PCB Assembly Services - Screaming Circuits: PCB Choices: THROUGH-HOLE OR SURFACE MOUNT?

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PCB Choices: THROUGH-HOLE OR SURFACE MOUNT?

Should folks building small numbers of circuit boards use surface-mount parts, or should they stick with through-hole components that are so much easier to handle and solder (not to mention see)?

I spend my days and nights surrounded by printed circuit boards (PCBs). I work in a quick-turn assembly house by day, and by night I live in what sometimes looks like an assembly house. As I write these words, within eyesight, I have close to a dozen PC boards of my own design, maybe more. That being the case, I'm often asked by people with a wide variety of levels of experience whether they should still use through-hole components, or if they should use surface-mount technology (SMT) wherever possible.

Deep in the mists of time, when I first put hot iron to solder, the primordial soup had only recently been discontinued at the local diner. In those days, we did have a version of "surface mount" assembly, but it just involved the use of Fahnestock clips, solder lugs, and point-to-point wiring on the surface of a pine board.

Today, surface-mount assembly involves copper pads on the surface of the PCB, parts that don't have leads going through the PC board, and robots that can place tens of thousands of these tiny parts per hour.

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Now, 48,000 placements per hour are great if you need to sell a million phones a month, but what about those of us who might need 1,000 boards for an early-stage startup, 100 units for a hobby business, or just one or two boards for a one-off design? Should folks like that still look at surface-mount parts, or should we stick with tried-and-true through-hole components that are so much easier to handle and solder (not to mention see)?

I personally advocate more use of surface-mount components. I do realize that doing so complicates things and that some parts are only available in through-hole packages. Sometimes the mechanical strength of a surface mount is not sufficient for the weight of the component or the stress that will be put on it. In those cases, I would definitely use a through-hole or through-hole/surface-mount hybrid.

A list of occasions when you might wish to use through-hole components is as follows:

  • Very large parts like transformers, relays, and capacitors that either doesn't have a surface-mount equivalent, or that require the extra mechanical strength provided by through-hole connectors.
  • Connectors that will be under a lot of mechanical stress.
  • If you aren't comfortable seeing and handling small surface-mount parts.
  • You just prefer through-hole-hole.
  • You're only building a few boards and/or you're using components that are already in your possession.

Beyond the above points, there are a lot of good reasons to design your PC board around surface-mount components, despite their small size. First, a lot of the newer and most advanced components simply aren't available in through-hole packages. If you want to stick with through-hole only, you're eliminating many of the newest and most versatile parts.

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The through-hole board on the right in the image above is one of the first PCBs I had made at a commercial fab shop. The surface-mount board on the left has the same functionality, plus a motor controller and USB connectivity. I hand-soldered the parts on both boards. The motor driver chip I used isn't available in a through-hole package.

In past years, most chips would be available in a variety of sizes, from 0.1" pitch through-hole DIP (dual in-line package) down through whatever was the smallest thru-hole package of the time. With so much competition today, coupled with a wide variety of specialized components, the cost of supporting so many variants is becoming a significant factor. Many chip companies are adjusting to these market conditions by consolidating on the most usable, smaller, packages.

Microchip Technology still does a really nice job of providing the same parts in 0.1" pitch DIPs down through super-small 0.5mm pitch QFN or BGA packages, but who knows how long this will stay? I've seen a lot of new power components -- like LiPoly chargers and Class-B amps -- that are only available in QFN or BGA packages. Some high-speed components are going that way too.

Tiny IoT (Internet of Things) devices and the proliferation of mobile computing is a big driver in this trend. A 3 x 3mm part can be used both in a tiny IoT device and in a big desktop system. By comparison, a 20-pin through-hole part -- or even a not much smaller SOIC -- can only be used in the desktop unit, but not in the IoT device, so manufacturers may decide to only make the smaller part. You may be able to find through-hole breakout boards for some of these components, but not all.

Surface-mount components are almost always less expensive than their through-hole counterparts. It stands to reason that a component that uses significantly less raw materials would cost less, and this rule holds true for many electronics components, especially passives. At Digi-Key, through-hole and surface-mount resistors both tend to start at $0.10 each, but by the time you buy 100, the surface-mount components are half the price.

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The size issue also helps with component storage. In the image above, I've got a few less than 500 surface-mount parts in the wound-up strip. Compare this to the space consumed by the handful of through-hole capacitors on the right.

Board real estate is an important consideration as well. You can either stuff more surface-mount functionality into the same PC board area, or you can reduce the board size and save on fab costs.

If you're ever planning on building in volume, starting with surface-mount is the way to go. This way, you won't need to redesign your board between your prototypes and production. This can be a big time and money saver.

The big question, however, is cost. Doesn't it cost a lot more to use surface-mount because you need to contract out the assembly work? Well, anytime someone is doing work for you, it's going to cost money, but the equation is a little more complex than that.

Most people, with some patience, a good magnifier (or high-power reading glasses), and something to rest a wrist on, can hand-solder down to 0603 or 0402 packaged passive parts. Some people have even figured out how to hand-solder QFNs and BGAs. Below a certain point, however, hand work just isn't practical without specialized equipment (it can be done, though).

If you are working on a hobby project, you'll need to stick to things you can hand-solder yourself, get a toaster oven set up, or find a shop that caters to low-cost rather than high reliability. When what you are doing starts to become a business, then time equals money, at which point you can trade the two factors off depending on your needs.

The bottom line is that, if you prefer through-hole parts, then by all means use them. There's no shame in it. Just understand that some of the newest and most capable parts may not be available to you. I personally recommend using surface-mount parts whenever possible.

 

Duane Benson,

Chief Technology Officer & Director of Marketing, Milwaukee Electronics

 

 

Comments

Of Course there are more good reasons to design printed circuit boards around surface-mount components.

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