Slump and Tackiness Testing Using Rheometry

Estimated reading time: 9 minutes

Increased use of solder paste calls for serious consideration of its importance within the context of quality management systems such as TQM, ISO 9001 and QS 9001. Understanding solder paste quality issues becomes a requirement, particularly when process improvements are required through SPC or other tools.

By Ineke van Tiggelen-Aarden

According to studies, 64 percent of all defects in the SMT assembly process originate in the printing process.1 Printing machines currently are advanced, and offer a high degree of process control and sophistication, such as computer control of all vital functions, closed print heads and even climate control.

The printing properties of a solder paste are defined by its rheological properties. Rheology is a relatively young sub-science of physics. It studies the deformation and flow behavior of substances. The characterization of the rheological properties of a solder paste generally has been reduced to the measurement of its viscosity.2 The latter is just one parameter of rheology. In fact, the simplification of a paste's rheological properties down to its viscosity is due to a lack of understanding. When the application of solder paste was originally pioneered, the industry soon realized that the measurement of viscosity did not provide the needed overall picture in terms of material rheology. Consequently, in analogy with similar inspection methods used in other industries, additional tests (such as slump and tackiness) were developed.

Viscosity — Rheometry

When solder paste was introduced through the application of hybrid technology several decades ago, the industry saw the need for characterization of its flow properties during printing. In comparison to the paint industry, the measurement of viscosity using an instrument with a rotating spindle was adopted. However, viscosity-wise, there is a world of difference between solder paste and paint. Solder paste is a more solid material with a higher percentage of particles, and these particles are many times the size of the particles of pigment found in paint (Figure 1).

Figure 1. A spindle device has an extremely small surface. This is one of the reasons why the reproducibility of measuring data is limited.3 The spiral pump instrument yields results with a moderate reproducibility when compared to a plate/plate rheometer. The latter is more accurate and offers possibilities for advanced rheometry, such as oscillation and temperature superposition.

More advanced rheometers have become available during the past 15 years. Thanks to more advanced and sensitive electronics, these newer units are able to interpret the torque on the measuring devices with a resolution of 0.01 µm and an angular resolution of <1 µrad. Additionally, they control the temperature of a sample more directly through close contact with the heating in the base plate. Temperature control is achieved via a Peltier element, providing a control within 0.1°C accuracy. The element also is capable of programmable temperature changes within seconds. The temperature can be set at a constant value, or it can vary in intervals or in a continuous ramp as much as 0.8°C/second. These systems also can measure in a rotational mode as well as in oscillating mode. The advanced software offers capabilities of an analysis procedure, including steps such as smoothing of the raw data, interpolation of data points in a defined range and calculation of mean values. Thus, these systems provide a more reliable output.

With this type of equipment, it is relatively easy to produce a reliable range of viscosity values at different shear rates, yielding a more reliable Thixotropic Index or enabling the performance of a relaxation test that closely simulates a breakdown period of a line, a print stroke at elevated speed and a pause.

Slump

Slumping is the permanent deformation by gravity, vibration or heat of a paste deposit after it has been printed on the solder pad. By definition, slumping is a rheological phenomenon. Although gravity and vibration affect slumping, temperature has the largest impact, and therefore is the most important factor. Heat makes the flux material in the solder paste much more fluid. Slumping is controlled by the elastic properties of a solder paste, and is a major contributor to defects such as bridging and solder beading.

From the rheological perspective, slumping can be defined by the elastic properties of a solder paste. In fact, the amount of strain the material can take before the rheological structure breaks is the measure of slumping.

Slumping occurs when the viscous properties become dominant over the elastic properties5 and the matter begins to flow. Flow is initiated when the quotient — also known as the damping factor — between the elastic properties and the viscous properties becomes larger than 1.

The slump potential can be quantified with the following equation:

Where: Sr = Rheometric Slump N = Number of interpolated data points taken at tan δ 0.95 up to and including 1.00 γ = Strain K = 1,000 tan δ = Damping Factor tan δ = G"/G' G' = Storage modulus (elastic properties) G" = Loss modulus (viscous properties).

A software-controlled analysis procedure includes steps such as smoothing of the raw data, interpolation of data points at a damping factor (tan δ) of 0.95 up to and including 1.00 and the calculation of the mean value of this range. The number found by the rheometer should be multiplied by a constant (1,000) to arrive at a score that is easier to handle.

Slumping is a rheological parameter; and although stencil design plays a key role in preventing beading, slumping may impact beading significantly. Only an enhanced slump test, for example, with a quantifiable result can illustrate that correlation.

Tackiness

Tackiness could be described with the concept of wetting and cohesive forces of the material concerned.

In the rheological perspective, it could be defined by the complex viscosity6 of a substance just before the structure breaks. The concept of slumping within the context of rheology was explained earlier. In fact, the same procedure can be used to characterize the degree of tackiness. The tackiness of a solder paste can be defined as the complex viscosity just before its rheological structure breaks.

Within this context the following equation can be used to calculate tackiness:

Where:Tr = Rheometric Tackiness N = Number of interpolated data points taken at tan δ 0.95 up to and including 1.00 η* = Complex Viscosity η* = Τ* / γ* Τ* = Complex Shear Stress γ* = Complex Shear Rate tan δ = Damping Factor tan δ = G"/G' G' = Storage modulus (elastic properties) G" = Loss modulus (viscous properties).

The actual test is performed with a plate/plate rheometer that is capable of controlling the temperature of the solder paste with an accuracy of ± 0.1°C. The test is run at a constant temperature at 25°C. The procedure falls into the category of amplitude sweep testing, which is done in an oscillation mode with a constant frequency of 1 Hz. The gap between the two plates is 2.0 mm. The amplitude is programmed in a continuous ramp from 0.1 down to 0.0001 percent strain. In Figure 2, the measuring data of two different types of solder paste are shown. The different types also show a varying tackiness force when measured with the traditional instruments as well as that experienced in the field.

Figure 2. A diagram of the rheometric determination of slump, which can be combined with the test for tackiness. It also shows a significant correlation to the slump index test.4 The markers on the green and red line show the damping factor at the critical values for each paste. The green marker on the X-axis shows the point of slumping of Paste "A," and the red marker shows the same point for Paste "B." The red marker on the blueline shows the complex viscosity of the paste with the higher tackiness, whereas the green marker marks the lower tackiness with its corresponding complex viscosity on the black line.

The data show a significant correlation between the traditional methods7 and the rheometric technique. Additionally, the latter features a smaller variation of the results, because it can provide more accurate readings. This accuracy is enhanced due to more accurate temperature control and the fact that the measuring plates have a much larger surface-to-volume ratio. Consequently, the impact of print quality of the paste deposit and the rectangularity of the measuring probe have been eliminated (Figure 3).

Figure 3. The principle of traditional tackiness measurement in gram-force [gf] with a probe attached to a load-cell.

For the end user, tackiness testing with traditional equipment or with a rheometer seems too remote and expensive. Usually, the user relies on data as certified by the vendor of the solder paste, who may or may not be testing religiously. For the end user, when paste does meet other testing criteria, tackiness testing in a separate operation is superfluous and too expensive (Figure 4).

Figure 4. There can be many reasons why the tackiness measuring probe surface may not have good contact with the paste deposit.

The best option is to rely on the certificate of compliance of the paste vendor provided that the vendor uses a testing method that can qualify as fundamentally significant and as long as the end user can audit SPC data.

Conclusion

Paste vendors should standardize on both slump and tackiness testing with rheometric procedures, providing more accurate and reproducible results. For the user, it is important to demand a certificate of compliance from the paste vendor regarding the testing of viscosity and other rheological parameters such as slump and tack, plus the standard tests such as solder balling, wetting, etc. There is significant correlation between the results of the slump index and the rheometric slump test.4 A set of inspection methods that could enhance the international standards8 is proposed in the table.

Table. Inspection methods, according to the current IPC standard. (Right) A set of inspection methods that could enhance the current material inspection for both the supplier as well as the end user of solder paste.

In modern business practice, transparency is a key word. Therefore, paste vendors should allow customers to audit their SPC data under certain conditions. This would help eliminate much of the myth and mystery associated with solder paste.

References

1. Charles-Henri Mangin, "Where Quality is Lost on SMT Boards," Circuits Assembly, February 1991.
2. Viscosity is the resistance of a substance to flow once forces are applied
3. "Correlation of Malcom Spiral Viscometer vs. Brookfield T-bar Spindle Viscometer," Kantesh Dos, Austin American Technology
4. Ineke van Tiggelen-Aarden, "A Simpler Approach to Cost-Effective Solder Paste Testing," APEX 2003 Conference Proceedings, March 2003.
5. The viscous properties of a substance are best described as: the mass does neither to any degree nor at any time restore its original shape once the shear forces are stopped. The elastic properties will cause a partial or complete restoration, with a small or large delay, of the original shape once the shear forces are stopped. An example is the rod climbing of dough or many gels on a stirrer shaft (also known as "Weissenberg effect").
6. The viscosity, shear rate and shear stress parameters that have been determined by sinusoidal shear loads through oscillatory testing are noted with an asterisk to indicate the complex form, and thus distinguish them from parameters that have been established by rotational testing.
7. Malcom TK1.
8. IPC ANSI-J-STD-005, IPC TM 650a and the Japanese Industrial Standard JIS Z Z3284.

Ineke van Tiggelen-Aarden, technical director, may be contacted at Cobar Europe BV, Aluminiumstraat 2, 4823 AL Breda, Holland; + 31 76 544 55 66; Fax + 31 76 544 55 77; E-mail: i.van.tiggelen@cobar.com; Web site: http://www.cobar.com