Analyse Phase Blog

Containment to Prevent Automotive Steering Column Electrical Failures

Analyse Phase

Date: 13 March 2016

Introduction 

Following on from our measure phase where we did a CMK evalution of our conformal coating machine to ensure that our process was capable and secondly a Design of Experiments (DoE) to ensure our process was operating at its optimum settings, we set about analysing the results.
Results of CMK analysis.
              X value   2.2333
              Y value   2.0872
               Z value   3.6545
CMK, also called initial process capability, was measured using 50 continuous samples produced on our machine, keeping all influences other than the machine parameters constant, ie operator , measuring equipment, etc. Formula for calculating CMK is the same as CPK, but here we need to use standard deviation calculation for the 50 data values obtained. Stability is verified by plotting this on a control chart to look at whether the process is stable or not. As our CMK values were greater than 1.67 which is the automotive industry specification, the machine was proven capable and the data also proved our process is stable as no data values are outside the control limits and also satisfy the other process stable criteria.
Results of DoE analysis.
Following on from the measure phase where the array of runs generated by Minitab software, using all variable factors at their high and low settings (See Table2 DoE1 Measure phase) were completed and presented for analysis. The key responses namely (1) Quantity of Bubbles were visually inspected and rated from 1-best to 10-worst , (2) Wet Thickness measured using a wet thickness measuring device (microns),
(3) Coverage of Conformal Coating rated from 1-best to 10-worst and (4) Width of strip measured using a calipers.
The results were analysed using Main Effects graphs generated by Minitab. See Figures 16-19

It is evident from Figures 16-19 above that three process variables have a significant effect on the key responses, the slope of the line being the significance indicator , ie no slope , no effect . The process variables are          1.Fluid pressure (psi)          2.Height of nozzle above pcb (mm)          3.Micro adjust (mono) A susequent DoE2 was to be designed using only these significant variables , all other variables remaining constant, and a trial conducted using the array of runs generated by Minitab in Table4. A total of 27 test boards were produced as per Table4 above and these were then sent to our Validations Department where they were assessed by means of a Thermal Shock Endurance Test. The test subjected the parts to extremes in temperature from -40 degrees Celsius to +85 degrees Celsius, a duration of 30 minutes at each temperature .The parts were assessed for visual deterioration during and after the tests at intervals of 250 cycles, 500 cycles, 750 cycles and finally 1000 cycles. The key responses mentioned above were also inspected. Fig 20.  It is clearly evident that after only 250 cycles of Thermal Shock Endurance Testing that we had a very high failure rate from our trial boards using the optimum settings derived from our Design of Experiments. At stage it became apparent that our existing process was not capable of achieving our requirement of adequately coating the Hall IC leads in order to prevent short circuits from the metal debris in the steering column assembly. This left us with no other alternative but to look at another means of solving our conformal coating issue. This will be discussed in the next phase of the DMAIC process.