Measurement of squeegee force

CmPrint

Measurement of the force-time curve during printing

The first process step in the production of electronic printed circuit boards (PCBs) is printing. Solder paste is printed on the printed circuit board by means of a stencil. The quality of the paste print has a decisive influence on the overall quality of the finished printed circuit board. An important influencing factor is the accuracy and reproducibility of the squeegee force.

Motivation for development

A frequent task for our measuring service is the checking of the printing accuracy (x, y-accuracy). The position accuracy of the printed depots in the CmController is measured.

If several printers are measured one after the other, it is very often found out that despite exactly the same material (board, stencil, squeegee and paste) one has to make changes to the print parameters in order to obtain an optimal filling of the depots. This indicates that the measured printers differ in terms of their force distribution.

For production, this means that if a board is moved to another line, which may even be identical, adjustments may have to be made in the print program.

The aim of the development of CmPrint was to create a measuring device that would allow an objective assessment of the squeegee force. This creates the prerequisites for optimum printer settings and avoids unnecessary adjustments.

Performing a force measurement on the stencil printer

The heart of the CmPrint system is a sensor board (see picture above), which can record the forces occurring on the entire surface. This is installed in the printer to be measured instead of the usual printed circuit board supports. As in production, the printer receives a printed circuit board and a printing stencil. The dummy circuit board is designed in such a way that the sensor board touches the pressure stencil in the pressurized position and can measure the forces unadulterated.

After the printer has been set up, several prints are measured. This allows a statement to be made about the accuracy and reproducibility. This is repeated with different force values to assess the linearity using different squeegee force settings.

Analysis of the measurements

With the associated CmForce software, the measured values can be conveniently evaluated:

  • Constant squeegee force over the squeegee stroke
  • Cutting and comparing the two squeegee directions (forwards and backwards)
  • Measure the squeegee force and compare it with the entered values
  • Analysis of reproducibility over multiple prints
  • Calculation of capability characteristics
  • Performance of machine capability test (MFU) for the parameter pressure
  • Automatic generation of reports for documentation of the results.

CmPrint FMP-04 (Standard)

Technical specifications

  • Measuring area of the sensor board: 330 x 240 mm
  • Height: depending on the printer
  • Measuring range: 2 - 500 N
  • Measurement Uncertainty (U): \(\pm 0.5\) N

Examplel

Below are two screenshots. On the left you can see the force-time curve of the entire measurement series. The right picture shows two force progressions of successive pressures (pressure direction forward and backward).