Introduction
- Definition of surface mount technology (SMT) soldering
- Advantages over through-hole technology
- Crucial process in PCB assembly
- Overview of steps from paste deposition to reflow
- Details on each stage of the process
Defining SMT Soldering
- Method of attaching SMT components to PCB pads
- Solder paste deposited on pads, then component placed
- Adhesive holds component in place temporarily
- Final solder joint formed by reflowing the paste
- Used for components like:
- Resistors, capacitors
- SOICs, QFPs
- BGAs, CSPs
- 0201 and smaller passives
- Enables:
- Miniaturization
- High component density
- Automated assembly
SMT vs Through-Hole Assembly
| Parameter | SMT | Through-Hole |
|---|---|---|
| Component types | Chips, passives, ICs | Leaded components |
| PCB requirements | Pads | Drilled holes |
| Paste deposition | Stencil | Wave/selective solder |
| Component attach | Surface pads | Inserted in holes |
| Soldering method | Reflow | Wave or selective solder |
| Process | Automated | Manual labor intensive |
| Density achievable | Very high | Limited by hole spacing |
| Reworkability | Challenging | Relatively easier |
| Thermal management | Vias | Large thermal planes |
| Cost | Higher initial | Lower initial, higher long-term |
SMT Process Step-By-Step
1. Print Solder Paste
- Use stencil to apply precise amount onto pads
- Stencil openings match copper pad layout
- Squeegee spreads paste evenly over stencil
- Common pastes:
- SAC305 (Sn/Ag/Cu)
- Sn/Pb (being phased out)
- Key parameters:
- Print speed
- Pressure/angle of squeegee
- Stencil wipe frequency
- Print gap
2. SMT Component Placement
- High speed pick and place machines
- Precisely place components on wet paste
- Split second per component placement
- Advanced vision systems for alignment
- Optimizing component placement:
- Group by package size
- Minimize tool changes
- Optimal board flow path
3. Adhesive Curing
- Reflow will happen later
- Adhesives cure components in place
- Avoid tombstoning before reflow
- Typically light heat or UV curing
4. Solder Paste Inspection (SPI)
- Verify paste deposits before reflow
- 2D or 3D optical inspection
- Detect defects like:
- Insufficient paste
- Too much paste
- Smears
- Blockages
- Take corrective actions or clean stencil
5. Reflow Soldering
- Heat applied to melt solder paste
- Forms permanent solder joints
- Conduction, convection, radiation heat
- Typical profile:
- Ramp up
- Soak zone
- Reflow peak
- Cool down
- Profile must suit components
- Common methods:
- Convection reflow oven
- Vapor phase reflow
- IR/radiant heat
6. Post-Reflow Inspection
- Verify all joints formed properly
- Look for:
- Shorts
- Opens
- Insufficient heat or wetting
- Other defects
- Automated optical inspection (AOI)
- X-ray inspection
7. Cleaning
- Remove flux residues after reflow
- Improve aesthetics and prevents issues
- Solvent cleaning with agitation
- May impact component susceptibility
SMT Line Optimization
- Balance speed vs yield tradeoff
- Component sequencing for minimum changeovers
- Optimization software to maximize uptime
- Maintenance and changeover procedures
- Real-time monitoring and feedback
Defect Prevention and Control
- Sources of defects:
- Design
- Materials
- Process
- Handling
- Tightest control at process level
- Defect elimination strategies:
- Statistical process control
- DOE optimization
- Anomaly detection
- Traceability systems
Quality and Reliability Testing
- ICT – tests shorts, opens, values
- Flying probe electrical test
- Burn-in stress testing
- X-ray and coefficient of thermal expansion analysis
- Vibration, drop, and shock testing
- Process audits and sampling
Conclusion
- SMT enables miniaturized, high density PCB assemblies
- Multiple stages from paste print to reflow
- Each step must be optimized for yield and reliability
- Overall process control and monitoring is crucial
- Defect elimination and quality testing ensures performance
Frequently Asked Questions
What is the most critical SMT process step?
Reflow soldering is generally considered the most crucial step since this permanently forms the solder joints attaching components. The profile must be optimized to suit all component requirements.
How small of components can be soldered with SMT?
SMT can reliably solder extremely small components like 01005 passives, microBGAs, and even microchips with tolerances down to 15 microns or below with advanced techniques.
What are common SMT defects?
Insufficient solder and open joints, bridges and shorts, tombstoning, skewed parts, solder balls/beading, heat damage, missing components, bent leads, are examples of potential SMT assembly defects.
What is the key advantage of SMT vs through-hole?
Higher component density, miniaturization, and suitability for automation give SMT a density and throughput advantage. However, through-hole assembly can utilize lower cost components and enables easier rework.
How can SMT solder joint quality be inspected?
Automated optical inspection and 3D SPI immediately after soldering can catch many defects. For internal inspection, X-ray laminography provides detailed 3D views of hidden solder joint quality.
To achieve the best electronics, one needs to solder. This process is often done with a soldering iron and leads to success if done properly. But how do you know how to solder? Here, I will introduce you with the SMT soldering process, including a step-by-step guide on how to put together a piece of electronics.
What is SMT soldering?
SMT soldering is the process of soldering electronic parts onto a circuit board, which is mostly done by hand. SMT is an abbreviation for “Surface Mount Technology”. This means that we do the solder assembly without a soldering iron, but instead with a machine called a “solder reflow oven”. The steps and materials used to SMT solder include the following.
When Is SMT Soldering Used?
SMT soldering can be helpful in many ways, including using as a hobby or at work. The use of SMT soldering will increase your knowledge in electronics and electricity. It’s also a good way to learn how to solder when you are starting out.
The reason SMT soldering is helpful at work is because of the low cost and high speed when producing the products.
What are the parts of SMT soldering?
There are two key things you need for SMT soldering: a circuit board and electronic parts.
For the circuit board, it is essential to use a silk-screened PCB (Printed Circuit Board) with plated holes (not tin). Usually, this type of circuit board can go for $1~$3 dollars in local Makerspace or online. To further save cost, you can also make your own printed circuit board, by drawing your circuitry on copper foil. Another option is to use a copper-clad board, which looks like a thin sheet of copper. It is acid-etched or sand-blasted with holes and traces.
For the electronic parts, you can find them online easily. For example, Rayming PCB & Assembly has a wide range of SMT parts at reasonable prices. Other places include eBay or Amazon. You should be careful when choosing these parts, as there are some which help specifically in prototyping and we cannot use in production parts.
Stuff you need
Before you start, you will need:
A soldering station
This is the current state of art in electronics manufacturing. It consists of a hot air furnace that heats up solder, a soldering iron that allows heating of the pasted joint, and a soldering tip which will melt the solder. The key to using a soldering station is to heat up pieces separately and avoid overheating them. You can use either the temperature mode or the time mode.
Some solder
This is the main ingredient in recreating this process. Solder consists of tin and lead, which are both an alloy with silver. Solder is normally a bit cheaper than other types of welding materials, and it has a better melting point than steel.
A soldering iron
This is the current state of art in electronics manufacturing. It consists of a hot air furnace that heats up solder, a soldering iron that allows heating of the pasted joint, and a soldering tip which will melt the solder. The key to using a soldering iron is to heat up pieces separately and avoid overheating them. You can use either the temperature mode or the time mode.
A piece of circuitry
An electronic circuit board is usually printed with various layers of metal with holes thats are full solder to connect to all the parts together. Circuit boards have a variety of shapes and sizes, depending on their function. You can use any type of printed circuit board; I will show you how to make a simple LED circuit board here.
A circuit diagram
There are many ways of using electronic components. Each design has its own merits, but most printed circuit boards require a schematic to read the connections of the wires and where they will connect to the solder. The circuit diagram is often presented as numbers on a grid.
Make SMT Soldering:
Step 1: Preparation
Assemble your tools appropriately (soldering iron, solder iron etc.) (see image below for more details). We must place solder in a temperature safe container to avoid damage from heat and oxidation once it becomes molten. Then find an area to work that is free from distractions. We can do this in the garage or workshop.
Step 2: Preparation of copper board
Find a copper board with the right size and shape for your circuit design. For example, a standard breadboard is good for prototyping, but if you want to make something more complex, you should use an integrated circuit board (or “chip” board). Chip boards are more expensive than breadboards, but it’s often easier to solder components on them because there are less wires that need to connect. For example, SMD LED soldering requires a chip led. We use solder paste here as well as it helps in stabilizing joints and making them watertight.
Step 3: Mixing of the solder paste
To mix the solder paste, first make sure that you switch off both of your soldering iron so you do not burn yourself. Remember, solder paste is extremely hot when it comes out of the nozzle. If you have a refillable hard solder gun, you can use the syringe to fill up some solder paste and simple push it down into a microwave safe container.
However, if you have an integrated circuit laser gun, make sure to screw off the tip off with a screwdriver before attempting this as it will burn your fingers. In this case, you will need to melt the solder paste first in a microwave or a small pot then mix it with the solder. It is best to hold your soldering iron with one hand and mix with the other as it is hot and can cause burns. After mixing, use a clean spoon or spatula to help melt the solder so that your circuit connects correctly.
Step 4: making SMT
Now we are ready for action! Switch on your soldering iron and put it on temperature 40°C~45°C. Heat up several pieces at once. Solder melts at 113°C. So, you must heat up several parts simultaneously. If you place them too close to each other, they will melt and you will have difficulty in moving the remaining parts.
As a test, take one of the parts and melt it on the tip of the soldering iron. If it is solid enough, then you can move on to more parts. If not, repeat until all your parts merge together.
Once you have all the parts joined, then use a piece of solder to connect all the pieces together. Take some time to test your work and check for any solder connecting to the iron. If so, remove by placing more solder on the joint.
Using this process, you can make your prototype. It does not require much skill, but it does take some time and patience. If it is your first time, it is probably best to make one part at a time and check for errors. The next step to making your own product using SMT is how the solder paste comes from the gun onto a pad. This video guide below will show you how to do this:
Conclusion
In this project, you have seen how you can use the basic components and simple soldering to make your own low cost PCB board. In addition, you have learned how to solder using a hot air soldering station and how to solder using an integrated circuit laser. You will also know the main ingredients in recreating the process of reflow soldering.
