When designing and manufacturing your PCB, one of the early decisions involves the type and style of components to be used. Traditionally, the option available was to insert components into plated through-holes on the PCB. Increasingly, the trend has been to use a method that does not involve drilling holes.
Let us look at the broad differences between the two technologies, namely Through-Hole and Surface Mount. This can act as a handy guide for PCB design to make the right choice of technology depending upon their bespoke requirements.
With older technology, there are enough and more reasons why it is still in vogue. One of the use cases of through-hole technology includes the prototype stage of the board. It is here that the board requires quick assembly to act as a proof-of-concept. Once the prototype is approved, it works better to revise the PCB layout and include SMT components. This approach saves costs as you do not have to deal with subcontracting small lots that command a price premium.
Through-hole technology is also used extensively by hobbyists. With generous spacing, it makes hand soldering far easier.
Cost savings offered by through-hole technology
There are several cost advantages that through-hole PCBs have to offer:
- First, the need to create a new solder stencil for every revision is taken away. You also tend to save money that you would otherwise need to spend on pick-and-place equipment that is required for SMT technology.
- Often you can conduct in-circuit testing manually for small batches, which keeps your cost low.
- It allows you to use low-cist tin-lead solder when working with through-hole boards.
- Cost is also kept low on account of easy reworks. For example, if there are any mechanical issues, you can correct them without too many assembly issues as faced in SMT assembly. A through-hole prototype can also reveal any design-related mechanical issues before they become too costly.
With fewer issues, it is also possible to maintain timelines that are critical for quick time-to-market. It is little surprise then that through-hole is preferred in the early development and validation stages, irrespective of whether you are a hobbyist or an established player.
Surface Mount Technology
SMT offers several advantages that through-hole cannot match. Most importantly, it is suited for today’s environment where miniaturization is a growing trend. This technology works well where component density is high, and efficient use of the available space needs to be made. Using SMT components makes it possible to drill small vias in the pads, allowing for interconnections, saving precious real estate. The space around the SMT components can then be used for placing additional components. In fact, with SMT, both sides of the board can be utilized for component mounting.
Surface mount design considerations
When going with SMT PCB Assembly, the following considerations, however, need to be kept in mind:
- Planar finishes such as ENIG, Immersion Silver, or Immersion Tin work well when dealing with fine pitch SMDs. Leaded solder used in HASL isn’t a great choice as it tends to puddle at one end.
- SMT boards need high soldering temperatures compared to through-hole boards, and this is mainly on account of the lead-free surface finishes that are used. Therefore, you need a material that stands up to high soldering temperature and multiple cycling shocks.
- In Gerber artwork, extra features such as a set of fiducial pads added to external layers are important so that the pick-and-place equipment has a reference for squaring the PCB. It is also prudent to eliminate solder mask openings for vias to minimize the potential for shorts.
- As a best practice, test point pads should be added for in-circuit testing.
- It is vital to use stack-ups which are symmetrical around the center of the stack.
Here is a quick comparison of through-hole versus surface mount technology characteristics that can act as a ready reckoner.
To sum up
The above parameters will come in handy when deciding through-hole versus SMT for your new PCB project. While SMT has gained increased popularity because of its apparent advantages, the fact remains that through-hole technology still finds relevance in certain use cases.
About the author
Suresh Patel has worked as a Sales Engineer and in other management roles at Mer-Mar Electronics. He brings 25 years of experience in printed-circuit-board sales and technical client service and managing business.
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Filed Under: Tutorials