Solder Reflow Ovens: Solder Paste and PCB Assembly
One of the more common themes I’ve noticed in holiday movies is the unfortunate person who destroys dinner in the oven. You’ve probably seen the same scenes; smoke rolling out of the oven, disappointed family members ordering a pizza, and nothing left but blackened cinders where there should have been a magnificent turkey, ham, casserole, or dessert. That’s because an oven produces a lot of heat, and if used carelessly will create a disaster. When the oven is an industrial version used to reflow PCB solder, it’s even more important to know how to use it.
The industrial ovens used to reflow solder on a printed circuit board are intricate tools. Using a combination of radiated heat and heat convection, the solder reflow oven delivers a carefully regulated flow of consistent heat to the PCB. This process completes the assembly of the board by melting the solder to form good solid solder joints between the components and the board. Let’s look at how the solder reflow oven is used for this process, and what you as a designer need to know to make sure that the process runs smoothly.
Introduction to the Solder Reflow Oven
Electronic components can be soldered onto a printed circuit board in a couple of different ways. First, there’s the traditional method of hand soldering them, where a technician uses a soldering iron and works on each connection separately. PCB assemblers will use wave soldering systems for mass production of boards or automated selective soldering processes for specific applications. The most widely used method of soldering components for mass production, however, is the solder reflow process, which is used primarily to solder surface mount components to the circuit board.
The solder reflow process first starts with applying solder paste to all of the surface mount pads on the board. Next the components are installed on those pads, and the solder paste will hold them in position until they are firmly soldered in place. We’ll talk more about solder paste and how it works a little bit later. With the components now installed in their correct locations, the circuit board is then run through the solder reflow oven.
An industrial solder reflow oven uses multiple heat zones for the soldering process. These zones are individually regulated for precise temperatures according to a user-controlled thermal profile that is pre-programmed into it. As the board travels through the oven on a conveyor belt, the duration spent in these different zones is also controlled by the thermal profile. The solder reflow process generally follows this pattern:
Preheat: This brings the board up to its first designated temperature.
Soak: Once the board reaches the first designated temperature, it will remain here for a predetermined period. This will activate the flux in the solder paste in order to remove the oxides on the metal soldering surfaces.
Reflow: The board now is heated again to melt, or reflows the solder.
Cooling: The board now goes through a controlled cooling cycle to solidify the newly formed solder joints.
Close up view of soldered components on a circuit board
What is Solder Paste?
We’ve already mentioned how solder paste is used to hold the components onto the board as it goes through the solder reflow oven, but it actually does much more than just that. Solder paste is a mixture of metal solder particles and sticky flux which has the consistency of paste. With its unique make up, solder paste performs three different and important functions during the solder reflow process:
Adhesive: The sticky nature of the paste holds the surface mount parts in place until the solder reflows to form a permanent bond.
Flux: This is a chemical cleaning agent that prepares the metal surfaces before soldering by removing oxides and impurities. It also helps the soldering process by promoting the wetting of the molten solder, and protects the metal surfaces from re-oxidation during soldering.
Solder: These are the metal particles that after melting will join the leads of a component to their corresponding pads on the circuit board. Because solder has a lower melting point then the metals it is joining, it provides a good solid connection between the two without causing any harm to the board or the components.
The solder paste is usually applied by screen printing it on the board using a stencil, or by jet printing it on. Jet printers are used a lot in prototype runs as it takes some time and expense to create a stencil. Once a circuit board is in full scale production however, a stencil is usually preferred as it can apply solder paste to a board in very little time. Solder paste can also be applied manually with a syringe if the board is being reworked.
Components land patterns designed for the solder reflow oven
Best PCB Design Practices for Solder Reflow
Solder reflow is the preferred method of mass producing surface mount boards, but there are some design practices that you need to follow for success:
Pad size: Surface mount land patterns must be made to the correct size. Pads that are too small could cause poor solder joints to be created during reflow. Pads that are too large could result in parts floating out of alignment causing clearance problems, or even possibly shorting together.
Balanced metal: It is important to balance the metal traces between the pads of smaller two-pinned surface mount parts. Unbalanced metal could lead to uneven heating, and that could cause the solder to reflow faster on one pad than on the other. This could result in a condition known as “tombstoning” where one side of the part is pulled by the faster-melting solder into a standing position instead of soldering flat as it should.
One way to make sure that you don’t end up with problems like these is to set up the design rules and manufacturing rules of your PCB design tools so that you are following the best design guidelines. Your tools should be able to set up a variety of rules and constraints so that you can control the different pad sizes, trace widths, and other clearances in your design.
The PCB design tools that will give you the greatest flexibility in setting up different design rules and constraints are available from the Cadence line of high performance EDA tools. OrCAD PCB Designer has a full and comprehensive set of rules and constraints that will guide you through your design so that your PCB design is ready for manufacturing.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.