformation Mechanism of Two-Phase Solder Column Interconnections Under Highly Accelerated Thermal Cycling Condition: An Experimental Study

Thermomechanical behavior of solder interconnections of a ceramic column grid array package assembly is characterized by high sensitivity moire´ interferometry. The in situ thermal deformations of the most distant column from the axis-of-symmetry are documented for (a) an initial isothermal loading of ΔT = −60° and (2) subsequent accelerated thermal cycles of (20°C)−(125°C)−(−40°C)−(20°C). The deformed shape of the column obtained from the displacement fields clearly delineates the effect of the eutectic solder fillet on the column deformation accumulated during accelerated thermal cycling. A deformation mechanism is suggested to explain the results, which is confirmed by the results from the actual accelerated thermal cycle test of the same assembly. The distributions of inelastic strains accumulated during thermal cycles are determined along the sides of the column. The results emphasize the importance of the solder fillet height for optimum reliability. The effect of specimen preparation required for the moire´ experiment on the column deformation is also discussed to validate the experimental data.

Parametric Finite Element Method for Predicting Shapes of Three-Dimensional Solder Joints

This paper discusses the application of the parametric finite element method for predicting shapes of three-dimensional solder joints. With this method, the surface of the joint is meshed (discretized) with finite elements. The spatial variables (x, y, z) are expanded over each element in terms of products of interpolation (blending) functions expressed in parametric form and element nodal coordinates. The element nodal coordinates which are not constrained by the boundary conditions are determined by minimizing the potential energy function which governs the joint formation problem. This method has been employed successfully in the past to predict the shapes of two dimensional fillet and axisymmetric joints. In this paper, the method is extended to three dimensional problems involving sessile drops formed on a rectangular pad and solder columns formed between two horizontal planes and subject to a vertical force.

Finite element analysis for Solder Ball Connect (SBC) structural design optimization

Solder Ball Connect (SBC) is a second-level surface mount electronics packaging technology in which ceramic modules containing one or more chips are joined to a circuit card (FR-4) by means of an array of nonhomogeneous solder columns. These columns consist of a high-temperature-melting 90%Pb/lO%Sn solder sphere attached to the module and card with eutectic solder fillets. The solder structures accommodate the bulk of the strain (which is due to the thermal-expansion mismatch between FR-4 and the 9211 ceramic of the modules) generated during power cycling. If the solder structures are not properly designed, the thermal strain can be a source of premature fatigue failure. In this work, finite element analysis is used to characterize the plastic strains that develop in the SBC interconnection during thermal cycling. Since plastic strain is a dominant parameter that influences low-cycle fatigue, it is used as a basis of comparison for various structural alternatives. Designed experiment techniques are used to systematically evaluate the thermal strain sensitivity to structural variables. Results are used to identify an optimally reliable structure that is robust in terms of assembly-process variables.

Creep of solder interconnects under combined loads

An exact analysis is presented for the creep deformation of solder interconnects subjected to the actions of bending moment and axial force. Dimensionless interaction curves and charts which relate the variables, axial force, location of neutral-axis, maximum bending stress and strain rate, bending moment and change in curvature rate are provided for engineering practice convenience. The tangent-modulus buckling loads of centrally loaded solder columns and a simple formula for the critical time to buckling are determined. The constitutive relationship of the solder interconnects is given.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>