For many years now, the hybrid electronic industry has been using the art of reflow soldering and refining it. Therefore, one cannot consider reflow soldering a new manufacturing process. The advent of Surface Mount Technology (SMT) and lead-free soldering has further expanded the methods and techniques the industry uses for reflow soldering.
Goals of Reflow Soldering
Reflow soldering aims to satisfy two basic goals. The first goal is more traditional, including:
- Achieving maximum flexibility in allowing soldering of a large number of components while minimizing the changeover time
- Achieving uniform, durable, and effective solder joints
The second goal has a wider scope and includes:
- Minimizing the stress and damage to the PCB and SMD components
- Minimizing the movement of parts during the soldering process
Achieving the above goals requires a good understanding of the reflow soldering process, and the methods to modify it to ensure the product remains protected.
Reflow Soldering—the Process
The basic reflow process consists of four broad steps:
- Depositing solder paste on specific pads on a PCB using a predesigned stencil
- Placing SMD parts in the paste
- Heating the assembly to allow the solder paste to melt (reflow) and wet the PCB pad and the ends of the SMD parts, resulting in the properly soldered connection
- Cooling the assembly to the cleaning temperature
The solder paste is the most crucial ingredient in the entire reflow process. Several types of solder pastes are available, developed for specific sets of processes and profiles. Therefore, it may be necessary to consult the supplier to obtain a product best suited to a particular requirement, and the selection may require some trial, inspection, and retrial.
Careful handling of solder paste will ensure it remains effective during both its shelf life and its operational life. Best practices include maintaining a first-in-first-out program, routine inspection of expiry dates for shelf life control, refrigeration at temperatures specified by the manufacturer for storage, and bringing the paste up to room temperature before opening the container. Incoming inspection programs should include the test temperature for assessing viscosity and slump of the solder paste.
The characteristics of flow and deformation of solder paste, also known as rheology, affects the quality of paste deposition on the solder pads. Several factors influence this rheology, including the amount and size of the metallic particles suspended in the paste, and the differences in the density of the metallic particles and that of the organic mix.
An ideal solder paste should deliver the shelf life and stability claimed by the manufacturer. It must not exhibit a tendency to separate and/or settle, demonstrate adequate tackiness to hold components after placement for at least 10-12 hours without slumping or sagging. Reactions of the solder paste with the stencil are also important, for instance, it should transfer with a clean release without adhering to the stencil and with a clean breakoff of fine needles.
Printed Circuit Board
The PCB is the second most crucial ingredient in the reflow process. For proper soldering, it is necessary to bake the boards at elevated temperatures for a certain period, prior to application of solder paste. This drives out the excessive moisture from the board, which may otherwise lead to a large number of defects in soldering.
Some PCBs come with a protective sealer to prevent the exposed surface of copper pads from oxidizing and preventing adherence of solder. The fluxing agent in the solder paste dissolves the sealer as the assembly heats up in the reflow process. Other boards may come with solder-coated pads.
Pad design is critical to proper soldering of SMD components. Designers typically follow one of the international standards specified by IPC, EIA, and others for designing PCBs. This includes the design of different types of vias and the spacing between the pads.
PCB manufacturers provide tooling holes on the PCB for accurate registration of stencils, and a proper deposit of solder paste on the pads. This requires a tight tolerance to these tooling holes. Manufacturers also provide fiducial markers/optical locations for accurate placement of SMD components on the PCB using pick-and-place machines.
Reflow Soldering Methods
Heat transfer methods differentiate various reflow soldering processes. Although the hybrid electronic industry used heated belts or convection tunnels in their early years, the advent of SMT has led to a concentration on infrared and vapor phase reflow soldering methods.
In reflow soldering, the heat transfer is a result of the combination of radiation, convection, and conduction. Manufacturers of reflow soldering ovens tailor the radiation bandwidth to allow the air surrounding the PCB to heat up, subsequently heating up the PCB and the SMD parts on it. The absorption coefficient of the PCB material also helps in absorbing heat through radiation.
As the PCB heats up, heat travels by conduction to the pads and the solder paste. This combination of radiation and conduction accounts for nearly 60% of the total heat transfer. Radiating energy directly reaching the materials makes up the rest of the heat transfer.
The absorption of heat energy is not dependent on the color of the components. Rather, the thermal capacity of individual parts defines the time different components take to reach the same temperature.
Infrared Reflow Soldering
Reflow soldering ovens of the infrared (IR) type typically use quartz tungsten lamps as near or short wave lengths for their IR energy source. Heat transfer in these systems is primarily through radiation and convection, with a small part occurring through convection as well. Another type of IR system uses area source emitters, utilizing long-wavelength heat waves.
IR reflow soldering ovens typically have several preheat zones, followed by a soldering zone, and finally a cooling zone, and the PCB assembly travels through all these zones. IR lamps in the preheat zone are run at much lower than the rated wattage to allow the PCB and SMD components reach the required temperatures simultaneously. The energy transfer in the preheat zones is primarily by conduction through the PCB material and it depends on the speed at which the board is traveling through the system.
As the boards reach the soldering zone, they face a rapid increase of temperature going beyond the melting point of solder. It is very important to minimize the total time the board remains in this high peak temperature, as the high heat is necessary only to melt the solder, and a longer duration may degrade material properties of the PCB.
The next zone, the cooling zone, may have an in-line cleaning system where an aqueous system helps to clean the boards and cool them as well. Other systems may have no-clean cooling zone that uses air-cooling to room temperatures.
The entire process requires thermal profiling each board assembly for obtaining optimum solder joints.
Vapor Phase Reflow Soldering
Vapor phase (VP) reflow soldering overcomes most limitations of the IR reflow systems. As lead-free solder pastes show a tendency towards reduced wetting and higher melting points, an oxygen free atmosphere is necessary. IR reflow systems achieve this by adding a flow of nitrogen gas through the oven, while a vapor phase reflow system itself provides this atmosphere. Soft vapor phase reflow systems are the latest, which, unlike the IR systems, utilize convection as the primary method of delivering heat to the PCB and its components. The system uses a liquid based on perfluoropolyethers to generate a vapor of a specific temperature when heated. The PCB is lowered in the vapor, which condenses and transfers its latent heat by convection. In general, vapor phase ovens need a small footprint so they occupy less space than do IR reflow ovens. They also consume far lower amounts of energy, and provide an automatic inert atmosphere promoting outstanding quality of soldering.
Knowing the temperature distribution across the PCB as it passes through the three zones of a reflow oven is critical to obtaining optimum solder joints. The most common method uses a data collection system utilizing multiple thermocouples attached to strategic positions on the PCB, with the entire arrangement traveling through the oven. The system wirelessly transmits the position and temperature data it collects to a neighboring computer, which plots the temperature profile for that run.
Operators need to tweak the temperatures at various zones until soldering on the board is satisfactory.
Contrary to popular belief, it is not difficult to achieve an ideal reflow solder profile, and conscientious SMT assemblers work to approach this for each product. This not only results in a trouble-free process, but also maximizes the attainment of all goals.