PCB board assembly is a tedious task because small components like SMD are hard to solder onto the board. So how do so many tiny components get on the boards with such high precision? Let’s find out.
PCBWay is one of the largest and most advanced PCB board assembly company. In their automated factory, PCBway applies advanced machinery to assemble the SMD and through-hole components. In this blog, we are going to learn about those types of machinery one by one.
Solder paste stencil machine
The first step in the surface mount soldering process is applying the solder paste. The solder paste stencil machine applies solder paste in the form of microscopic balls of solder, suspended in a flux paste. It uses a mechanism similar to the silkscreen printing so that solder paste to go only on those pads, where the components are going to be soldered down.
Solder paste inspection machine
The solder paste inspection machine checks whether the solder paste printed by the solder paste stencil machine meets the IPC standards or not. It monitors the thickness and amount of the solder paste so that there is no defect due to solder paste. Only a few PCB companies use such kind of machine and PCBWAY is one of them.
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Pick-and-place circuit board assembly machine
The pick and place machine is one of the most important parts of the PCB board assembly process. This robot picks tiny components out of a reel containing a lot of SMD components and puts them on the board, in the solder paste that has already been applied, and it does it really quickly.
Some of the models have extra features required for comparatively larger components. They have a camera mounted on top of the head along with a vacuum tip which detects fiducial marks on the circuit board. There is a bottom camera as well. The machine moves the component over, and then the bottom camera takes a picture of the component and they could see exactly how it’s rotated or shifted, relative to the head. Once we know the board position and the component position, then we can precisely align them before it drops the component onto the board, into the solder paste that’s already been applied.
Any slightly misaligned component is nudged over after visual inspection through handwork.
These are similar to conveyor belt toaster ovens, only bigger and much fancier. Inside the oven, the board is heated on a temperature curve to a maximum temperature where that solder melts, and then gradually cool them off on a specific curve. Once it comes out this end, it’s completely soldered.
Automatic Optical Inspection (AOI) Machine
AOI uses all different colors of rainbow light to automatically inspect all of the different solder joints. They first take a board that they have tested and visually inspected. Then they take a whole bunch of pictures of every single solder joint, using all of the rainbow light inside the machine, that’s shining at different angles. The reflected lights from the surface of the shiny solder allow them to see the 2d picture as a 3-dimensional surface.
This machine then knows what are the colors for all the different solder joints for that golden master. And what do they look like when they’re correct. And compares it to the board being inspected. If a defect is detected, it is marked for inspection arrows by the tester. The defect is manually fixed with a soldering iron in the reworking room.
Moreover, one of the rare defects they find is tombstoning. It’s where a small surface mount component flips up on one end and looks like a tombstone in a graveyard.
This machine is used for BGA soldering, where all of the pins for the chip are underneath the chip. It inspects every single BGA, to make sure the solder is correct, hence detecting errors like; connected together, smeared, not enough solder, missing a solder ball, presence of an air bubble, etc.
Wave soldering circuit board assembly machine
It is a giant pool of liquid solder. The boards come along over the top of the liquid solder, and the liquid solder, in a wave, splashes on the bottom of the board, and it sticks to anything that’s metal. Hence, it creates these nice tent-like joints between the component legs and the board. It doesn’t stick to the green solder mask because it is solder repelling in nature.
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