Surface Mount Technology Advantages and Disadvantages for PCB Assembly

Did you know that choosing which tires to use on a Formula 1 race car has a critical effect on the car’s performance? The tires normally used don’t have any treads on them. These give the car the best grip on the pavement, but they will hydroplane in the rain. The rain tires, on the other hand, have treads to conduct the water away from the tire to give it a better grip, but the heat and friction of dry pavement will rip these tires apart. Making the right tire decision for the track conditions can be a difficult job.

When preparing to design a printed circuit board, you may need to decide between surface mount and thru-hole packages for some components. Surface Mount Technology components, or SMTs, have a lot of advantages over traditional thru-hole components. There are, however, some disadvantages that you should be aware of too. Let’s take a closer look at some of these surface mount technology advantages and disadvantages.

Surface Mount Technology Advantages that You Should be Aware of

SMT parts have been around now for a long time now, and have become the standard when it comes to components used on printed circuit boards over plated thru-hole parts (PTH). Here are some of the reasons for the popularity of the SMT package style:

• Automated assembly: Most SMT parts can be easily installed on circuit boards using automated pick & place equipment. High volume parts, such as passive components, are loaded into the pick and place machine from reels while other parts are loaded from tube feeders or trays. This is a very important difference over PTH parts which often have to be manually assembled.
• Commonality of parts over PTH: SMT parts are usually less expensive than their PTH counterparts due to their size and volume. SMT parts are also more available than PTH parts because of how much they are in demand.
• Size: SMT parts are typically smaller than their PTH counterparts since they don’t have long leads that need to be inserted and soldered into holes. This allows the PCB designer to fit more components in less space on the board then can be done with PTH parts.
• Electrical performance: With more SMT parts being able to fit on the board than PTH parts due to their size, you also end up with shorter signal paths on the board. This will give your PCB better signal integrity, and less generated heat than with PTH parts.

You can see why surface mount parts are used so much, but there are some circumstances where thru-hole parts have the advantage.

High volume components are loaded into pick & place machines from reels like this

Some of the Disadvantages of Surface Mount Technology

Even though most printed circuit boards will benefit from being designed with SMT parts, there are some reasons why PTH parts may be more desirable:

• Easy to work with: Although rework technicians are very skilled with tweezers and magnifiers in order to rework small SMT parts, the larger PTH parts are easier to work with. Some SMT parts are so small that they present quite a challenge to find, de-solder, replace, and then re-solder. There is also the concern that there could be collateral damage by handheld soldering irons to nearby parts with the STM parts being so small and close together.
• Electrical performance: Although the SMT parts will give better signal performance due to their small size, the larger PTH parts are still needed for high-power applications. There often isn’t an SMT equivalent part, and in cases like this, you will need to use a PTH package instead.
• Mechanical strength: Connectors for single-use, such as I/O devices on a motherboard, are often an SMT part. A more robust connector, however, should be used in applications where they will constantly be connected and disconnected. Here is where a PTH part can be the best choice as its soldered thru-hole leads will give much better mechanical strength over a surface mount soldered part. The same goes for switches or any interface device that will be getting regular use.

Now that you know some of the advantages and disadvantages, let’s take a quick look at how best to decide.

How to Decide which Technology is Best for Your Design

The best time to decide between an SMT or PTH part is when you are choosing the components that you will use in your schematic. By incorporating these parts early in the design, you will make sure that the PCB Layout is created with the correct component package footprints. Here are some questions to consider when making your decision:

• Use: Will this component get “touched” much or be subjected to other stress?
• Power: Will this component be conducting a lot of power and heat?
• Performance: Which package style will give your circuit the best performance?
• Price: Is there a price benefit by choosing one package style over the other?
• Availability: How quickly will the part be available for prototypes?
• Rework: Are you expecting a lot of rework of the circuit, especially during prototyping?
• Assembly: Which package style will give you the highest quality assembly for the lowest cost?

For the most part, a surface mount part will be the best way to go, but don’t be afraid to choose a thru-hole part if it is really called for. Here is where your PCB contract manufacturer can really help with your decision. They will have the experience of working with all kinds of PCB component package styles in order to give you the best recommendations.

At VSE, we have been doing this now for over 30 years. We have a team of component engineers that keep their pulse on the component supply chain so that they know which parts are available now and in the future, and for what price. Meanwhile, our manufacturing engineers will be able to advise you on which parts will work best for the manufacturing process that will be used for your board.