Modelling the self-alignment of passive chip components during reflow soldering

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Abstract

In my research a 3D model was created to investigate the restoring force arising and the self-alignment occurring during reflow soldering; and simulations were performed to examine the assumptions given by the model. Besides, experiments were carried out to verify both the assumptions and the simulation predictions. Passive components with the size of 0603 (1.5 × 0.75 mm) were placed with intended misplacements and their position was measured before and after soldering. Three cases were examined: how misplacements perpendicular to the longer sides of components affects the restoring force, how parallel misplacements affect the same, and how a sidewall metallization on the component influences that. Based on the results, it is shown that the degree of restoring force is higher in the case of misplacements perpendicular to the longer side of components (x-direction) than in the case of misplacements parallel to that (y-direction). However, in the case of y-direction misplacements, the restoring force increases when sidewall metallization on the components is present.

Highlights

  • Self-alignment of surface mounted passive components was investigated.

  • Solder profile of surface mounted components was simulated with Surface Evolver.

  • Solder profile simulations were verified with cross-sections of solder joints.

  • The degree of self-alignment parallel to longer side of the components is lower.

  • Self-alignment parallel to longer side can be improved with sidewall metallization.

Introduction

Reflow soldering is generally used for fastening components mechanically and connecting them electrically to electronic circuit assemblies [1], [2], [3]. Concerning today trends, the passive discrete components (resistors and capacitors) are getting smaller and smaller, as it is demanded by the continuous development of surface mount technology. Consequently, automated placement machines are facing real challenges since the reduction of components sizes leads to a lower relative positioning accuracy at the same placing speed. Nevertheless, it is an empirical fact that the inaccuracy of placement can be reduced to a certain extent due to the self-alignment of the components. However, the self-alignment models for passive chip components suffer from serious weaknesses, e.g. they are 2 dimensional. 3D models are available only for complex circuit packages such as BGA or CSP packages [4].

At the beginning of surface mount technology, the examination of the self-aligning movement of the components during reflow soldering was limited to passive discrete components of larger sizes, e.g. components with size code 1206 (3 × 1.5 mm). That time, the applied models were two-dimensional and mainly focused on the tombstone effect (when one of the component’s terminations lifts from the pad). The first force model has been described by Wassink and Verguld [5]. It is a simple two dimensional force model, which aim was to predict the moments acting on the component during soldering in order to prevent the tombstone effect. The model assumes that there is no solder on the left face of the component and it considers the solder fillet as a straight line instead of a curve. In addition, the model, due to its simple manner, does not take the hydrostatic pressure of the liquid solder into consideration.

A more complex model has been described by Ellis and Masada [6], which takes the hydrostatic and capillary pressure of the molten solder into account, and considers the solder fillet as a curve. However, it was a two-dimensional model like the Wassink–Verguld model. The model comprises further simplifications; it assumes that the component is brick-shaped (i.e. rectangle in 2D) and its mass centre is in the geometrical centre of the body. In addition, the model presumes that the corner of the component is always in contact with the soldering surface (pad), and the component rotates around that point. Although the model includes many specific details – the meniscus of the solder is not considered to be a straight line, the force due to hydrostatic pressure is taken into consideration, and the chip component is allowed to be displaced along its pad length to illustrate the effect of component misplacements –, it is still a two dimensional model, so three dimensional motion of the components cannot be described.

Newer models describe mainly the motion of high lead count integrated circuits packages, such as QFPs (Quad Flat Pack) and BGAs (Ball Grid Array) [7], [8], [9]. Movements of flip-chips were also investigated [10], [11], [12], [13] where the diameter of the solder bumps is smaller (50–100 μm) compared to BGA packages (400–800 μm) [14], [15]. According to these models, the same forces support the movement of the components during soldering, as in the case of the passive chip components: namely, the surface tension force of the molten solder and the force of the hydrostatic pressure [16], [17].

Although the high lead count IC packages have a great interest today, the size decrease of passive discrete components (e.g. 01005 – 400 × 200 μm) induces increasing positional offset due to the inaccuracy of placement machines. Therefore, a detailed analysis of the self-aligning movement during soldering of small passive components based on a 3D model is absolutely necessary.

Section snippets

Theoretical background of profile calculation

As can be seen from the aforementioned models, predicting a component movement during soldering is based on the solder profile calculation. Thus, investigations were performed defining the shapes for various boundary conditions [18], [7], [19]. After the profile calculation, the forces acting on the component can be determined. Two main methods spread to determine the solder profile; one is based on the principle of pressure continuity, while the other one is based on minimizing the energy

3D self-aligning force model of passive chip components

Based on my model, mainly five forces are acting on the chip components during reflow soldering (Fig. 3). The force originating from surface tension (Fst) is acting on the boundary contact line of the three phases which are the solder, gas, and component metallization. The forces originating from hydrostatic (Fh) and from capillary (Fc) pressure are acting on the area of the component metallization; while the force originating from dynamic friction (Fν) depends on the mass of the liquid solder

Experimental

In this research, simulation and experimental measurements were carried out regarding the self-alignment of passive chip components. Self-alignment of two component types were compared; one type was a 0603 (1.5 mm × 0.75 mm) chip resistor which does not have sidewall metallization, while the other one was a 0603 chip capacitor which has sidewall metallization. A testboard (Fig. 10) was designed and fiducial points were placed around all components for later positional measurements. The substrate of

Results

The results of the calculated restoring force are illustrated in Fig. 15, Fig. 16. It can be said that the restoring force in the case of x-direction misplacement is about two to three times higher than the y-direction restoring force, which is in accordance with the prediction of my model discussed in Section 3.2. The restoring force for the highest degree of misplacement is 450 μN and 150 μN for the x and y-direction offsets respectively.

Concerning the y-direction restoring forces for both

Conclusion

In my paper the self-alignment of surface mounted passive discrete components was investigated. The degree of restoring force was predicted by a 3D model and calculated with Surface Evolver. Besides, experiments were performed to make deduction for restoring forces which arise during reflow soldering. Based on the results, it can be said that the degree of restoring force is higher in the case of misplacements perpendicular to the longer side of components (x-direction – 450 μN) than in the case

References (28)

  • Ruijun Chen, Brett Fennell, Daniel F. Baldwin. Flip chip self-alignment mechanism and modeling. In: International...
  • D. Josell et al.

    Misaligned flip-chip solder joints: prediction and experimental determination of force-displacement curves

    Trans ASME – J Electron Packag

    (2002)
  • Qinong Zhu et al.

    Optimization of design and manufacturing parameters for solder joint geometry and self-alignment in flip-chip technology

    IEEE Int Conf Solid-state Integrated Circuit Technol

    (1998)
  • C. Gary Tsai et al.

    Self-alignment of microchips using surface tension and solid edge

    Sens Actuators A

    (2007)
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