A well rounded technique

Solder pastes are vital in the electronics industry, but to be useful these metal suspensions must contain smooth spherical particles. Here we learn how image analysis can deliver the ideal solder

Used extensively throughout the electronics industry for the construction of printed circuit boards and assemblies, solder pastes are suspensions of metal alloy particles in a flux-containing medium. They serve primarily as attachment media joining the interconnection features of a device to the PCBs themselves. During automated production, solder paste is applied through a screen or stencil, or using a syringe. The PCB is then heated to melt the metal particles, which fuse to form a smooth metal bond.

A number of properties determine whether or not a solder paste produces a successful bond and therefore a reliable product. The composition, size and shape of the metal alloy or solder particles, for example, all influence how the solder behaves when it is being applied. A new automated, high sensitivity characterisation technique is now being used to deliver both size and shape information on the particles involved, and this type of characterisation is helping to improve manufacturing processes and more closely define materials specifications.

Table 1: Summary of size results for the 3 samples of solder

The particle size of a solder paste for a given application is limited by the minimum size of the aperture openings of the stencil used in printing the paste to the board or substrate. Excessively large particles can easily clog the stencil apertures, resulting in poor printing quality and requiring frequent cleaning, which slows production. Particle size becomes even more critical as the amount of solder to be deposited becomes smaller.

The industry standard ANSI/J-STD-005 describes the size classifications applied to solder and the proportions of larger particles that are acceptable in each case. Fine particles tend to flow easily through a screen or syringe. These encourage the use of finer screens, which improve definition - the accuracy with which the solder is applied. However, fine particles also present a relatively high surface area to volume ratio for oxidation, and oxides inhibit soldering. Both fine particles and oxides may also encourage ‘solder balling’, the formation of discrete solder spheres rather than a smooth fillet. A fine balance must therefore be achieved.

In terms of shape, smooth solder spheres are generally preferable. Having the smallest surface area to volume ratio of any shape, spheres will have the lowest oxide content for a given thickness of oxide layer. Non-spherical particles also have the drawback of tending to block or jam the screen, stencil or dispensing needle, through which the paste is being applied. Aggregates, because of their size and shape, pose particular problems. Optimising and controlling both particle size and shape is extremely important during solder paste manufacture.

Figure 1: Overlay of the size distributions for the three samples of Solder

To compare the size and shape distribution of different products, three samples of solder were measured using a new automated particle characterisation system. The Morphologi G3 (Malvern Instruments) provides microscope quality images and delivers statistically significant data through the analysis of hundreds of thousands of particles.

Describing a 3-dimensional particle can be much more complex than it first appears. While it can be convenient to use a single number, if the particle is not a perfect sphere there are many different ways in which its size could be described. Image analysis captures a 2-dimensional image of a 3-D particle, from which it calculates various size and shape parameters. Principal among these is circle equivalent (CE) diameter, which can be used to calculate particle size. The captured image is converted to a circle of equivalent area and while differently shaped particles will influence the CE figure, it has the virtue of being a single number that becomes larger or smaller as the particle does so. Importantly, it is both objective and repeatable.

Particle shape is an even more complex challenge and many parameters may be used to build a complete picture. Calculating multiple shape parameters for every particle, and generating distributions for each, allows the identification and quantification of even subtle differences.

Figure 2: Classification result comparing the proportion of spherical particles to misshapen/fused particles within each of the solder samples

In this study, more than 20,000 images were captured for each of the three solder samples in around 30 minutes. To optimise data quality, particles with an area of less than 100 pixels were excluded from the results and the classification function of the system was used to define and exclude touching particles.

Table 1 summarises the size data recorded for the three samples, showing that particle size and distribution for each is markedly different. Figure 1 shows the overlay of the size distributions. Sample 1 is trimodal, with a large mode at 58.9 microns, a second at 73.9 microns and a third at 84.1 microns. Samples 2 and 3 on the other hand are both monomodal cantered at 36.9 and 57.2 microns respectively. Sample 2 contains noticeably smaller particles than the other two.

Since spherical particles are preferred for solder applications, circularity was also investigated. Circularity is defined as the ratio of the circumference of a circle of equivalent area, to actual particle perimeter. A perfect sphere has a circularity of 1, while convoluted, irregular shapes have circularities closer to zero. The three samples showed quite different distributions for circularity, with sample 3 containing the highest percentage of particles with low circularity.

The proportions of misshapen and spherical particles in each sample were compared (see figure 2) by classifying on the basis of shape parameters.

Figure 3: Example images of misshapen/fused solder particles

This confirms that solder 1 has the highest proportion of spherical particles while sample 3 contains more that are misshapen and fused. This classification was also used, in combination with a CE diameter classification, to investigate the properties of each of the three size populations in solder 1. The results revealed that both of the smaller size brackets in the first sample contain highly spherical particles. By contrast, the larger size bracket has a bimodal circularity distribution, containing not only the most spherical of particles but also dumbbell shaped particles which appear to be two spheres fused together.

The performance of solder pastes can be related directly to a number of properties of the materials themselves, including particle size and particle shape. Conventionally, solder particles have been classified for size by sieving through meshes. Now a new tool for particle characterization using image analysis allows improved size and shape monitoring for quality control purposes with both parameters being determined in a single measurement. Using filter and classification functions, the proportion of misshapen or oversized particles can be identified and individual images for every particle allow further investigation if required.

By Deborah Huck. Deborah is product technical specialist, morphological imaging at Malvern Instruments