Introduction to PCB Assembly Methods

Printed circuit board (PCB) assembly is the process of soldering or mounting electronic components to a PCB to create a functional circuit. The two main methods for PCB assembly are through hole and surface mount technology (SMT). Each method has its own advantages, disadvantages, and best use cases.

Understanding the key differences between through hole and SMT assembly is important for optimizing the design, manufacturability, cost, and reliability of electronic products. This article will provide an in-depth comparison of through hole vs SMT PCB Assembly across 10 important dimensions.

What is Through Hole PCB Assembly?

Through hole PCB assembly is the traditional method where component leads are inserted into drilled holes in the PCB and soldered to pads on the opposite side. The soldering is typically done by hand or using a wave soldering machine after all the components are inserted. Through hole components include resistors, capacitors, inductors, connectors, and more with wire leads.

Advantages of Through Hole Assembly

• Strong mechanical bonds between component leads and PCB
• Easier manual assembly and repair
• Better for large/heavy components
• Higher power capacity
• Lower cost for low volume production

Disadvantages of Through Hole Assembly

• Consumes more PCB area and limits component density
• Requires drilling holes
• Slower assembly process
• Higher cost and lower throughput for high volume production
• Mostly limited to single-sided component placement

What is SMT PCB Assembly?

Surface mount technology (SMT) PCB assembly is the newer method where components are soldered directly onto pads on the surface of the PCB without through holes. The components are typically smaller, lighter, and can be placed on both sides of the board.

SMT assembly is an automated process using pick-and-place machines and reflow ovens to solder all the components simultaneously. SMT components include chip resistors, capacitors, inductors, small outline ICs, and more.

Advantages of SMT Assembly

• Higher component density and smaller PCB sizes
• Faster and more efficient automated assembly
• Lower cost and higher throughput for high volume production
• Allows double-sided component placement
• Better high frequency performance

Disadvantages of SMT Assembly

• Requires precise component placement
• Not well suited for large/heavy components
• More difficult manual assembly and repair
• Requires specialized equipment and processes

10 Key Differences Between Through Hole and SMT PCB Assembly

Now that we’ve defined through hole and SMT PCB assembly, let’s compare them head-to-head across 10 important factors that impact the design, manufacturing, cost, and reliability of circuit boards and electronic products.

1. Component Sizes and Types

One of the biggest differences between through hole and SMT is the size and types of components used. Through hole components have long wire leads that are inserted into holes, so they are generally larger. SMT components are smaller, lighter, and have flat contacts that are soldered directly to the board surface.

Here are some common types of through hole and SMT components:

The smaller size of SMT components enables much higher component density and more compact circuit designs compared to through hole. However, some larger components like connectors, switches, heavy capacitors, and high power devices are still better suited for through hole mounting.

2. PCB Footprint and Density

Another key difference is the PCB footprint required for through hole vs SMT components. Through hole components consume more board area because they need drilled holes plus annular rings and pads for soldering. The holes also limit routing flexibility and take up space on multiple layers.

SMT components have a much smaller footprint with just flat contacts and pads on the surface. This allows placing more components in a given area, shrinking the overall PCB size. SMT pads are usually smaller than through hole and can be nested between pins.

To quantify the difference, here is a comparison of some common PCB footprints:

As you can see, the SMT footprints are much more compact, often less than half the size of the equivalent through hole component. This directly translates to higher component density and smaller PCB sizes with SMT.

3. Assembly Process and Equipment

The assembly process and equipment required are very different for through hole and SMT. Through hole assembly is a slower, mostly manual process whereas SMT is faster and highly automated.

Here’s a typical through hole assembly process:
1. PCBs are loaded onto a fixture
2. Through hole components are inserted manually or with machines
3. The loaded PCB is passed through a wave soldering machine
4. The leads are trimmed on the bottom side
5. The soldered PCB is cleaned and inspected

In contrast, SMT assembly uses pick-and-place machines to automatically place components and reflow ovens to solder them all at once:

1. A stencil is used to apply solder paste on the PCB pads
2. High-speed pick-and-place machines place all the SMT components
3. The populated PCB is passed through a reflow oven to melt the solder
4. The soldered PCB is cleaned and inspected, often using automated optical inspection (AOI)

The SMT process is much faster, with placement rates up to 50,000-100,000 components per hour compared to a few hundred per hour for through hole. However, it requires more specialized equipment and a precisely controlled process.

4. Manufacturing Cost and Scalability

The manufacturing cost and scalability are important considerations when choosing between through hole and SMT PCB assembly. In general, SMT is more cost effective and scalable for higher volumes, while through hole may be cheaper for smaller volumes.

Here are some of the key cost factors:

The tradeoff is that through hole has higher variable costs that scale with volume, while SMT has higher fixed costs but much lower incremental costs at high volumes. As a result, there is often a breakeven point where SMT becomes more economical.

For example, here is a rough comparison of assembly costs for a simple PCB:

As you can see, through hole is cheaper for prototypes and small production runs up to ~500 units. But as volume increases, SMT becomes much more cost effective due to the economies of scale with automation. Most consumer electronics today are assembled using SMT for this reason.

5. Mechanical Strength and Reliability

Mechanical strength and reliability are other important factors to consider. Through hole mounting provides a stronger mechanical bond because the component leads are physically inserted into holes and soldered. This makes through hole better suited for applications with high vibration, impacts, or heavy components.

SMT components are held in place only by the solder joint between the contacts and pads. While SMT solder joints are generally reliable, they can be more prone to cracking or detachment under mechanical stress. The smaller size of SMT components also makes them more fragile.

However, SMT does have some advantages for reliability:
– No hole drilling reduces the risk of PCB defects and shorts
– Smaller component size and mass reduces the effect of vibrations
– Automated process provides more consistent solder joints than hand soldering

Ultimately, the optimal choice depends on the specific mechanical requirements and operating environment of the product. Through hole is often used for automotive, aerospace, and military applications, while SMT dominates consumer electronics.

6. Electrical Performance

The choice of through hole or SMT also affects the electrical performance of the PCB, particularly at high frequencies. SMT generally provides better high frequency performance due to several factors:

• Smaller component packages have lower parasitic inductance and capacitance
• Shorter lead lengths reduce impedance mismatch and signal reflections
• Smaller loop areas between component and PCB reduce electromagnetic interference (EMI)
• Planar geometry allows controlled impedance routing and ground planes

Through hole components have longer leads that can act as antennas, radiating or picking up interference. The through hole vias also create discontinuities and stubs that can cause signal integrity issues at high frequencies.

Therefore, SMT is preferred for high speed digital circuits, RF/wireless applications, and other designs that are sensitive to parasitics and require tight control of impedances. Through hole is sufficient for lower frequency analog circuits and power electronics.

7. Heat Dissipation

Heat dissipation is another consideration, especially for power electronics and circuits with high power density. Through hole components have an advantage here because the leads provide a direct thermal path to conduct heat into the PCB and to the other side for heatsinking.

SMT components only dissipate heat through their small contacts and surface area. The thermal resistance is higher, so SMT is more prone to overheating in high power applications. Thermal vias and heatsinks can be used to improve heat transfer, but they add cost and complexity.

The power dissipation limits depend on the specific component package and PCB design, but here are some rough guidelines:

As you can see, through hole allows higher power dissipation per component, often 5-10 times more than SMT. High power SMT devices like MOSFETs use special packages with exposed pads for heatsinking. But in general, through hole is preferred for power circuits above a few watts.

8. Rework and Repair

Rework and repair are inevitable in electronics manufacturing, but they are quite different for through hole and SMT assembly. Through hole is generally easier to rework and repair because the components and solder joints are accessible on both sides of the board.

Through hole components can be desoldered and replaced using manual tools like a soldering iron, solder sucker, or desoldering station. The process is slower but can be done with basic equipment and skills.

SMT rework is more challenging because the components and solder joints are smaller and only accessible from one side. Special tools and techniques are needed, such as:

• Hot air rework station to heat and remove SMT components
• Microscope or magnifier for visual inspection
• Precision tweezers and vacuum pickup tools for handling
• Stencils and solder paste dispensers for adding new solder

SMT rework requires more skill, time, and equipment, so it is usually done only by trained technicians. It also carries a higher risk of damaging the PCB or neighboring components due to the high heat and small size.

In general, through hole is better for prototyping, testing, and small scale production where manual assembly and rework are common. SMT is better for high volume production where the goal is to minimize defects and rework in the first place.

9. Design Tools and Guidelines

Designing PCBs for through hole or SMT assembly requires different tools and guidelines. Through hole design is simpler and can be done with basic PCB layout software. The main considerations are:

• Component hole sizes and pad dimensions
• Minimum hole spacing and edge clearance
• Drilling tolerances and aspect ratios
• Soldermask and silkscreen markings

SMT design requires more advanced tools with features like:

• Large component libraries with footprint data
• Automated part placement and optimization
• Design rule checking (DRC) for SMT manufacturability
• 3D visualization and collision detection
• Panelization and fiducial generation

SMT also has many more design guidelines to ensure reliable assembly and yield, such as:

• Minimum pad size and spacing for each component package
• Solder mask and paste mask apertures
• Component orientation and placement for machine vision
• Thermal relief and via design for soldering
• Clearances for pick-and-place nozzles and reflow ovens

Following SMT design rules is critical for avoiding defects like tombstoning, bridging, or insufficient solder joints. Through hole is more forgiving of design variations, but still benefits from following best practices.

10. Environmental Impact and Sustainability

Finally, environmental impact and sustainability are growing concerns in electronics manufacturing. Through hole and SMT have different implications in this regard.

Through hole assembly uses more raw materials and generates more waste due to:
– Larger PCB size with drilled holes
– Larger component packages
– Lead trimming and waste
– Higher energy consumption for wave soldering

SMT assembly is generally more eco-friendly because:
– Smaller PCB size and less material usage
– Smaller component packages and less packaging waste
– No lead trimming waste
– Lower energy consumption for reflow soldering

However, SMT does use some problematic materials like lead-based solder paste and harsh cleaning chemicals. Lead-free solder and no-clean fluxes are becoming more common, but they have their own tradeoffs in terms of process complexity and reliability.

Both through hole and SMT PCBs can be recycled at end of life, but the process is not perfect. The best approach is to design for sustainability from the start, using techniques like:

• Specifying RoHS compliant components and materials
• Minimizing PCB size and layer count
• Choosing recyclable substrate materials like FR-4
• Avoiding underfilled or potted components that are hard to separate
• Providing clear labeling and instructions for recycling

Frequently Asked Questions

1. Can through hole and SMT components be used on the same PCB?

Yes, it is common to use a mix of through hole and SMT components on the same PCB, depending on the specific requirements of each component. Called “mixed assembly”, this approach allows utilizing the benefits of both methods where appropriate.

For example, a power supply PCB might use through hole components for the high power section and SMT components for the low power control circuitry. A mixed assembly PCB typically goes through both reflow and wave soldering steps.

2. Are SMT solder joints as strong as through hole?

SMT solder joints are generally strong enough for most applications, but they are not as mechanically robust as through hole joints. The strength of an SMT joint depends on factors like the pad size, solder material, and reflow profile.

Properly designed and reflowed SMT joints can withstand significant shear and tensile forces, but they are more susceptible to cracking under vibration or bending than through hole. For high reliability applications, additional strain relief methods like underfill or conformal coating may be used.

3. What is the smallest size of SMT component?

SMT components come in a wide range of sizes, from large chip carriers and BGAs down to tiny chip resistors and capacitors. The smallest commonly used SMT sizes are:

• 0201 (0.02 x 0.01 inches): resistors, capacitors
• 01005 (0.016 x 0.008 inches): resistors, capacitors
• SC-70 (0.04 x 0.02 inches): diodes, transistors
• DFN-2 (0.02 x 0.01 inches): diodes, transistors

Even smaller sizes like 008004 and 0.4mm pitch BGA are used in some ultra-compact designs, but they require specialized equipment and processes. In general, 0402 and larger sizes are preferred for most products to balance density and manufacturability.

4. What is the cost difference between through hole and SMT assembly?

The cost difference between through hole and SMT assembly depends on many factors, but in general SMT is cheaper for high volumes and through hole is cheaper for low volumes.

Some key cost drivers are:

• Through hole has lower fixed costs for equipment and setup, but higher variable costs for components, PCBs, and labor
• SMT has higher fixed costs for equipment and setup, but lower variable costs due to automate