Script – Solder Paste Printing
All programming for machines is done, components are kitted into feeders and we are ready now to start the mounting process.
Our first production step is to apply solder paste to the printed circuits boards. We have two ways to apply solder paste to the PCB, paste printing and paste jetting.
For the paste printing we have manual, semi-automatic and fully automatic screen printers. For jetting we use Mycronic MY700 jetters.
The eC-stencil mate is our own development, part of the eC-equipment we offer for electronics designers to assemble their own prototypes.
It is very useful for small batches, using very economical board-size laser cut stainless steel stencils. The tool gives excellent registration and repeatability without a complex set-up.
The maximum panel size we can print with this tool is 250 x 300 mm. Our semi-automatic screen printer can handle larger panels. The table and squeegee are motor driven, but positioning the board is still a manual process.
When the batch size is increasing, or the technology more demanding we use a fully automatic screen printer.
We have the machine connected to an SPI inspection station. The printer gets feedback from the SPI to modify the print parameters automatically to improve the result.
Once set up correctly the machine runs automatically, but setting up the machine is more complicated and takes more time.
The prepared solder paste layer is the basis for the SPI inspection.
We import the prepared Gerber and component placement file into an editor for our Koh Young 8030-2 Solder Paste Inspection system and set the properties of the board.
Finally in another program we set the inspection conditions and parameters such as tolerances for the volume, the offset and height of the solder on the pads. This results in an inspection program loaded to the machine.
The operator needs to teach the board to be inspected by the machine. First he indicates the fiducials, and then scans a bare board to create a zero – level for paste measurement.
The results of the SPI inspection is shown on the monitor of the machine.
All our printers use laser cut stainless steel stencils we burn in our PCB factory.
To achieve the right volume of solder paste on the pads, we use 3 different stencil thicknesses, 70, 100 and 130 microns. The eC-stencil mate uses smaller – board size – stencils. The other printers use 545 x 545 mm stencils mounted in stretch frames.
We use pastes that flow easily . For all standard jobs we use IPC type 4 solder with particle sizes of maximum 38 micron. For finer technology with a pitch under 0.5 mm we use IPC type 5 solder with particle sizes of maximum 25 micron.
Solderpaste needs to be stored in a fridge at a temperature of 5°C. It is very important to define the right volume of solder alloy deposited on the pads.
We covered this topic in our video about paste date preparation. Too much solder paste will create shorts, too little causes poor soldering.
A new way of applying solder paste is by direct printing or jetting. We invested recently in 2 MY700 Mycronic jetters. These jetters are equipped with dual heads and an advanced motion system.
This way it is possible to dispense till 300 dots per second with high accuracy. The dot volume, size and shape are easy to adjust, and can be optimised for each individual pad on the board.
The minimum dot size is 215 micron. Where we inspect the paste deposition with an SPI inspection tool after printing the paste, in case of jetting we use the internal 2D vision system of the MY700 for registration, inspection and automatic correction of the jetting parameters.
For complex printing we can combine stencil printing and jetting. For example if we get a component that needs printing with a 70 micron stencil, and also components on the same board that need more paste, we can use combined printing.
We print the critical apertures with a 70 micron stencil, and jet extra paste on the pads that require it.
The boards are printed with solder paste and ready to go to the pick n place machine for placement of the components.