In my previous post I went over all the steps to fabricate a PCB. This post is all about taking the finished PCB bare board and assembling it with components.
PCB Assembly Process
There are two main type of PCBs, namely, through-hole and surface-mount. The assembly process for any board is basically this:
1. Populate the board with components
2. Solder the components
But there is some difference between through-hole and surface mount.
Through-Hole Assembly Process
In addition to the steps above there is also trimming the through-hole component leads and cleaning the flux.
Flux is applied before solder to break down the oxide that may be on the pad and allow the solder to flow.
Some solder includes flux in it. For hand soldering this is called flux core solder.
Solder can be either hand soldering or some type of automated soldering.
Wave soldering is the most common type of automated soldering for through-hole boards. What happens is the wave soldering machine contains a large reservoir of molten solder that is being agitated to create two waves. The first wave is a turbulent wave that helps to break the surface tension of the solder and get it into the holes. The second wave is a laminar or smooth wave that has a finishing effect.
Another method to solder through-hole components is to spread solder paste into the holes, populate the components and then run it through a reflow oven. The disadvantage with this is the components are being heated where with wave soldering the whole component is not heated, just the leads.
Flux has to be cleaned off of the board after soldering. This is done with a solvent chemical or an aqueous (mostly composed of deionized water) method.
Some flux is considered no-clean, it has the advantage of being faster and more environmentally friendly. However, it can affect the appearance of the board and interfere with test probe contact as well as adversely affecting rework efforts and lessening the adhesion of conformal coatings.
Surface Mount Technology Assembly Process
The basic surface mount process is as follows:
1. Apply solder paste to the component lands
2. Place the components via pick-and-place machine
3. Reflow the board to melt the solder and secure the components.
For a multilayer board, these steps are done on the bottom side and then the board is flipped over and the top side is done. The only difference is since the bottom side components are reflowed twice, the second time while upside down, there is a chance components could fall off. The components will only fall off if the weight of the component is such that it is greater than the surface tension of the solder holding it on. If this is the case, an adhesive is added to the component to hold it on during the second reflow cycle.
Apply solder paste
Solder paste is a mixture of tiny solder balls, flux and other chemicals. It is most often applied by stencil or screen printing. It can also be dispensed directly to the board.
Some components are not well suited for solder paste. Direct chip attach, like flip chip technologies typically have area array connections that are too small to use solder paste. In this case, flux is applied to the circuit board pads and is used to hold the chip in place. Then, during reflow the solder balls on the chip provide the solder to hold it on the board. Another method to apply flux is to dip the chip solder balls into a bath of flux and then place it on the board.
There are two types of common pick and place machines, turret or gantry. The turret type is where a board is on a conveyor and stops by a round turret that spins a picks up components off of another line and puts them on the board. This is more often used for passive components. The gantry machine is where the board is stationary while an X-Y type placement machine goes and gets the correct part from the feeder supply. This is used for larger components like ASICs.
Reflow is done as either a batch or inline process. In an inline oven, the board will go in one side unsoldered and pass through increasingly hotter temperature zones before passing out the other with the soldering complete. The objective of the temperature profile is to get the board hot enough that all components are soldered but not so fast that they are damaged by the heat. Batch furnaces provide a vacuum condition where inline usually do not. The inline types are more often used for high volume production.
Cleaning considerations are similar to those of through-hole boards above.
Finishing and Underfill
For some very fragile components or boards that will be used in unusually harsh environments, it may be necessary to use some encapsulant material to cover all the components and protect them. Underfill is done after soldering and is the process of filling the void under chips, especially direct chip attach technologies like flip chip that need additional mechanical stability. Underfill is applied with a capillary action. Capillary action is where the underfill is dispensed next to a chip on a board and the surface tension of touching the chip pulls it under to fill the void.
The most important aspect of encapsulant and underfill is to ensure there are no voids after it is applied. This can leave air and moisture inside that can cause all kinds of reliability problems and failures.
The methods of assembling a through-hole and surface mount circuit board were presented. The main tasks in assembling a board is populating components and soldering those components in place. There are other considerations like, cleaning and encapsulation.