The title of this post is what happens at this stage of the design. The development path can go one of two ways or get split between two people. One path is to generate a board layout from the schematic. I will talk about that later, if I can. The other is to generate a Bill of Material from the schematic and begin figuring out what this will cost. Actually I had already checked that the parts were available, not discontinued or on back order and not outrageously priced. I also had to download the datasheets to check that I had the right part for the footprint on the board about to be laid out.
The Bill of Material or BoM generation is automated in EAGLE. During the schematic capture phase of the project I will often run a BoM organized by values to find any parts that should have the same information but don’t. For example if I gave a resistor the value of 10.0k instead of 10.0K. (Yes capital letters mean the part gets put into a different list. The other reason to run a BoM, to find typographic or other errors in the schematic.) Here is an example:
Notice that there are two lines with 10.0K resistors, something that needs to get fixed. But it is also an example of what a values organized BoM looks like. Often I will save this file with the ending of “Check BoM” since I will also use it later to verify that I have the correct number of each part.
The other organization of the Bill of Material is by Part. Here is an example:
This Bill will get imported into a spread sheet program and given the title of Raw BoM. It is a relatively laborious process because there is no easy way to automate the import. So it is mostly done by hand. Later this document will be used to help create the hand assembly instructions, mostly as a check off list that all the parts have been accounted for in the hand assembly. The other thing I do with this particular spread sheet page is collect the library information.
Library information is critical because of a trap within the EAGLE software. If you load a new version of EAGLE into the same directory as your existing version it removes all the custom libraries and over writes all of the EAGLE provided libraries with new copies. So if you want to be able to updated this version of your board I will need to make an archive of all the files used including the library files. I create the archive after the boards are ordered anyway. Yes, I was burned by this once. So now I am careful.
The “Side” column gets filled in after the layout has finished. One of the reasons I care what side a part is on tells me how many copies of that side of the board I need to make for my hand assembly instructions. The other reason is that several automated assembly houses charge each time you use a particular part on a side. Thus if you use a part both on the top and on the bottom of the board you get charged twice. I like to prevent that cost when I can.
The next step in the Bill of Material analysis leads to what I call the “Info BoM”. In this listing or bill I am not concerned by each individual part but more for each part type, how many of each type there are and how I order them. How many of each part, or Quantity, comes from tallying up all the instances of a particular part on the Raw BoM and then checking that count with the Check BoM. If there is a discrepancy it is probably a typo on the schematic that needs to be fixed. There have been times when generating an accurate quantity has taken two or more iterations.
Once the quantity is correct then I look at the other specifications of the parts. What tolerance do I need? What other information is critical? In the example below the capacitors have a choice of 25 Volt or 50 Volt. The choice may be from information in the circuit or it might be that I can simply not get that part with that parameter in a reasonable time. (There is an example of that coming.) So here are the early columns of the Info BoM:
Moving right along the row the next type of information is the source of the part or the Vendor. Also what number the vendor uses for the part that I want. In the case of this capacitor the Venkel company (www.venkel.com) comes up most often. In decoding the part number I see that one of the 50 Volt versions of this capacitor is an X5R dielectric and the other is a X7R dielectric. I won’t discuss the trade offs between the two here, but sometimes it is important. (A beginning discussion can be found at http://www.circuitsvilleeng.com/sabertc.htm
Moving further right shows some of the reasons why the 25V capacitor was selected instead of the 50V. These columns show what the individual cost of the component is, the Unit Cost, and how many I have to purchase to get that price. In this example the part cost is $0.01 or one penny each but only if I buy a reel of 4000. This means an upfront investment of $40 for these parts. That price is relatively low, for example I had to purchase for a customer 390 of a part for $2.59, or over a thousand dollars of up front cost even before any were sold.
The next two columns are ones that I find most useful for a project. The first is what these parts cost on this board, or the individual part cost times the quantity on the board assembly. The costs in this column are totaled at the bottom of the list and used in the next column. The column heading is “Percentage of Total” and is the cost of this part type is as percentage of the total assembly cost. This column is useful because it tells me where reducing part cost would have the most impact on assembly cost. For example using one less capacitor saves me only a penny and less than 1%. But a buzzer at $3.49 each is 13.9% of the total, so if I could find a cheaper buzzer I could save some significant money on the assembly. This savings could be used to reduce the cost of building the product and thus either reduce the price so as to sell more units or simply to increase the profit margin.
The final two columns answer the question I posed earlier in this discussion. The question was why use the 25 Volt capacitor instead of the 50 Volt version. The Lead Time is how much time the vendor needs to build the parts before they can be shipped to me. For the 50 Volt capacitors the lead time was 10 weeks. (Yes, almost 3 months.) The 25 Volt capacitors could ship the next business day, so after checking the electrical requirements waiting 10 weeks was not necessary.
The final column is an inventory I keep on parts for each client. Not only do I have to find all the information about these parts but when the order comes to make an assembly I need the parts, either to build a small quantity by hand or take to an assembly house to run a large batch. If I don’t have enough parts I better get the order out in time to keep the schedule moving.
That was the basics of Bills of Material, how I use them. Next I get to try to explain board layout.
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