Standard Surface Mount Assembly Research Articles

Development of a High Temperature Interconnect Solution as an Alternative to High Lead or Gold Content Solders

AbstractA collaborative research programme between project partners Microsemi, the National Physical Laboratory (NPL) and Gwent Electronic Materials (GEM), has successfully developed innovative materials specifically designed to offer an alternative for high Pb or Au content materials to increase the operating temperature of electronic assemblies. Currently, for electronic assemblies to operate at high temperature, they must use a high lead solder or a very expensive gold based solder to withstand these temperatures. The ELCOSINT project has developed an inexpensive lead-free alternative for joining high temperature electronics suitable for operating at temperatures above 250°C utilising standard surface mount assembly processes. This paper summarises the work undertaken by the authors to develop and better understand this new family of electrical interconnection materials. The project brought together a materials supplier (GEM – Gwent Electronic Materials), an end-user (MSL - Microsemi) and an technology research organisation (NPL – National Physical Laboratory) to jointly develop, test and implement in production, the solution based on silver-loaded silicone materials. This paper focuses on the testing and materials evaluation undertaken at NPL to determine the long term performance of these alternative materials including high temperature ageing up to 300°C, thermal cycling and damp heat testing. Details of the shear strength and electrical performance of interconnects between the substrates and components during the test regimes are given. The manufacturing process is outlined including details of the test vehicles utilised. The processing temperature for the conductive adhesive is 250°C which offers additional advantages in potential improvements in component and substrate reliability compared to soldered solutions which would typically be processed at temperatures above 300°C.

A 7–21 GHz Dual-Polarized Planar Ultrawideband Modular Antenna (PUMA) Array

The design, fabrication, and measurement of a 16 × 16 dual-polarized planar ultrawideband modular antenna (PUMA) array operating over 7-21 GHz (3:1 bandwidth) are presented. The array is comprised of tightly coupled dipoles printed on a grounded dielectric substrate and are excited by an unbalanced feeding scheme that eliminates external wideband baluns and feed organizers. The array can be assembled modularly, where each low-profile, fully planar, low-cost tile is fabricated using standard multilayer microwave PCB techniques. A unique solderless, modular interconnect mates the array to a dilation fixture that facilitates measurements using standard surface-mount assembly (SMA) connectors and terminations. After presenting the most critical design trends, simulation results of the final array in infinite, infinite × finite, and finite × finite models are compared with measurements. This prototype array exhibits a measured active VSWR <; 2.1 and close to ideal gain at broadside, and VSWR <; 2.8 with low cross-polarization out to θ = 45° in all planes, showing close agreement with simulations.

Effect of Gold Content on the Reliability of SnAgCu Solder Joints

Electroplated Ni/Au over Cu is a popular metallization for printed circuit board finish as well as for component leads, especially wire-bondable high-frequency packages, where the gold thickness requirement for wire bonding is high. The general understanding is that less than 3 wt% of Au is acceptable in SnPb solder joints. However, little is known about the effect of Au content on the reliability of SnAgCu solder joints. The purpose of this paper is to determine the acceptable level of Au in SAC305 solder joints. Three different package platforms with different Au thicknesses were assembled on boards with two different Au thicknesses using a standard surface mount assembly line in a realistic production environment. The assembled boards were divided into three groups: as-built, isothermally aged at 125 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C for 30 days, and isothermally aged at 125 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C for 56 days. All boards were then subjected to accelerated mechanical reliability tests including random vibration and drop testing. The results show that solder joints with over 10 wt% Au are unacceptable. If Cu is available to dissolve in the solder joint, then an Au content under 5 wt% will not significantly degrade the reliability of the solder joint. When Ni layers are present on both the board and the component sides of the interface, this limits the ability of Cu to dissolve into the solder joint, and hence an Au content under 3 wt% is acceptable. The failure mechanism for solder joints with high Au content is fractures through the AuSn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> intermetallic compound. Additional findings confirmed that there is a danger of placing parts near high-stress areas and that a high level of voiding reduced reliability.

Displacement Theory for Fixturing Design of Thin Flexible Circuit Board Assembly

The compliant nature of flexible substrates subject to assembly forces can result in severe misregistration of the component leads and substrate bond pads, leading to assembly process defects. Specially dedicated tooling for fixturing thin flexible substrates in standard surface mount assembly equipment is gaining importance. This paper focuses on developing a theoretical foundation for implementing Smart Tooling of fixturing thin flexible substrates. The primary goals are to determine the impact of fixturing tooling on assembly process quality, to develop a displacement theory to predict transverse displacements, and to analyze the optimum perimeter fixturing configuration. The predictive capabilities of the transverse displacement model are verified.

Smart tooling for assembly of thin flexible systems

Emerging electronic assemblies are demanding lower cost, lighter weight, miniaturized packages mounted on thin flexible circuit boards and/or flex circuits. However, the compliant nature of the flex substrates poses new processing technology challenges for standard surface mount assembly equipment. A particular challenge is fixture tooling. The flexible substrate experiences significant transverse displacements under perpendicular assembly and/or fixturing forces during solder paste printing and component placement processes. The transverse displacements result in misregistration of the component leads and substrate bond pads, leading to severe assembly process defects. The solder reflow process further complicates the issue due to the thermo-mechanical warpage induced. Conventional assembly equipment utilizes dedicated tooling designed to handle rigid circuit board assemblies. As electronic assemblies move toward very fine pitch surface mount packages, chip scale packages, and flip chip attachment assembled to thin flexible double-sided circuit boards, reengineered and specialized dedicated tooling for fixturing flexible substrates in standard assembly equipment are becoming extremely important. This paper focuses on developing analysis methodologies and theories for implementing machine dedicated Smart Tooling. The primary goals being to determine the impact of fixturing on assembly process quality and to determine optimum fixturing configurations for thin flexible circuit board assemblies based on circuit design data. A mathematical model to describe both transverse and perpendicular displacements of flex substrates is developed, and its closed form solution for transverse displacements is obtained. Fixturing configurations based on a perimeter support technique of flex substrates is analyzed to minimize transverse displacements.

Surface mount assembly of BGA and μBGA

A ball grid array (BGA) is a surface‐mount device and is processed in a standard surface‐mount (SM) assembly line. However, there are some special features which make it different from other assemblies. The peculiarities of PCB layout, screen printing, placement, soldering, and inspection in BGA processing are presented. The failures of BGA assembly during ramp up and series production start‐up are analysed in detail. An assembly quality better than 1dpm was achieved. The scale of repair is determined by the electrical quality of the devices. If small packages with higher pincount and better electrical performance are required, the µBGA is a sound choice. Compared with other high pincount packages with a small formfactor as TAB, flip‐chip and chip on board, the µBGA may be processed in a standard SM assembly line. The results of µBGA‐assembly feasibility studies are discussed.