Last year, my family had a Fourth of July Celebration at my brother’s place. He has a sizable backyard, so we cooked out and even watched a movie outside on a projection screen. Everyone had a wonderful time, but by far, I enjoyed throwing horseshoes the most. Horseshoes is one of those games that is cheap, low-tech, and a bit physical. All you need is two stakes, four shoes, and the ability to toss 2-3 pounds a few dozen times (depending on you and your opponent’s skill levels). And when you get a ringer, Eureka! With today’s high-tech games, such as Wii, it is easy to forget that you can have just as much fun playing one of the oldies, but goodies, like horseshoes.
With electronic components getting smaller and smaller and circuit boards getting denser and more compact, PCB design is becoming increasingly dominated by multilayer fabrication principles for assembling surface mount devices (SMDs). While this is not a bad development, overreliance on surface mount technology can lead to the underutilization of through-hole technology, even when it may be the best design and assembly option. Knowing when the through hole technology process should be used alone or in conjunction with surface mount technology begins with clearly understanding the strengths and weaknesses of both processes.
Through Hole Technology Process versus Surface Mount Technology Process
Through hole technology (THT) assembly has been around for the better part of a century (since the 1940s) and has been demonstrated to be highly reliable, able to secure large components, and maintain connectivity in the face of high power and high temperatures. Surface mount technology (SMT) developed to accommodate smaller and lighter components, which it does well. SMT also provides for more complex trace routing than THT and is more cost-effective.
Regardless of what assembly method is used, there are standards for solder joint quality that must be met, such as J-STD-001, J-STD-002, and J-STD-003. The basic process steps for assembly are essentially the same whether you employ THT or SMT. These basic steps for printed circuit board assembly (PCBA) are:
- Preparing the bare board from the fabrication process.
- Component placement.
- Soldering of components.
- Inspection and correction.
- Cleaning the board.
Where THT and SMT assembly differ is in how the components are placed and soldered, as the lists below indicate.
Through Hole Technology Process Steps
A. Component placement
For THT, the component leads or pins are inserted through the board.
B. Inspection and correction
Any errors in placement will be corrected here.
C. Wave soldering
For wave soldering, one side of the entire board is exposed to a “wave” of solder. As the board traverses the wave, the through-hole components are soldered simultaneously.
Surface Mount Technology Process Steps
A. Solder paste application
For SMT, an initial layer of solder paste is applied to hold the components in place for soldering.
B. Component mounting
Components are placed on footprints that consist of conductor pads that will be soldered to provide the electrical connections.
C. Reflow soldering
Reflow soldering is performed in an oven where temperatures can reach 235°C.
The table below provides a head-to-head comparison that can be used to determine which PCBA process may be best for your design situation.
Comparison of Through Hole Technology and Surface Mount Technology Processes
Advantages of Through Hole Technology
Advantages of Surface Mount Technology
Stronger component bond
Faster and more cost-effective
Better for larger components
Allows for smaller, denser boards
Better for components subject to mechanical stress
Allows for more complex routing
Better for high power and high temperature components
Allows for double-sided component mounting
Whether or not THT is better than SMT depends upon your design and other specific factors. However, in many cases, you can make use of the advantages of each on the same board.
Through Hole Technology and Surface Mount Technology Process
The design process when SMT is used is more complex than for THT. In particular, there are more trace routing options that require additional choices to be made. However, for small, complex boards SMT is probably the better option. The THT process takes longer and is more expensive. Yet, for large components, such as transformers when designing power supplies or high power industrial PCBs that may be subject to high temperatures and vibrations, THT is usually the better choice. In most cases, the choice is not as clearly determinable and your design may call for the use of both PCBA methods. In these cases, you can make your selection based upon the following general guidelines:
THT and SMT
Small components, sparse spacing, low power
Small components, sparse spacing, high power
Small components, dense spacing, low power
Small components, dense spacing, high power
Large components, sparse spacing, low power
Large components, sparse spacing, high power
Large components, dense spacing, low power
Large components, dense spacing, high power
The above design attributes should always be considered; however, others such as the severity of vibrations that your board will be subjected to may need to be included, as well.
The best PCBA choice for your design depends upon a number of factors. For some cases, it may seem obvious that either the through hole technology process or surface mount technology process is the better option; however, you should always make this decision in consultation with your contract manufacturer (CM).
|Tempo‘s Custom PCB Manufacturing Service|
Tempo Automation, the industry leader in fast, high-quality PCB prototyping and low-volume production is well-equipped to assemble your boards using the best process. And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.
If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on the through hole technology process or designing to best utilize it, contact us.