Masters Theses

Date of Award

8-1997

Degree Type

Thesis

Degree Name

Master of Science

Major

Environmental Engineering

Major Professor

Chris D. Coz, R. Bruce Robinson

Committee Members

Terry Miller

Abstract

The Printed Wiring Board Industry manufactures circuit boards to house wiring layouts containing electrical components. Holes through these boards connect different sections and serve as coupling locations for resistors and various components on the boards. The Making Holes Conductive (MHC) process applies a thin conductive layer to the insides of these holes that facilitates electroplating. The typical MHC process consists of several chemical baths which prepare and clean the boards and then apply this conductive seed coating to the holes. This thesis compares the respective worker and population health risks associated with seven MHC processes. The MHC alternative technologies reviewed here are: Electroless Copper, Non-Formaldehyde Electroless Copper, Palladium (Tin and Organic Stabilized), Graphite, Carbon, and Conductive Polymer. The risk comparison is achieved by estimating chemical exposure to workers and the general public and then combining these exposures with current toxicity information. The exposure routes quantified in this work are inhalation of chemicals from MHC processing baths, and dermal contact with chemicals in those baths. Risks are quantified for line operators, lab technicians, and the general public living near PWB facilities. The risks derived in this paper should be viewed as relative comparisons between MHC processes, and are used here as screening-level estimates only. There were many variations associated with the MHC processes, and thus several important assumptions were made to characterize them. Some of the key assumptions were: conveyorized processes had no emissions to inside air; line operators did not wear protective gloves/gear in the process area; reactions and speciation of chemicals in MHC baths were not quantified; and conservative upper-end estimates of exposure variables for receptors were used in the absence of other available data. An uncertainty and sensitivity analysis was performed on the air transport models used in determining emission rates from MHC baths. The three air transport models utilized in this work were: Surface Desorption, Bubble Desorption, and Aerosol Transport. A Monte Carlo approach was employed to determine which parameters contributed the most to air modeling variance (as measured by Spearman Rank Correlation Coefficients). Several runs were made with varying MHC chemicals and bath configurations in this Monte Carlo analysis to ensure that results were representative of all processes. The parameters which consistently had the greatest effect on modeling outcome for all chemicals and baths were:

  • Air Turnover Rate (ventilation rate divided by room volume) - contributed approximately 58% to model variance
  • Process Room Volume - contributed approximately 37% to model variance
  • Bath Area - contributed approximately 1-6% to model variance
  • Chemical Concentration in MHC Bath - contributed approximately 1.5%
The air turnover rate assumption originally chosen for calculating air concentrations in the MHC process area was revised based on these results. The corrected air turnover rate which provided the 90th percentile frequency distribution was found to be 0.021 air turnovers/minute. A comparison of common design standards was made to validate the revised air turnover rate. The only known MHC process which had an excess lifetime cancer risk was electroless copper: 1.19 X 10-3/sup>. All other processes were evaluated for noncarcinogenic risks only because electroless copper had the only carcinogen with an established slope factor. The noncarcinogenic risks to Line Operators (the driving receptor) from MHC process baths were derived in terms of Hazard Index (HI) and Margin of Exposure (MOE). These risks are summations (reciprocal summation for MOE) of risk estimates across all pathways and chemicals for the receptor. HI is defined as the potential dose rate (exposure) divided by the reference dose, while MOE is defined as the potential dose rate divided by the No or lowest-observed adverse effect level. The total risks per process are presented in the table below.

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