Optimisation of the plated through hole (PTH) process using experimental design and response surface methodology

The process of producing side-to-side and innerlayer connections for printed circuit boards by metallising with copper is called the plated through hole (PTH) process. The copper deposition rate of the PTH process is one of the major quality characteristics. In this project, experimental design and response surface methodology were employed to investigate the PTH process behaviour and perform the process optimisation. First, a 25v5-1 fractional factorial design was used to screen out the significant factors as well as their interactions on the response of the copper deposition rate of a PTH process. The location factors were identified to be operation time, temperature, concentration of copper additive and the interaction of operation time and temperature. A normal probability plot of the residuals was used to determine the factors which give the minimum thickness variability of deposited copper in the PTH process. The interaction of operation time and temperature was found to be a dispersion factor. Model adequacy checking was conducted to check the normality, independence, and constant variance assumptions of the model. It is then followed by employing response surface methodology to locate the optimal operating point within the process window for the minimum thickness variability of deposited copper. First, a first-order model was regressed on the standard deviation of copper thickness of the four corners of the process window. The path of steepest descent was found and a new first-order model was then regressed at the point. The lack of fit of the new first-order model indicates that a second-order model is required. A central composite design (CCD) method was used to fit the second-order model. The stationary point obtained by the partial differentiation of the second-order model is 10.623 min in operation time and 24°C in temperature which was characterised by canonical analysis to be a minimum point. The predicted minimum variation of copper deposition rate calculated, based on the second-order model, is 1.02 μin. A confirmation experiment was conducted based on the recommended optimal setting. The minimum variation of deposited copper was found to be 1.08 μin.