Probe force analysis of Polar GRS500
This application note contains the results
of an investigation into the force applied by the test probe on a Polar
Instruments GRS500 Graphical Repair System. The investigation was undertaken on a GRS500 serial no.16047 fitted with a standard probe head serial no. 259|
Concept of GRS500 probe operation
The solenoid driven probe head is mounted on a vertical (z axis) slide that is driven up and down by a precision stepper motor drive.
The probe is driven up or down by solenoids and if not activated is suspended by springs in mid travel.
The travel of the solenoid driven motion is 10mm and this is not adjustable: rubber cushions limit the travel and suppress noise. The probe carrier runs on a linear ball bearing and is connected to the solenoid mechanism by an optical switch assembly that allows detection of when the probe is touching the PCB under test.
During testing the actual compression to which the pin is subjected is set by the position of the z axis slide. While testing, only the solenoid is used to retract the probe when moving around a component (e.g. an integrated circuit) but for across the board moves the Z-mechanism is also employed to increase the clearance or fly height.
The replaceable probe pin consists of a shell containing a spring and a test pin or needle. The spring is preloaded so the needle does not move until the applied force reaches a certain level. Pressure increases with travel up to maximum compression. The graph below shows the characteristic of the pin tested.
In normal operation the pin is compressed by about 2mm which allows a reasonable variation in solder height between test samples while maintaining a firm contact pressure of about 0.6 Newtons (60 grms wt. or 2.11 ounces
the pressure due to the spring in the test
To assess this the velocity of the probe is first measured at the height at which the board would normally
be mounted. This was measured at 1.5m/s.
The impact of the probe needle on the board is approximately equivalent to dropping the needle from 100mm.