Surface mount technology (SMT) has become the dominant assembly method for printed circuit board (PCB) production from high volume consumer goods to low volume industrial equipment. Compared to older through-hole assembly, SMT provides many practical advantages in manufacturability, miniaturization, quality, and reliability.
This article examines the key benefits of SMT manufacturing and why it has largely displaced through-hole assembly. We&#;ll look at how SMT enables smaller, higher performance, and more reliable PCB designs compared to through-hole. Cost and manufacturability advantages are also discussed.
SMT Manufacturing Overview
SMT components have small metal termination pads or leads on the underside. The components are placed onto conductive pads on the surface of a PCB, then soldered in place by reflowing solder paste.
- Solder paste printing - Solder paste is printed on pads where components will be placed.
- Component placement - Robotic pick and place machines position components precisely onto the pads.
- Reflow soldering - The board passes through an oven profile to melt the solder and attach components.
- Cleaning/inspection - Residual solder flux is cleaned off and solder joints are inspected.
SMT eliminates the need for drilling and through-hole plating, allows components on both sides, and enables miniaturization not possible with through-hole parts. Let&#;s examine the advantages this provides:
Space Savings with SMT Components
A key advantage of SMT is the ability to assemble smaller components and achieve much higher component density on a PCB. Some examples of miniaturization:
- passives - 0.4 x 0.2 mm, less than 1/4 size of .
- Fine pitch ICs - Components with lead pitches below 0.4 mm.
- 0.3 mm via holes - Microvias versus ~1 mm with through-hole.
This size reduction is driven by a few key SMT advantages:
- Elimination of through-hole diameters
- Small termination pads versus through-hole leads
- Assembly on both sides
- Precise component placement
The drive towards miniaturization continues, with sizes down to passives now common. SMT enables this dramatic space savings versus through-hole designs.
Weight Savings
Related to the size reduction, SMT assembly also provides a weight savings benefit. A few examples:
- A 20 pin SOIC in SMT format weighs about 36% less than the equivalent through-hole DIP package.
- A miniature SMT relay weighs around 60% less than an equivalent through-hole relay.
- A PCB assembled entirely with components can weigh 70% less than the same board with size components.
For portable and weight-sensitive products, the reduced mass of SMT assembly is highly desirable. The weight savings also has cascading benefits in the final packaged product, allowing lighter housings, smaller batteries, etc.
SMT Allows High Component Density
The small size of SMT components allows fitting a tremendous number of parts into a compact PCB footprint. For illustration:
- A PCB measuring just 50 x 100 mm assembled with components and 0.5 mm fine pitch ICs can hold well over 500 separate components.
- Complex processors and FPGAs in BGA packages have over 1,000 terminations in a 35 x 35 mm footprint.
- Stacking components on both sides doubles available space.
This high density interconnect capability enables advanced miniature systems. Through-hole assembly severely limits component density.
Improved Electrical Performance
Several factors contribute to better electrical performance with SMT designs:
- Shorter traces - Higher component density allows shorter signal paths. This improves speed and reduces noise.
- Finer lines and spaces - SMT assembly can reliably handle traces and spaces down to 125 μm (0.005"), versus 200 μm (0.008") typical for through-hole.
- Fewer drilled holes - Elimination of through-hole vias removes a source of impedance discontinuities.
- Double-sided routing - Allows trace routing on inner layers for impedance control. Not feasible with through-hole.
- Lower inductance - SMT termination pads have lower inductance than through-hole leads.
Together these attributes provide better high frequency response, signal integrity, reduced noise and crosstalk, and superior electrical performance overall.
Improved Manufacturability and Yield
SMT offers significant advantages in manufacturability and manufacturing yield:
Faster assembly - Highly automated SMT lines achieve much faster assembly rates than manual or semi-automated through-hole assembly.
Fewer solder defects - Solder pots used in through-hole wave soldering create bridging risks. SMT reflow avoids this.
Easier rework - Replacing defective SMT components is simpler than desoldering and reworking through-hole parts.
Lower skill requirements - Automated SMT lines reduce reliance on specialized manual assembly skills.
No hole drilling - Eliminates all through-hole drilling, a common manufacturing defect source.
Fewer process steps - Through-hole requires drilling, masking, hole plating, wave soldering, cleaning, etc. SMT consolidates multiple process steps.
Overall SMT manufacturing offers reduced processing steps, faster assembly, lower defect rates, and easier rework - resulting in higher manufacturing yields and throughput. This reduces production costs.
Improved Mechanical Robustness
SMT assembly provides better resistance to shock, vibration, and flexing loads versus through-hole designs:
- No leads or clinching - Avoidance of through-hole component leads improves reliability under vibration and flexing. SMT joints are inherently more robust.
- Lower mass - The lighter weight of SMT components reduces inertial loads under shock/vibration.
- Smaller boards - Miniaturization enabled by SMT allows more compact board dimensions. This improves structural stiffness.
- Double-sided joints - Solder joints on both sides of the board better secure components against dynamic loads.
- Lower I/O forces - Lighter, smaller SMT components exert less leverage force on solder joints when plugging and unplugging connectors.
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For products seeing significant shock, vibration, and flexing during use, SMT assembly offers clear reliability benefits over through-hole construction.
Thermal Management Advantages
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SMT assembly improves thermal design and reliability in several ways:
- Ground pad contacts - Many SMT packages have a grounded bottom pad to aid heat dissipation into the PCB. Through-hole parts lack this.
- Better airflow - The flat profile of SMT boards enables straight airflow channels versus obstructed air channels around tall through-hole components.
- Double-sided cooling - Allows SMT components on both sides for maximum heat spreading into the PCB.
- Smaller PCBs - Reduced footprint of SMT boards simplifies heat sinking, shields, and other cooling measures.
- Better simulation - It is easier to create accurate thermal models of SMT circuits to optimize heat flow vs through-hole designs.
In summary, SMT manufacturing provides options to enhance thermal performance and reliability that are difficult or impossible to achieve with through-hole assembly.
Environmental Resistance Capabilities
SMT assembly offers good environmental and operating flexibility:
- Improved sealing - Flat SMT boards allow better gasketing, sealing, and potting versus complex through-hole component profiles.
- Conformal coating - Entire SMT assemblies can be conformally coated for protection. Not practical on dense through-hole designs.
- Operation in any orientation - SMT boards can be used in any position without gravity stressing component leads. Through-hole has orientation limitations.
- Excellent shock/vibration - As previously covered, SMT assemblies outperform through-hole in dynamic environments.
- Moisture resistance - The elimination of through holes removes moisture ingress points present with through-hole PCBs.
Together these attributes let SMT assemblies operate reliably in demanding mechanical, thermal, and moisture conditions that exceed through-hole capabilities.
Design Flexibility Benefits
SMT fabrication allows design and layout options not practical with through-hole:
- Component placement - Components can be placed freely on both sides for optimal placement. Through-hole has strict component side restrictions.
- Routing - Unrestricted routing on inner layers for impedance control and high density routing. Not possible on through-hole multilayers.
- Dense stacking - Double-sided SMT allows vertically integrated assemblies and 3D flex stacking. Through-hole is essentially 2D.
- Edge contacts - Direct SMT attachment of edge card connectors for plug-in modules. Through-hole requires a separate edge card bracket.
- Mixed through-hole - Some through-hole components can still be used, allowing design flexibility. But not vice-versa.
The lack of through-hole drilling constraints allows SMT complete freedom in electronics packaging innovations not possible with older through-hole technology.
Summary of Key SMT Manufacturing Benefits
- Extreme miniaturization capability
- Lightweight and compact assemblies
- High component density
- Improved electrical performance
- Faster manufacturing throughput
- Lower production costs
- Enhanced mechanical robustness
- Thermal design flexibility
- Environmental resistance
- Highly scalable and adaptable
SMT Disadvantages and Limitations
SMT does have some disadvantages versus through-hole assembly:
Lower repairability - Replacing defective SMT components is generally more difficult than swapping out through-hole parts. And rework causes more heat stress.
Manual prototype assembly - Through-hole allows easy hand assembly and tweaking of prototypes. SMT requires solder paste and oven reflow equipment.
Higher startup costs - SMT assembly lines require stencils, paste printers, reflow ovens, and other equipment. Through-hole needs little investment.
Skills gap - SMT process control requires staff training on equipment maintenance, soldering, inspection, etc. Through-hole assembly uses common skills.
Not suitable for some components - Larger power devices, connectors, transformers, and electromechanical parts may still need through-hole formats.
So through-hole still maintains a niche role for very low volume, manually assembled products focused on serviceability rather than miniaturization. But the overwhelming advantages of SMT make it the technology of choice for nearly all contemporary PCB manufacturing.
Conclusion &#; SMT As the Modern Assembly Standard
In summary, SMT manufacturing provides transformative advantages in space savings, performance, manufacturability, robustness, design flexibility and cost. This has established surface mount technology as the standard electronics assembly method for modern PCB production.
SMT enables levels of system miniaturization, operating reliability, design flexibility and functional density that are impractical or impossible with older through-hole assembly techniques.
The rapidly expanding capabilities of SMT, from microcomponents to high-density package interconnects, will continue to enable innovations in manufacturing electronic products across all industries and applications.
Frequently Asked Questions
Q: Is it possible to convert a through-hole design to SMT?
A: Yes, many through-hole designs can be adapted to SMT. This requires substituting SMT component packages, adding pads and rerouting. Not all designs can be easily converted however.
Q: Can through-hole components still be used on an SMT board?
A: Yes, it is common to have some connectors, transformers, and other parts in through-hole packages on an SMT board. The through-hole components are wave soldered.
Q: What are the most common SMT component packages?
A: SOIC ICs, quad flat packs, resistor/capacitor arrays, BGAs, QFNs and // chip components encompass most SMT formats. Standardized packages ensure component availability.
Q: How small can manufacturers assemble SMT components?
A: Assembling passives and 0.3 mm pitch ICs is common nowadays. Some advanced manufacturers can place and even smaller components.
Q: Does solder paste shelf life affect SMT assembly quality?
A: Yes, solder paste should be kept refrigerated and has a shorter shelf life than through-hole solder. Managing paste condition is vital for maximizing SMT yields.
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