SMT Process Introduction

What is SMT?

SMT stands for Surface Mount Technology. It is a method for producing electronic circuits in which the components are mounted directly onto the surface of printed circuit boards (PCBs). An electronic device so made is called a surface-mount device (SMD). In industry, this approach has largely replaced the through-hole technology construction method of fitting components, in large part because SMT allows for increased manufacturing automation which reduces cost and improves quality. It also allows for more components to fit on a given area of substrate. Packaged ICs (Integrated Circuits) are mainly produced with SMT methodology.

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SMT vs Through-Hole Technology

SMT components are usually smaller than through-hole components because they have smaller leads or no leads at all. They are designed to be handled by machines rather than by humans. SMDs can be one-quarter to one-tenth the size and weight, and one-half to one-quarter the cost of through-hole components.

Feature	SMT	Through-Hole
Component Size	Smaller, 1/4 to 1/10 size	Larger
Component Weight	Lighter, 1/4 to 1/10 weight	Heavier
Cost	Lower, 1/2 to 1/4 cost	Higher
Placement	Machine placement	Manual or machine placement
Connection	Surface mounted	Leads through holes
PCB Real Estate	More components fit in same space	Fewer components fit

Through-hole mounting provides stronger mechanical bonds than SMT, so it is better for components that are subject to mechanical stress such as connectors or transformers. SMT is the main method used for high volume production where components are smaller, lighter, and cheaper.

Advantages of SMT

The main advantages of SMT over through-hole technology are:

Smaller components
Both sides of PCB can be used
Higher component density (components per unit area)
Fewer holes need to be drilled
Lower resistance and inductance at the connection
Better mechanical performance under shake and vibration conditions
Faster and more automated assembly
Lower initial cost and time of setting up for production
Simpler and faster design changes
Easier product updating

These advantages make SMT the main method used today for high volume PCB manufacturing and assembly.

SMT Process Overview

The SMT manufacturing process involves several steps, each of which requires specific equipment and materials. The main steps in the SMT process are:

Solder Paste Printing
Pick and Place
Reflow Soldering
Inspection
Rework & Repair

Here is an overview of each step:

1. Solder Paste Printing

The first step in SMT is to apply solder paste to the PCB contact pads with a screen printer. The solder paste consists of tiny solder particles, flux, and binding agents. It is applied to the PCB using a laser-cut stainless steel stencil and a squeegee blade.

The stencil is aligned to the PCB, the solder paste is placed on the stencil, and the squeegee pushes the paste through the holes in the stencil and onto the PCB pads. The amount and position of solder paste must be precisely controlled. Too much paste can cause bridging and too little can lead to open connections.

2. Pick and Place

After the PCB has solder paste applied, the components are placed onto the PCB by pick and place machines. These robotic machines pick up components from feeders or trays and place them on their designated locations with high speed and accuracy.

Modern pick and place machines can have multiple placement heads and place tens of thousands of components per hour. They use computer vision to identify and align components. Some machines have special nozzles for odd-form components or adhesive dispensing heads for components that need to be glued.

3. Reflow Soldering

After component placement, the PCBs go through a reflow soldering oven. This oven has multiple thermal zones that preheat, soak, reflow, and cool the board in a controlled way. The solder paste melts in the reflow zone and creates permanent solder joints between component leads and PCB pads as it cools.

The reflow oven uses a specific temperature profile that is tailored to the solder paste and components used. It is important to control the ramp rate, soak time, peak temperature, and cooling rate to ensure good solder joints without damaging components.

4. Inspection

After reflow, the assembled PCBs need to be inspected for quality. There are several inspection methods used:

Manual Visual Inspection
- Operators visually check for defects
Automated Optical Inspection (AOI)
- Camera-based system checks for correct component placement and solder joints
X-ray Inspection
- Used to see solder joints under BGA and QFN components
In-Circuit Testing (ICT)
- Electrical probe testing of circuit functionality
Functional Testing
- Testing the final product functionality

Inspection is critical for identifying assembly defects early in the process. It is much more cost effective to catch and fix defects at this stage rather than later in the product lifecycle.

5. Rework and Repair

If defects are found during inspection, they may need to be repaired. This can involve replacing wrong components, adding missing components, or redoing poor solder joints.

Rework can be done by hand with soldering irons, but there are also specialized SMT rework stations that use focused infrared or hot air to melt solder locally. BGA (ball grid array) components have special procedures for removal and replacement that can be quite intricate.

Equipment Used in SMT Assembly

SMT assembly requires specialized equipment for each process step. Here is some of the key equipment used:

Screen Printer

Used to apply solder paste to PCB
Has a frame to hold stencil and PCB
Uses a squeegee to force paste through stencil openings
May have vision system for alignment
Precision of 12.5 μm

Pick and Place Machine

Robotic machine for placing components on PCB
Uses vacuum nozzles to pick up components
Has feeders for component reels or trays
High speed, e.g. 50,000 CPH (components per hour)
Placement accuracy of 25 μm
Vision system for component alignment

Reflow Oven

Oven with controlled heating zones
Heats PCB and components to melt solder
Ramp rate, soak time, peak temp, cooling rate are key parameters
Uses convection, IR, or vapor phase heating
Nitrogen environment for better soldering
Has cooling zone to solidify solder joints

AOI Machine

Automated optical inspection machine
Uses cameras and computer vision to detect assembly defects
Inspects component presence, position, polarity, solder joints
Faster than human visual inspection
Can be 2D or 3D imaging

X-ray Inspection

Uses X-rays to image inside objects
Good for BGA, PoP, QFN solder joints
Can detect solder bridging, voids, shorts, opens
More expensive than AOI

Rework Station

Used to repair assembly defects
Has tools for component removal and replacement
Hot air or IR heating for localized solder melting
Microscope and fine tools for small components
Solder paste dispenser and cleaning tools

Materials Used in SMT Assembly

In addition to equipment, there are several key materials used in SMT assembly:

Solder Paste

Mixture of solder powder, flux, and binder
Solder alloy is usually tin-silver-copper (SAC)
Flux removes oxides and aids soldering
Binder gives paste its viscosity and tackiness
Solder particle size and shape affects printing and melting properties
Paste is thixotropic – viscosity decreases under shear

Stencil

Metal foil with openings that match PCB pads
Used to deposit solder paste in precise locations
Usually made of laser-cut stainless steel
Thickness, aperture size and shape are key design parameters
Smooth walls and flat surface are important
Coated to improve paste release
Periodic cleaning needed

Squeegee

Flexible blade used to push solder paste through stencil
Usually made of polyurethane
Hardness, shape, and edge quality affect print quality
Metal squeegees sometimes used
Angle and pressure affect solder deposition

Flux

Chemical that improves solderability
Cleans oxides from metal surfaces
Prevents reoxidation during soldering
Promotes solder wetting and flow
Can be water-based, solvent-based, or no-clean
Flux residue may need to be cleaned

Cleaning Chemicals

Used to remove flux residue after soldering
Water-based for water-soluble fluxes
Solvents for rosin-based fluxes
Need to be compatible with components and PCB
Removed by spraying, brushing, or immersion

SMT Design Considerations

To ensure a successful SMT assembly process, the PCB and component design must follow certain guidelines. Here are some key design considerations:

PCB Footprint

Copper pads must match component package
Pad size and spacing affect soldering
Thermal pads for heat dissipation
Soldermask and silkscreen markings
Fiducials for vision alignment

Solder Mask

Polymer coating over bare copper
Prevents solder bridging
Protects from oxidation and contamination
Improves electrical insulation
Provides durability and appearance

Panelization

PCBs grouped into arrays for assembly
Rails, tiebars, and V-grooves for separation
Edge rails for handling
Fiducials and tooling holes
Panel size fits assembly machines

Component Placement

Sufficient spacing for pick and place
Consideration for component height
Orientation for polarity and readability
Tall components placed inward
Thermal considerations

Design for Manufacturability

Avoid small passives (0201, 01005)
Minimize rework requirements
Limit component variety
Use standard packaging
Consider testability

By following good DFM (design for manufacturability) practices, the time and cost of PCB assembly can be reduced.

SMT Quality Standards

There are several industry standards that provide guidance on producing high-quality SMT assemblies. Two of the main standards are:

IPC-A-610

Acceptability of Electronic Assemblies
Covers end product inspection of PCBAs
Workmanship defect criteria
Visual quality standards
Inspection methods and tools

J-STD-001

Requirements for Soldered Electrical and Electronic Assemblies
Covers materials, methods and verification for producing soldered PCBAs
Solder joint acceptability criteria
Cleaning, coating, and handling processes
Supported by IPC-A-610

By understanding and applying these standards, manufacturers can improve their SMT assembly quality and reliability.

Frequently Asked Questions (FAQ)

1. What are the most common defects in SMT assembly?

Some of the most common SMT defects are:
– Tombstoning – one end of the component lifts up
– Bridging – solder connects adjacent leads
– Insufficient solder – poor connection strength
– Solder balls – small spheres of solder stuck to PCB
– Skewed/misaligned – component is rotated or offset from pads

2. What causes tombstoning?

Tombstoning can be caused by:
– Uneven solder paste deposits on pads
– Uneven heating of component during reflow
– Component geometry or mass imbalance
– Vibration during reflow

3. How can solder bridging be prevented?

Some ways to prevent solder bridging are:
– Ensuring proper stencil aperture size and shape
– Controlling solder paste viscosity and rheology
– Optimizing reflow profile (time and temperature)
– Using a nitrogen reflow environment
– Sufficient pad spacing in PCB design

4. What is the difference between no-clean and water-soluble flux?

No-clean flux is designed to be left on the PCB after soldering. It is not corrosive and does not require removal. Water-soluble flux is designed to be removed with a water cleaning process after soldering. It has higher activity but can cause corrosion if not cleaned.

5. How often should stencils be cleaned?

Stencils should be cleaned periodically to prevent solder paste build-up and printing defects. The cleaning frequency depends on factors like paste type, print volume, and stencil design. Generally, stencils should be cleaned every 1-4 hours of printing, or when print quality degrades. Understencil cleaning systems can extend time between cleanings.

Conclusion

SMT is the dominant method of PCB assembly today due to its advantages in miniaturization, automation, and cost. By understanding the SMT process, equipment, materials, and design guidelines, a high-quality and efficient electronics manufacturing process can be achieved. Continuous improvement and adherence to industry standards will ensure successful SMT assembly for a wide range of products.