I worked at TI in Houston in 1980. My job was to test/repair boards that came from the line that put them together. On a break, I watched the machine put the parts in the board, then hit the wave solder machine (that was cool to watch!). After that, it went to a line of ladies that would solder the wires for connectors etc. THEN the boards would come to us. First step was to check the resistance between the board ground (12 v supply for the board), and the digital ground. If it was less than 40meg ohm, it had to be fixed. Usually it was the left over/burned rosin/flux. Washing that off (we used I think it was Carbon tetrachloride or similar) usually fixed it.