The ability of a probe to hit the target is affected by two main factors: Fixture Offset and Probe Pointing Accuracy.
Fixture Offset is the sum of all the tolerances and deviations resulting from the PCB assembly and Fixture manufacturing processes. This group includes offsets in the PCB assembly print and tooling holes location as well as offsets in the position and straightness of the tolling pins and socket in the fixture.
Fixture Offset is mainly determined by the quality of the drilling and milling process. The higher the standard in this process the higher the accuracy and the smaller the test pad that can be contacted. Using high precision Drilling and Milling Machines supported by regularly calibration process and the appropriate Drilling and Milling Process are absolutely essential for achieving a low Fixture Offset.
T-5 Photo close up showing few test (2-3) pads with several probes hit marks
Temperature and Delamination are the main factors that affect the quality of the drilling and milling process. Controlling temperature in the drilling and milling tools as well as on the plate during the entire drilling and milling process is indispensable to achieve straight drills and minimal tolerances when drilling for the tooling pins and sockets. Delamination is a specific problem associated with the drilling of fiber composite material typically used as a Fixture plate material. It reduces the structural integrity of the fixture plate and results in drills with high tolerance and poor assembly properties Measures to control temperature and avoid delamination is a muss in any fixture drilling and milling process. Based on more than 10 years of experience, we have developed a Drilling and Milling Process specially designed for manufacturing Fixtures that includes several techniques to prevent delamination and control temperature during the entire drilling and milling process.
Disperse Drilling Sequence Process. When drilling the fixture plates our drilling and milling software automatic develop a drilling sequence that disperses drilling along the plate preventing to drill holes in successive orders that are too close to each other. This technique prevents heat from building up in the fixture plate areas with high-density probes.
T-6 Video showing an Agilent3070 probe plate been drilling at different areas – CNC drilling head moving to several parts of the probe plate and drilling holes
Dry Drilling and Milling. Most people know coolant made of oil-based substances that leave drilled and milled parts dirty and oily after the process. Instead of oily coolant, we use a technique that combines ethanol and forced airflow to cool down both tooling and fixture plates. When ethanol comes in contact with the airflow, it evaporates very fast cooling the air and material it is in contact with the process. This technique allows a faster cooling down of tooling and plates by continuously blowing them with cooled air.
T-7 Short Video of a milling process with a section in slow motion showing the cleaning process
Clean Drilling. The main factor causing delamination is the wrong tooling speed and/or feed rate; however, selecting the correct tolling speed and fee rate is not enough to avoid delamination. We implement in our Drilling and Milling Process additional rules:
Maintain clean tooling during the entire milling and drilling process. Drilling deep holes increase the tendency for debris material to fill and obstruct the drill flutes. This increases the heat generated in the drilling process and prevents the airflow from conducting heat away from the tooling point, where the drilling occurs. In addition, excessive debris on the drill flutes, cause the drill cutting edges to fracture resulting in premature tooling failure.
Step drilling, or the practice of withdrawing the drill after drilling a short distance. This technique increases the debris removed during the drilling process and reduce their accumulation at the bottom of the hole. In addition, step drilling permits cleaning and cooling drilling tools and plates during the drilling process
T-8 Video close up and in slow motion showing air cleaning the tooling and step drilling in a sequence of several drilling