When selecting a soldering iron tip, you need to be sure that it is the proper size/diameter for the application. By selecting the proper soldering iron tip, the following benefits are attainable:
- Heat is efficiently transferred by the tip which is important for good wetting fo the solder
- Becuase of the efficient heat transfer and good wetting, lower temperatures can be used
- Lower temperatures mean lower oxidation rates and longer tip life
- Longer tip life results in lower operating costs
To select the right size/diameter tip, you must consider the size/area of the land/pad the tip will contact. If you maximize the contact area between the tip and the land/pad, heat will transfer more efficiently than if you has a smaller contact area. An easy way to picture this is like a freeway with cars. The cars represent the heat energy moving from one point to another. If the contact area is large like in a highway with many lanes, the flow of cars (or heat energy) is high from one point to another. If the highway is narrow with only one or two lanes, the flow of cars (or heat energy) is reduced.
Below is an example demonstrating how the size of the tip affects the transfer of heat into a solder joint. For this example, we will be making 5 solder joints on an FR-4 board. Each joint will be heated until it reaches 250°C. We will be using lead-free solder (SAC 305) and the soldering tip temperature will be set at 360°C. The test will be repeated using D12, D16, and D24 shapes.
As you can see from the results above, the wider tip (D24 shape) transfers the heat more efficiently, which results in shorter dwell times, increasing the overall speed of the process. So from this information alone, a really big tip is better, right? Not really.
Let's look at another example demonstrating how the size of the tip affects the transfer of heat into a solder joint. But this time we will be making 5 solder joints on an FR-4 board but we will set the dwell time on each joint at 3 seconds. We will use the same lead-free solder (SAC 305) and the soldering tip temperature (360°C) as we did in the previous example. The test will be repeated using D12, D16, and D24 shapes.
As you can see from the results above, the D24 shape tip did not transfer the heat as efficiently as the D16 shape tip. You can see this in the temperature drop of the tip. Heat is not leaving the tip and going to the solder joint. This inefficiency can cause longer dwell times on the solder joint where the hot tip is in contact with the PCB and increases the possibility of damaging the PCB and/or component.
So, considering these two examples and the affects they have on the process time and the temperatures of the solder joint, PCB, and component, it is best to choose a tip that provides maximum contact area with the land/pad as shown in the diagram below.
But what if it is not possible to use the right size tip because there is no room? In that case, you will need to use a tip that will fit within the limited space you have to work in, but it is best to be sure you use a tip that just fits. Going too small will not heat efficiently. It is also best to consider the use of preheating of the PCB. This helps by 'preloading heat energy' into the PCB and components so that the limited heat transfer from an undersized tip can reach the necessary temperatures for quality soldering in a shorter time than if the same process was done without preheating. It has also been shown that soldernig with nitrogen does contribute to preheating of the solder joint and will increase the wettability of the solder.
It is also important to select a tip that has the highest heat energy capacity as possible. Why? Look at the next example...
Which Tip would you choose?
Going back to a car analogy, consider the soldering iron tip the car and the heat energy is the fuel you put in the tank. The larger the tank (or more massive the tip) the more heat energy is stored. When the tip makes contact and the solder joint is made, heat energy transfers out of the tip. This heat energy is replenished by the heating system in the soldernig iron. This is a basic fact regardless of what heating system is used to heat the tip.
If the tip has enough mass, there is typically enough heat energy remaining in the mass so that when you move to the next solder joing
Although both of the tips above are sized properly for the land/pad they will be contacting, the heat energy capacity of the two tips are different.
Let's look at an example of how these two tips perform...
In this example, we will be soldering 5 terminals on an Bakelite board. Each temrinal has an 8.5mm O.D. and a 4mm I.D. and will be heated to 250°C. We will use the same lead-free solder (SAC 305) and the soldering tip temperature (360°C) as we did in the previous examples.
As shown in the example above, in higher heat energy capacity of the DL12 shape tip means that there is less loss of heat energy from the tip as it moves from joint to joint. The lower loss of heat allows for a faster process time.
So, here we have seen that by selecting the proper tip for your soldernig application, you can improve process times allowing for more work to be done and the increased efficiency of the heat transfer allows for lower temperatures to be used, which reduces the risk of damaging the component or PCB, and also lowers the rate of oxidation of the tip as well as improves tip life.
NOTE: Test data above provided uses T12 Series Composite Tips. T12 Series and T15 Series equivalent in performance.
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