I spent this whole day researching on the different types of solder paste and tried to find an appropriate solder paste that we can use for our experiment.

Basically, solder paste is used to connect the leads of surface mount integrated chip packages to attachment points on a printed circuit board. The composition of solder paste varies, depending on the intended uses. Some properties of solder paste are:

• Viscosity: The degree to which the material resists the tendency to flow
• Slump: The characteristic of a material’s tendency to spread after application
• Working life: The amount of time solder paste can stay on a stencil without affecting its printing properties

http://en.wikipedia.org/wiki/Solder_paste#Properties_of_solder_paste

As I did some more research on the internet, I figured out that even though there are hundred of different types of solder alloys, only a few are commonly used by the industry and they are listed in the following link:

http://www.kester.com/kester-content/uploads/2013/06/Alloy-Temperature-Chart-15Feb11.pdf

Among those solder alloys, only 3 are available in solder paste form: Sn63Pb37, Sn96.5Ag3.0Cu0.5 and Sn62Pb36Ag02. As we use the air to flow the solder, it’s hard to get silver to high temperature quickly enough to get a good joint without damaging the chip. Therefore, the Sn96.5Ag3.0Cu0.5 and Sn62Pb36Ag02 are not good options as they both have some silver as their components. Thus, the Sn63Pb37 is our best option as it’s a composite of Tin and Lead, which makes it easy to get to high temperature. Furthermore, with melting point at 361 Fahrenheit (183 Celcius), Sn63Pb37 can be melted using our toaster oven as it provides maximum heat up to 450 Fahrenheit. Also, the solder paste that we found on the fridge is also of type Sn63Pb37, confirming that we have been using this solder paste before on student projects.

For the question yesterday, we want the oven to take a short time to get to high temperature because if it takes too long then there is a high chance that the solder paste will damage the chip and the circuit components. Therefore, the heat transferring efficiency of our oven is the most important aspect that we want to achieve in our project.

I also reworked on the experiment with the toaster oven yesterday, but this time with the thermometer put on the metal base. We want to see if there is any difference in the heat transferring rate with and without the metal base because realistically, when we work on the circuit board, we will need to put it on a firm base as well. I only made experiment with the toast mode as we have studied yesterday that it is in fact the optimal mode. And the resulted graph is shown below:

 There are in fact some significant differences between the two methods. Without the metal base, the thermometer can only reach up to a maximum temperature of around 470 Fahrenheit but when put on the metal base, it reaches to the peak temperature of 526 Fahrenheit, which is a lot higher than the maximum of only 450 Fahrenheit written on the oven. This incident happens because when we use the metal base there is more surface contact between the thermometer and the base, making the heat easier to be transferred. Nonetheless, the graphs show that in both ways it takes the same amount of time to get to the maximum temperature, showing that except for the peak value, the rate of heat transferring is basically the same.