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To propose a solution procedure to minimize/eliminate tombstoning defects in small chip components with different micro via‐in pad designs for high density module assembly.

Four different micro via‐in pad designs were compared (via‐hole diameter): ultra small via‐in pads (10 μm), small via‐in pads (20 μm) and large via‐in pads (60 μm), as well as designs with no via‐in pads and capped via‐in pads. Two process variables were also evaluated for the goal of achieving a high‐yield assembly solution in micro via‐in pad and lead‐free solder conditions. Potential factors such as the preheat conditions of the reflow profile and stencil aperture size, which might affect tombstoning in components with micro via‐in pads, were investigated.

The results indicated that the micro via‐in pad design significantly increased the tombstoning; thus, tombstoning did not occur in components with both no via‐in pads and capped via‐in pads. Capped via‐in pads exhibited the best results in preventing tombstoning and provided a wide process window for the selection of process parameters. The results showed that tombstoning was found to decrease with both increasing stencil opening ratio and use of reflow profile with long‐preheat condition.

The paper's findings provide certain process guidelines for high density module assemblies with via‐in pad design. The strategy is to prevent tombstoning by adopting capped via‐in pad design if possible when employing micro via‐in pad technology.

Tombstoning and voiding have been plaguing the surface mount assembly industry for decades. The recent global move toward lead‐free soldering and the extensive adoption of microvia technology further aggravate the problems. The present study investigates the impact of SnAgCu (SAC) alloy composition on these important issues.

In this study, tombstoning and voiding at microvias are studied for a series of SAC lead‐free solders, with an attempt to identify a possible “composition window” for controlling these problems. Properties which may be related to these problems, such as alloy surface tension, alloy melting pattern, and solder wetting behaviour, were investigated in order to assess the critical characteristics required to control these problems.

The results indicate that the tombstoning of SAC alloys is greatly influenced by the solder composition. Both the wetting force and the wetting time at a temperature well above the melting point have no correlation with the tombstoning frequencies. Because the tombstoning is caused by imbalanced wetting forces, the results suggest that the tombstoning may be controlled by the wetting at the onset of the paste melting stage. A maximum tombstoning incidence was observed for the 95.5Sn3.5Ag1Cu alloy. The tombstoning rate decreased with increasing deviation in Ag content from this composition. A differential scanning calorimetry (DSC) study indicated that this was mainly due to the increasing presence of the pasty phase in the solders, which result in a slower wetting speed at the onset of solder paste melting stage. Surface tension plays a minor role, with lower surface tension correlating with a higher tombstoning rate. The voiding rate at the microvias was studied by employing simulated microvias. The voiding level was lowest for the 95.5Sn3.8Ag0.7Cu and 95.5Sn3.5Ag1Cu alloys, and increases with a further decrease in the Ag content. The results indicate that voiding at microvias is governed by the via filling and the exclusion of fluxes. The voiding rate decreased with decreasing surface tension and increasing wetting force, which in turn is dictated by the solder wetting or spreading. Both low surface tension and high solder wetting prevents the flux from being entrapped within a microvia. A fast wetting speed may also facilitate reducing voiding. However, this factor is considered not as important as the final solder coverage area.

In general, compositions which deviate from the ternary eutectic SAC in Ag content, particularly with a Ag content lower than 3.5Ag, exhibit a greater solid fraction at the onset of melting, resulting in a lower tombstoning rate, presumably due to a slower wetting speed. The SAC compositions with an Ag content lower than 3.5 per cent, such as 2.5Ag, resulted in a lower tombstoning rate with minimal risk of forming Ag₃Sn intermetallic platelets. On the other hand, ternary eutectic SAC exhibits a lower surface tension resulting in an easier solder spread or solder wetting, and consequently exhibit a higher tombstoning frequency and a lower incidence of voiding.

Provides a solution to the tombstoning problem in lead‐free soldering.

The present study provided a solution to the tombstoning problem encountered in lead free soldering by controlling the SAC solder alloy compositions.

This paper aims to study the influence of reflow atmosphere and placement accuracy on the solderability of 01005 capacitor/SAC305 solder joints.

The 01005 capacitors were mounted on OSP-coated pads, and the samples were fabricated in four different atmospheres, i.e. 200 ppm O₂/N₂, 1,000 ppm O₂/N₂, 3,000 ppm O₂/N₂ and air. After the reflow process, visual inspection and X-ray detection were carried out to examine the solder joint shapes and possible defects. Some of the samples fabricated in different conditions were cross-sectioned and the solder joint microstructures were analyzed. On the other hand, besides placing the components on their normal pad positions, a 50 per cent offset of the x-axis (long axis) or y-axis (short axis) was introduced into the chip mounter programs to evaluate the 01005 capacitor’s assembly sensitivity to placement accuracy. The process-induced defects were investigated.

Experimental results indicated that an N₂-based protective atmosphere was necessary for 01005 type assembly, as it could obviously improve the 01005 solder joint quality, compared with the air condition. The protective atmosphere had little effect on the appearance, quality and microstructure of solder joints when the oxygen concentration was below 3,000 ppm. But a very low oxygen concentration could increase the risk of tombstoning defects for the assembly process. The N₂-based protective atmosphere containing 1,000-2000 ppm O₂ was acceptable and appropriate for the assembly of tiny components.

The results of this work provide a set of reflow process parameters and recommendations for 01005 size component assembly in manufacturing.

A reflow profile is proposed which is engineered to optimize soldering performance based on defect mechanism analysis. In general, a slow ramp‐up rate is desired in order to minimize hot slump, bridging, tombstoning, skewing, wicking, opens, solder beading, solder balling, and components cracking. A minimized soaking zone reduces voiding, poor wetting, solder balling, and opens. Use of a low peak temperature lessens charring, delamination, intermetallics, leaching, dewetting, and voiding. A rapid cooling rate helps to reduce grain size as well as intermetallic growth, charring, leaching and dewetting. However, a slow cooling rate reduces solder or pad detachment. The optimized profile favors that the temperature ramps up slowly until reaching about 180°C. Implementation of the optimized profile requires the support of a heating‐efficient reflow technology with a controllable heating rate. Emergence of the forced air convection reflow provides a controllable heating rate. In addition, it is not sensitive to variation in parts’ features, thus allows the realization of the optimized profile.

The potentially highly automated process of surface mounting electronic components directly onto a substrate or printed circuit board possesses a very weak link. Component movement subsequent to placement and before or during solder reflow leads to defect conditions such as tombstoning or rotational misalignment. This work investigates the feasibility of replacing this ‘weak’ assembly step(s) with ultrasonics. The selection and modification of suitable ultrasonic equipment is described as in the bonding of chip components onto PCBs. Reliability analysis of the resultant bonds along with bond quality in terms of shear strength and appearance under scanning electron microscope and optical microscope is studied. The results show that, with certain preferred directions of ultrasonic weld, weld preload and weld time bond strengths obtained compare very favourably with those achieved with the present surface mount technology reflow process, hence establishing the feasibility of ultrasonics for this application.