Opportunity Of Conducting Soldering Biology Essay


This experiment provided an opportunity of conducting soldering, and of learning analysis technique using spectroscopy and microscopy. The samples used in analysis are Sn- Bi alloys with different compositions. Cooling curves were depicted by cooling down these alloys from 300 ℃, and were used to draw phase equilibrium diagrams. SEM and EDS were applied to generate surface topographical images and chemical compositions of the specimens, making comparisons with the calculated results from the phase equilibrium diagram. Results showed that electronic analytical devices may differ from conceptual derivation in quantitative determination of compositions.

1.0 Introduction

The phase equilibrium lab introduces basic techniques of soldering and analyzing microstructures of alloys with scanning electron microscope (SEM) and Energy Dispersive X-ray Spectroscopy (EDS). The soldering part of the lab helps students develop a hand-on experience of joining two copper pipes with Sn-Bi alloys. In the meanwhile, a 90Sn- 10Bi alloy is cooled down from 300 ℃, and its temperature changes with time are recorded with a thermocouple. Afterwards cooling curves are drawn to develop a phase diagram of the alloy.

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The second part of the lab involves the use of SEM and EDS to observe microstructures of Sn- Bi alloys with different compositions. The SEM is responsible for producing images of the magnified structure of four samples, namely, eutectic, hypoeutectic, hypereutectic and solder alloys. The specimens are placed in the SEM chamber which can be seen from a monitor; observers are able to switch among the four specimens. A beam of electrons are bombarded toward (scan) the specimens and reflected beams are collected, which are displayed at the same scanning rate on a cathode ray tube. [1] EDS is used to determine the chemical composition of the solder and each phase by analyzing the emitted x-rays after hit by charged particles. [2]

2.0 Materials

2.1 Soldering and Cooling


Propane torch

Copper pipes

Solders (5:95 Bi:Sn)


K type (Nickel-Chromium Vs. Nickel-Aluminum)

Data Logger Thermometer:

Make: Omega

Model #: HH306A

Temperature Range: -200C - 1370C

Hot Plate:

- Make: Corning

- Model #: PC-400

2.2 SEM


- Make: Hitachi

- Model #: S570 & S4500

3.0 Procedures

3.1 Soldering

Firstly, clean copper pipes until there is no debris and obvious oxides on their surface of both ends. Flux was then brushed to wet the joining interface, inside and outside, to allow formation of permanent bond after solidification of the molten solder. [3] To avoid flaming, only small amount of flux is used. Afterwards, operator used the outer flame of the propane torch to heat the overlapping section of the two pipes, and the solder wire in the opposite direction to touch the surface of the joint until the solder was melted followed by melting of flux. Molten solder joined with the two pipes by capillary force; and cooling in water furthered the solidification. Pressure test was applied to examine the sealing of the pipe connections. Tap water ran into the pipes; there was no leakage implying that the soldering was done perfectly.

3.1 Cooling of Bi-Sn Alloy

The Bi-Sn alloys were cooled down from about 300 degrees to room temperature. The solder alloy is placed inside a cinder block and the decrease of temperature was detected by a thermocouple and recorded by a data logger thermometer. Cooling curves of the Bi-Sn systems are depicted with temperatures against compositions (Fig. 1).

3.2 Scanning Electron Microscopy

Preparation of samples: 1. Use rigid saws to cut off the sample

2. Heat sample pieces in Bakelite

3. Polish the sample with sandpaper

4. Polish with electro polish [4]

The prepared samples are placed in the SEM chamber for analysis. By zooming in, one can clearly see magnified images of the microstructures of the alloys.

4.0 Results

4.1 Cooling Curves

The curves are developed after cooling the Bi-Sn alloy of different compositions from ~300℃.

Fig. 1 Cooling curves corresponding to different compositions of the Bi-Sn system.

3.2 Bi-Sn Phase Diagram

Fig. 2 Phase diagram of Bi-Sn system with their hypereutectic and hypoeutectic compositons

3.3 Compositions







Overall composition from

EDS [wt% Bi]




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Composition of primary phase from EDS [wt% Bi]





Area fraction of primary

phase, calculated from SEM micrograph [%]





Weight fraction of primary

phase calculated from area

fraction [wt%]





Weight fraction of primary

phase calculated from tie line drawn on phase diagram [wt%]





Composition of Sn-rich phase from EDS [wt% Bi]





Composition of Sn-rich phase as determined from phase diagram [wt% Bi]





Weight fraction of total Sn-

rich phase calculated from tie line drawn on phase diagram [wt%]





Composition of Bi-rich phase from EDS [wt% Bi]





Composition of Bi-rich phase as determined from phase diagram [wt% Bi]





Weight fraction of total Bi-rich phase calculated from tie line drawn on phase diagram [wt%]





Table 1: Composition from EDS analysis and area and phase fractions for Bi-Sn Alloy

Sample calculations:

(The density of Sn used is 6.99g/cm3)

4.0 Discussion

4.1 Analysis of the cooling curves

The cooling curves in Fig. 1 shows that the freezing point of the Bi-Sn alloy decreases with increasing proportion of Bi. For all the cases, the rate of cooling slows down before reaching the freezing point, and under cooling, where liquids exist below the freezing point, occurs just before transforming into solids. There are five curves worth noting. In the case of 95Sn5Bi, the first arrow pointing to the freezing point of the alloy, while the second one indicates the position of maximum solubility of Sn in Bi in the phase diagram, which is slightly below 150 ℃. However, the time for 40Sn60Bi to turn from liquid to solid is relative longer than the others, and the graph is almost horizontal in the freezing process where energy is released; the temperature corresponding to the line is eutectic temperature. This special case is resulted from the composition of 40Sn60Bi alloy approximates to that of eutectic mixture (43Sn57Bi). Curves four and five as labeled in Fig. 1 have similar trends of cooling down, with upper arrows indicating some solid Bi, but Sn stayed molten and lower arrows showing solidification of mixtures. [5]

4.2 How cooling curves are used to construct phase diagrams

Draw a graph with composition being x-axis and temperature y-axis, and the temperature scales lie on both ends of the graph

Decide the coordinate of the eutectic point by alloy composition and eutectic temperature

Determine the freezing temperatures against the proportion of the mixture.

NOTE: For non-eutectic alloys, the freezing temperature corresponds to the "small hill" after the under cooling point.

To orient the maximum solubility point of Sn in Bi, find the cooling curve of 95Sn5Bi in Fig. 1, and the lower arrow indicate the point where solidus line and solvus line intersect. The maximum solubility point of Bi in Sn can be found by the cooling curve of 20Sn80Bi with the same method.

Solidus and solvus lines are determined by the freezing point at extremities and the maximum solubility points while liquidus by eutectic point.

Draw a horizontal line passing through the eutectic point and ends at maximum solubility points.

4.3 Accuracy of EDS

No substantial discrepancies are found in the results of EDS and phase diagram calculations for the composition of Bi-rich phase, most significant percentage error is within 7%. Whereas, there are notable differences for hypoeutectic and eutectic mixtures when comparing compositions of Sn-rich phase using EDS and phase diagram, with a disparity of about 14 wt%Bi for both cases. The discrepancies could be resulted from two parameters. For sample A, thin layers of Bi in the eutectic structure also reflect the electron beams of X-ray and thus cause the sharp increase in the amount of Bi present in primary phase. Also, one can see the black spots, which are impurities, in the SEM image of sample B. Most of the impurities are near the boundaries of primary and eutectic structure, and thereby accounted as primary phase by reflecting X-rays to the detector.

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4.4 Reliability of SEM Image

A 28.29 wt % gap between the weight fraction of primary phase derived from SEM image and tie line indicates that the data manipulated from SEM disagrees with conceptual ones. The major cause is that the inaccuracy of the grid counting technique of determining the weight fraction. The number of intersections of lines that fall in the particles are substantially affected by the number of grids drawn and magnification of the image. The rough approximation of this method results in unreliable data produced.

4.5 Comparison of the stated composition and EDS data

The results from EDS are generally accurate except for sample B, which is the eutectic mixture. EDS showed 9.72 wt % Sn higher than stated. This deviation could be explained by impurities involvement. As noticed before the lab, the alloys may constitute of more than two components and trace of impurities will also be present in the alpha + beta phase, resulting in a misrecognition of Sn, whose peak displayed may overlap with those of impurities. Thus, this biased analysis gives a high proportion of Sn in the mixture.

4.6 Practical use of pipe soldering

The Bi-Sn solder is largely used in pipe soldering and circuit board printing as the melting point of this alloy is relatively low, for instance, SnBi58 melts at 138 °C. Building codes nowadays prohibit using lead solder for water pipes, though traditional tin-lead solder is still available. "Studies have shown that lead-soldered plumbing pipes can result in elevated levels of lead in drinking water. "[6]

5.0 Conclusion

Based on the observation and analysis stated above, a conclusion can be drawn:

Microstructures of Bi -Sn alloys determines the properties (strength) of the solder, which contribute to its common application in pipe soldering. Meanwhile, EDS and SEM are effective in investigating the microstructures of alloys, graphically and quantitatively, if the technique of interpreting the results is reasonable and reliable.