Improve Phase Blog

Containment to Prevent Automotive Steering Column Electrical Failures

Improve Phase

Date: 3 April 2016

Introduction 

Due to the failure of our DoE2 samples at Thermal Shock Endurance Testing, it was necessary with immediate effect to come up with an alternative solution to our containment issue. It was proposed as a team that we consult with the supplier of our existing coating material, explain our problem and seek any advice they may have. It was brought to our attention that other customers of theirs had used a new type of material , a Dam and Cure Gel, to help overcome similar issues. This was to be trialled immediately, initially using a hand dispenser as we did not have the correct applicator in our existing conformal coating machine for this material. A revised Gantt Chart was created to schedule all tasks involved in the delivery of the newly proposed process trial.

Revised Gantt Chart

The trials involved three different methods of Dam and Cure Gel application

       1. Hall IC coated with Dam and Cure Gel only

       2. Hall IC coated with Dam and Cure Gel first, then coated with our normal Himiseal UV 40-250

           conformal coating

       3. Hall IC conformal coated with Humiseal UV 40-250 first, then coated with Dam and Cure Gel.

Initial inspection of the hand trials showed a marked improvement in the coverage of the Hall IC's . It was clearly evident that the samples with the Gel placed last gave far superior results as there was complete coverage and better adhesion to the printed circuit board due to it being placed on top of a lower viscosity material. The coverage was put to the test by placing metal debris on top of the coated Hall IC, which was proven to be well protected. These were then sent to our Validations Department for visual inspection and Thermal Shock Endurance Testing.

Metal Debris test on Gel coated Hall IC

Gel location on printed circuit board

When eventually we received the results from the Validations Department, it was worth the wait . All parts had passed validation and we could now progress with automating the process by installing a second Nordson conformal coating machine inline with our existing one. The original machine would not have been capable of applying two different materials. A provisional order has been generated for the new machine at a cost of €76000 and delivery should be in house within two weeks . 

Section of Validation Report for Dam and Cure Gel samples

A layout proposal incorporating the new Nordson Coating machine into the existing line has been generated and plans for actual installation have been drawn up.

Existing Layout Schematic

Proposed Layout Schematic

A risk management plan was put in place to foresee risks, estimate impacts and define responses to issues as a result of the relayout. It also contains a risk assessment matrix. A risk is defines as "in uncertain event or condition that, if it occurs, has a positive or negative effect on a project's objectives. The resultant risk profile is a quantitative analysis of the types of threat the proposed line changes are exposed to.

Risk Assessment Matrix

Process and Equipment Layout Comparison

Risk Profile Evaluation

The Risk profile has been reduced from a rated risk of 69 to 37 through the use of mitigating actions. The Risk Profile is now deemed acceptable by the project team.

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.