|MadSci Network: Computer Science|
Dear Emliga, Thanks for the interesting question! While it sounds simple, It seems that you may be trying to relate to your students that appearances can be deceiving; and you would be very correct. To answer your question, I am going to make the assumption that you are not limiting it to the actual board, but also to the components mounted upon it. I will also include some explanations where the materials may not be readily apparent, and will annnotate those materials that are present in trace amounts with the suffix (tr.) I will start with the motherboard and follow with the mounted components. Motherboard - fibreglass, phenolic resin, epoxy resin, tin, copper (board and cladding and internal power/ground planes/wire traces), tin and antimony and/or cadmium, and possibly lead (solder), rosin and/or ammonium chloride (flux), brass/gold/silver (contacts), plastic, aromatic solvents (tr.) - typically trichloroethylene or acetone(for removing flux), lacquer, varnish, paint, ferric chloride (tr.)(residual etchant) and various oxides (tr). and sulfates (tr.) resulting from exposure to the environment before sealing. Components - silicon/fibreglass (component substrates), carbon, nickle/chromium wire, high-carbon iron wire(resistors), mica/polyester/polyethylene/polypropylene/paper/aluminum(capacitors), ammonium and/or mangenese chloride(capacitor electrolyte), copper (wires/inductors), selenium/chromium/palladium/gallium/silicon/aluminum oxide(transistor and diode P-type and N-type junction materials and doping materials, gold/platinum/copper(semiconductor internal connections, aluminum indium gallium phosphide(or arsenide)(light emitting diodes), lithium/silver (battery), glass/epoxy resin/rubber/silicone/fibreglass (encapsulation,insulation), Barium and/or beryllium oxide and silicone grease(heat sink heat transfer compounds, Aluminum/steel(heat sinks). Also, given the various methods of semiconductor growing and laser trimming of capacitors and resistors, trace amounts of shielding and anti- oxididation gases may be present in the form of nitrogen, argon and helium. Silver nitrate may also be present as a result of incomplete removal of the un-polymerized areas of the board prior to etching. Very briefly, the copper-clad board is covered with a photo-sensitive material. A negative mask is positioned over the board and the resulting 'sandwich' is exposed to ultra-violet light. Those areas where the coating is exposed to the U.V. will polymerize, and remain unaffected by the wash with sodium carbonate; which removes the un-polymerized coating. Therefore, the un- polymerized areas will be etched away by the ferric chloride. If your mask has very dense blacks AND your transparent areas are very transparent, AND your exposure time and intensity is correct, AND your processing solutions are fresh AND your processing times are within accepted limits, you will end up with a high quality board. If any of the above are not up to standard or dust is present in the air, your board may end up with only partially etched areas resulting in, at the very worst, short circuits; and at the very least, stray capacitances which can adversely affect circuit performance; especially at the high frequencies now in common use with very fast processors. As you may (correctly) surmisedue to their complexity, there is a relatively high rejection rate with motherboards. I'm sure there are other other materials present due to the rapid changes in technology and manufacturing processes, but these are the ones that immediately come to mind. I hope this has answered your question without being too boring (considering the level of detail you asked for, and what you got)! I would be very interested in knowing how you are going to use this information, and enjoyed corresponding with someone from a country I would very much like to visit. As for myself, I live in Milwaukee, Wisconsin. You can contact me directly at Karl-Kolbus@email.msn.com Regards, Your not-so-mad scientist, Karl Kolbus
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