What is Reflow Soldering?
Reflow soldering is a process used in printed circuit board (PCB) assembly to attach surface mount components to the board. It involves applying solder paste to the PCB pads, placing components on the pasted pads, and then heating the entire assembly in a reflow oven to melt the solder and form permanent solder joints. Reflow soldering enables the efficient, automated assembly of high-density PCBs with hundreds or thousands of tiny components.
The reflow soldering process generally follows these key steps:
- Solder Paste Application
- Component Placement
- Reflow Heating
- Cooling
Solder Paste Application
The first step is to apply solder paste to the component pads on the PCB. Solder paste is a mixture of tiny solder particles suspended in flux. It is sticky so components will adhere in place.
There are several methods for solder paste application:
| Method | Description |
|---|---|
| Stencil Printing | A laser-cut stencil is aligned over the PCB and solder paste is spread across it, depositing paste on the exposed pads |
| Jet Printing | A jet printer dispenses tiny dots of solder paste directly onto the PCB pads |
| Dispensing | For low-volume jobs, solder paste may be dispensed manually using a syringe |
Stencil printing is the most common method for volume PCB Assembly. The stencil thickness and aperture sizes must be precisely designed to deposit the right amount of paste on each pad.
Component Placement
After the solder paste is applied, the PCB is conveyed into a pick-and-place machine that automatically places the surface mount components onto their respective pads. The components are usually supplied on tape reels or trays.
Advanced pick-and-place machines have multiple placement heads and can rapidly place hundreds of components per minute with extreme precision. Some machines also have on-board cameras for automated optical inspection (AOI) to verify component presence and alignment before reflow.
Proper placement is critical – the component leads must be aligned and centered on their pads or the solder joints may be poor. Placement parameters like pressure, speed and component rotation are programmed for each component.
Reflow Heating
After all the components are placed, the populated PCB is run through a reflow oven to melt the solder paste and permanently attach the components. The reflow process must be carefully controlled to achieve good solder joints without damaging the components.
The PCB passes through several temperature zones in the oven, following a specific reflow profile that is tailored to the thermal requirements of the solder paste and components. A typical lead-free reflow profile looks like this:
| Zone | Temperature | Duration |
|---|---|---|
| Preheat | 150-200°C | 60-120 sec |
| Thermal Soak | 150-200°C | 60-120 sec |
| Reflow Spike | 240-250°C | 30-90 sec |
| Cooling | < 100°C | 30-120 sec |
The preheat and thermal soak zones activate the flux and allow it to clean the metal surfaces. The spike zone melts the solder and allows it to wet the surfaces and form intermetallic bonds. Peak temperature and time above liquidus (TAL) are critical parameters.
Cooling is carefully controlled to solidify the solder joints without inducing thermal shock or stress. Cooling too quickly can cause cracking.
Inert atmosphere (N2) may be used in the reflow oven to minimize oxidation. Oxygen levels are typically kept below 50ppm.
Inspection
After reflow, the PCBs are inspected to verify solder joint quality and identify any defects. Common reflow soldering defects include:
- Bridging – Solder shorts between adjacent pins
- Insufficients – Solder joints that have not fully melted and wetted
- Tombstoning – Components that have partially lifted off the pad due to uneven heating
- Voids – Gas bubbles trapped in solder joints
- Whiskers – Thin strands of solder extending out of joints
There are several inspection methods used after reflow:
| Method | Description |
|---|---|
| Manual Visual | Operators visually check the PCBs, often with the aid of magnification |
| Automated Optical Inspection (AOI) | A machine scans the PCB and uses computer vision to analyze the solder joints |
| X-Ray | Used to detect voids and other defects hidden under components |
Caught early, many reflow defects can be repaired. However, the goal is to tune and control the process to prevent defects and achieve a high first-pass yield.
Advantages of Reflow Soldering
Reflow soldering has become the standard method for surface mount assembly because it offers several advantages:
High Throughput
Reflow enables much higher assembly speeds than older methods like Wave Soldering. Modern pick-and-place machines and reflow ovens can process thousands of components per hour.
Automation
The reflow process is highly automated, reducing manual labor and human error. Stencil printing, pick-and-place, and reflow can all be done by machines with minimal human intervention.
High Density
Reflow allows for the use of tiny surface mount components and fine-pitch leads that would be impossible to solder by hand. This enables much higher PCB densities.
Repeatability
With proper process controls, reflow soldering is very repeatable. Solder joint quality is consistent across a high volume of boards.
Low Defects
While reflow defects can occur, the process generally yields far fewer solder defects than manual soldering, and most defects can be reliably prevented.
Reflow Soldering at RAYPCB
At RAYPCB, we specialize in Turnkey PCB assembly services using state-of-the-art reflow soldering equipment. Our facility features several SMT Assembly lines with automated solder paste printers, high-speed pick-and-place machines, and multi-zone reflow ovens.
We maintain strict process controls and traceability throughout the reflow soldering process. Our technicians are highly trained in SMT setup, monitoring and troubleshooting.
Some additional capabilities we offer for reflow PCB assembly:
- Lead-free and leaded solder alloys
- Nitrogen reflow for sensitive components
- Fine-pitch components down to 0.3mm
- Ball grid array (BGA) and chip scale package (CSP) components
- Automated optical inspection (AOI)
- X-ray inspection for hidden solder joints
- Functional circuit testing
- Conformal coating and potting
With our extensive component inventory and relationships with parts suppliers, we can source components for your PCB assembly, even during times of shortage. We also offer PCB Design and layout services to optimize your board for manufacturability and reflow soldering.
Whether you need a few prototype boards or a full production run, RAYPCB can help bring your PCB assembly project to life. Our reflow soldering experts are ready to discuss your specific requirements.
Frequently Asked Questions
What is the difference between reflow soldering and wave soldering?
Reflow soldering is used for surface mount components, while wave soldering is used for through-hole components. In reflow soldering, the entire board is heated to melt the solder paste. In wave soldering, the board is passed over a wave of molten solder to coat the component leads. Reflow allows for smaller components and higher density than wave.
Can reflow soldering be done by hand?
In theory, yes – the basic principles of heating solder paste to reflow temperature could be done with a handheld heat gun or hot plate. However, this is only practical for the simplest boards. Proper reflow requires precise control of heat and time that is best achieved with a programmable reflow oven. Hand reflow also risks damaging components with uneven heating.
What is the shelf life of solder paste?
Solder paste is perishable – the flux vehicle can degrade over time, reducing its effectiveness. Refrigerated paste typically has a shelf life of 6 months, while unrefrigerated paste may only last 1-2 months. Expired paste can cause soldering defects. Use paste before its expiration date and keep it refrigerated when not in use.
How fine a pitch can be soldered with reflow?
With a precise stencil and a high-quality solder paste, pitches as fine as 0.3mm are possible. However, for pitches below 0.5mm, the margin for error becomes very small. The reflow process and paste printing must be tightly controlled. Solder paste particle size must be scaled down for fine pitches.
Why are some components unsuitable for reflow?
Some through-hole components are not designed to withstand reflow temperatures. The plastic bodies of connectors and large capacitors may melt or deform. Heat-sensitive components like batteries and LCD displays can also be damaged by reflow. Large components may also have issues with CTE mismatch, causing tombstoning. Any incompatible components must be hand soldered after reflow.
