Double Sided Printed Circuit Board - late 1960s?
This is a simple extension from single sided PCB but requires a way to connect traces on opposite sides of the board. I worked with some early DS PCBs and we used a shoe eyelet machine to install eyelets (like very small rivets except they have a hole through the center).
Later a reliable "via" was developed so that a PCB board shop could plate through the via holes.
A version of this is called "cord wood" where instead of mounting components to a single board they are mounted between two boards. This results in a very dense packing of the components and allows non planar shapes, like a hollow cylinder to be made. The RS-49 was the first CIA made radio to use this method and that was in the 1963 - 1964 time frame (they called it "sandwiched"). The GRA-71 keyer uses cord wood.
Another version of this is the flex circuit. Instead of using FR4 or fiberglass for the board material a thin flexable plastic is used. This is great for things like wiring the controls on the panel of a PRC-68 radio.
Yet another version is "Chip On Board" (COB) where the semiconductor die is attached directly to the board. It could be with the bonding pads up and conventional wire bonding used to connect between the board and the chip. It could be a bumped chip where there is a solder drop on each bonding pad and the chip is soldered to a flex circuit. The flex circuit and chip are cut out and the flex circuit attached into a package or to a board. This is used for things like greating cards and other low cost high volume applications.
Bread boarding and small run prototyping could be done using wire wrap technology. The component side of one of many PCBs lookes like this Stanford Telecommunications 5001 GPS simulator board. This instrument is serial number 4.
Another option was to make a PCB by making a 4X trace layout using Mylar grid paper and black tape with special "donuts" for the component holes and have life size a litho negative made. The PCB shop would use this to etch the board. Double sided boards required alignment marks on each negative done is such a way that they would not be used flipped or reversed.
This method was used for Silicon transistors and was very popular for ICs. Multifunction ICs and early Application Specific Integrated Circuits (ASIC) used this method. A PCB with hundreds of general purpose ICs could be replaced by a signle board with just a few ASICs greatly reducing the number of holes and therefore the cost. Since ASICs are not general purpose, repairs become harder if not impossible.
Nuvistors were tubes made using ceramic cylinders instead of the conventional vacuum tube glass bulbs. They could run on lower voltages and were promoted as an alternative to transisitors by some tube manufacturers.
Remote control was by means of RS-232 or HP-IB (IEEE-488, GP-IB) interface. The controls were no longer mechanical point to point but instead momentary contact switches (normally open) with some type of indicator to show the state of each switch.
HP developed 10Base-T, a way to use existing premises wiring for computer networking as a much lower cost alternative to installing coax cables. This developed into 100Base-T (CAT5) and higher speed versions that need special Unshielded Twisted Pair (UTP) cabling.
Many things were made using general purpose parts that could be repaired. These are in my opinion the "Golden Era" products because they can be maintained almost forever. When a newer design fails the only option is to get replacement modules form the factory, and if the factory is no longer selling them it's very difficult or impossible. There are a few parts in the PRC-68 series radios that are ASICs or programmed microcontrollers that can not be purchased new now, but most of the rest of the radios are made from general purpose parts and are repairable.
Some PRC-68 radios were made using two double sided boards separated by 1/16". At that time miltilayer boards were not available.
The Austron 2100F LORAN-C Frequency Monitor and 2100T LORAN-C Timing Receiver use double sided Printed Circuit Boards and have documented HP 5004 Signature Analyzer data to aid in troubleshooting to the IC level. This is probably the last of the equipment designed so that individual ICs can be replaced. After this troubleshooting is done by the board swap method or even equipment swap, like for a modern TV set.
Multilayer Printed Circuit Board - 1970s?
Once a via can be made, it's a natural extension to make a PCB with multiple layers which is how computers and other complex electronics boards are made now.
Complete function on a chip packages are used with this method as well as all of the above. It is impossible to repair a multiple layer board and even replacing a component can be extreamly difficult or impossible depending on the packaging method. Most of these are replace instead of repair.
Vacuum tube technology developed so that a tube would last as long as a transistor. The PRC-25 Viet Nam vintage radio has a final 2D24 tube thats known to never fail under normal conditions.
Later versions of the PRC-68 use a single multilayer PCB instead of the two seperate baords.
ExpressPCB offers 4 layer boards where the center two layers are always power and ground planes. The top and bottom layers are normal user wiring. The capactance between the power and gound planes is a good thing. Also both power and ground planes are connected by capacitors so both are good RF grounds which is a good thing if you need to meet FCC radiation standards. Whenever there's a hole from the top to the bottom that should not connect to either power or gorund, the center layers have a hole in the metal patters so that the via will not make electrical connection to the center layers. It's much easier to layout a 4 layer board since there's no power traces to work around.
Surface Mount Technology (SMT)- 1970s
Rather than drill holes in the board, the parts are mounted on the surface. This greatly reduces lead lengths thus extending the upper frequency of operation. The assembly method is to use an automated machine to put a dab of glue under where each part will end up, then use a pick and place machine to put each SMT part on the board, the use a production line soldering method like vapor phase reflow to make all the solder joints. Some thorough hole parts and be automatically placed, but then may need hand soldering. Parts can be placed on both sides of a board. This is the lowest cost method as of 2004 for making boards in volume.
Throwaway Products (2000+/-)
Prior to W.W. II watches were hand made and included jewels as bearings. They were purchased from a Jewelry store that could clean and regulate the movement. When bomb fuses were made into high volume products the same production equipment was later used to make Timex watches which had no jeweled bearings and were not serviceable, so jewelers would not carry them. Instead they were sold by drug and other discount stores.
A similar thing has happened to consumer (and other) electronic equipment. It used to be that a TV or radio was repairable and sold by a store that could maintain it. Now consumer electronic equipment is sold by discount stores and the cost is so low that it is a throwaway item.
2006 - the local paper is advertising new VCRs for $25 and DVD players for $35.
On Line PCB houses
There are a number of on line PCB fabrication houses, like ExpressPCB, that have a pricing structure based on:
- Job setup
- board setup Putting multiple boards on one panel saves money, but then you need to seperate them. Note the boards can be of any number of different designs.
- board area The key to minimizing cost is to reduce board area. The early 0.050" pitch SMT parts can be manually soldered and so they are an option for hobby projects, the finer pitch parts are much more difficult to work with. So there's a trade between ease of assembly for home use and what parts are choosen. You can get SOT-23 type packaged logic gates where there's only one gate in the package. This will save board area when compared to using a standard 6 pack of gates with some gates not being used. Microchip now has a very small SMT micro controller, the PIC10 family in the SOT-23 size package.
PCB Software
Professional PCB design is done using a layout program that has output files in Gerber format for the traces and also a drill file. The advantage of this is that it's a universal format that's accepted by a number of PCB manufacturing houses. The disadvantage is that it's a lot more complicated than the free software provided by ExpressPCB.
The Express PCB software is in two parts. The old one is just the mechanical board layout software. The problem with using just the layout is that you may or may not connect all the pads needed for each node. It seemed that on the simple boards I was making there would always be either a missing trace or a trace shorting out something. Now they have a schematic capture program (which can be used all by itself) that can be linked by the layout program. This makes it very easy to confirm each network node is properly connected.
Note that they do NOT count the number of holes for standard pricing (except there is a maximum number for any board, and if exceeded they either will not make the board or charge extra for each hole in excess). I found this out after designing a "prototype" board that had a lot of holes, and when trying to use a number of small holes to make seperating a panel easier.) The holes are drilled by a robot and the machine time is not that much of a factor for a normal board. If you had them make a board with a grid of holes there will be a surcharge. |
