To be completely honest, I used to not give a lot of thought to the process of soldering components onto a printed circuit board. As a PCB layout designer I concerned myself with the particulars of the components used in my design so that I could create their footprints and floor-plan their locations on the board. Then I spent a lot of time fine-tuning their placement to give myself the best possible chance of success when routing the traces together. After that, though, I didn’t give it much more thought. Oh sure, I created the solder mask and the solder paste layers in my Gerber files, but the rest was someone else’s problem. Right?
Wrong. Thankfully my way of thinking has grown over the years so that I look at PCB design from all perspectives and not just what is in front of me at the moment. This allowed me to start giving the process of soldering the attention it deserves.
To put it simply, if the components don’t solder correctly to the board, then all the time spent in design has been wasted effort. That revelation made me realize that I needed to understand a lot more about the PCB soldering process so that I could better design for it. Here’s a brief snapshot of the different soldering process your PCB design will go through.
For years the primary method of soldering a circuit board in production has been to wave solder it. This process is designed primarily for thru-hole parts, although with the correct preparation it will also handle some surface mount parts as well. Typically though manufacturers will use the solder reflow process for surface mount parts, and will create a pallet to mask off these parts on the back of the board so they don’t pass through the wave.
The solder used in the wave is made up from combinations of different metals such as tin, lead, and other elements. In cases where restrictions on hazardous substances is required (RoHS), the lead in the solder will be replaced with other materials that are not deemed hazardous.
A conveyor belt moves the circuit board, or panel, through the wave soldering process. A pan contains the molten solder that is pumped up into a wave which the boards or panels will travel over. This wave will coat and solder all the metal surfaces that it comes in contact with.
The wave solder process also includes spraying a cleaning flux onto the surfaces that are to be soldered. This removes any dirt and oxides contaminating the metal surfaces which would decrease the quality of the soldering. The wave process will also preheat the board to help guard against thermal shock to the components as well as to help bring the board up to the correct soldering temperatures.
Printed circuit boards are now mostly made up of surface mount parts however, and for these boards the solder reflow process is preferred.
The goal of any soldering process is to melt the solder between the metal pins of the component and the metal pads on the circuit board so that when it hardens there is a strong metal bond between the two surfaces. With the wave process the board and its pins to be soldered were passed over a wave of molten solder. With solder reflow, a powder version of the solder and flux materials is applied to the metal pads on the circuit board.
This solder paste is sticky and will hold the parts in place until the board or panel is passed through an industrial convection oven. While heating in the oven, the solder paste will melt and “reflow” providing the solder joint between the pins and the pads. Although primarily intended for surface mount parts, thru-hole parts can be reflowed as long as the proper solder paste stencils are used to apply the paste. Combining different technologies of parts into one process like this can help to reduce manufacturing costs.
The solder reflow process involves different phases. During preheat the board will be brought up to temperature to activate the fluxes for cleaning. During reflow, the greatest temperatures are used to melt and reflow the solder. Finally the cooling process is used to control the reduction of temperature to avoid thermal shock to the components.
Both wave soldering and solder reflow use thermal profiling techniques to determine the best process profiles to use. A circuit board will have several points within it measured to determine the correct temperature and wave or reflow duration in order to achieve the best soldering quality. But not everything will be waved or reflowed, there are some other methods in use as well.
Another great tool that PCB manufacturers have available is the selective solder system. This machine applies solder individually to thru-hole leads as a technician with a soldering iron would do, but it is an automated process. The selective solder machine pumps molten solder from a reservoir up through a nozzle for each solder joint. This entire assembly is on a chassis that can move in any direction to hit any location on the board, and the solder points are programmed into the machine using input from the same manufacturing image files that are used to build the board. Selective solder is a great bridge between full automated wave soldering, and manual hand soldering.
There is still times though where the skill of rework technicians is required to hand solder parts on a board. Speciality parts that don’t easily fit the automated soldering process profiles, or low volume runs of circuit boards are usually candidates for manual soldering.