What is the BOM list?

BOM is short for Bill of Material, it is a list of raw materials, components and parts needed to manufacture a specific final product; it mainly includes the part number, name and quantity. It also may consist of the manufacturer name or supplier of the components, other function columns and a comment section.

In PCB assembly, BOM means the list of all the components needed in a printed circuit board, such as manufacturer, manufacturer part number, brief description, package, quality needed., etc. It is the critical link between the customer and manufacturers for PCBA production.

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    BOM list composition

    In the PCB assembly processing, the BOM needs to list the PCBA that is formed with what components, also the detailed description, quantity etc., for each component. For example, below are some common compositions:

    • Serial number. ( You can see how many components this PCB has according to this information. )
    • Part number of the components. ( Normally it means Manufacturer Part Number. )
    • Part Name. ( It refers to what components this part is, some BOM lists will not have this, but you also can know what this part is according to the designator.)
    • Specification / Description. ( This item will describe the detail of this part, like the value of the part, the size of the part, the colour etc. )
    • Package / Foot print . ( Packaging is divided into DIP dual in-line and SMD chip packaging, for example, the SMT capacitor package: 0603, 1005, 2012, 3216 etc. Some BOM lists will not have this item; you need to check according to the part number. Some BOM lists will put this in Specification / Description. )
    • Brand / Manufacturer. (Some parts will designate a brand, like some IC, while some features are not designated brands. You can use the common brand, like some capacitors and resistors etc. )
    • Quantity. ( This item will show how many parts are used in one PCB. For example, 2 of R1 means one PCB must use two R1 components.)
    • “ R “ means Resistor.
    • “C” means Capacitor.
    • “D” means Diode.
    • “U” means IC chip.
    • “SW” means Switch.
    • “F” means Fuse.
    • “JP / CN” means connector.
    • Substitutable Part. ( This item will show you the substitutable part. )
    • Comments / Notes ( This item will put some special requirements or notes for the parts.)

    The importance of BOM in PCB assembly

    PCB manufacturing involves a variety of raw materials. So how to determine the raw material requirements of PCB? The answer is the BOM list. The appropriate PCB component material list lets you know the components and materials required, their brand, quantity, size and other detailed information. Typically, in the case of PCB assembly, the BOM list includes cables, PCB board, surface Mount Assembly components and a list of circuit requirements, etc.

    Why are BOM lists so crucial for PCB assembly?

    From the PCB design, production, then assembly in PCB are very complex and lengthy processes. So, you must fill in the list accurately. Any mistake is a huge loss. Below are some reasons why BOM lists are essential for PCB assembly. 
    1
    BOM list is very convenient in that you can intuitively know all the parts the assembly needs, like quantity, part number, etc.
    2
    BOM list will help you estimate the number of employees required for a specific assembly according to the purchasing components and difficulty level.
    3
    BOM list is conducive to correct purchasing, production planning, and stable operation.
    4
    Before actual production will need to audit all details. At that time, BOM helps track purchased parts and parts available in stock.
    5
    It is crucial to accurately obtain the parts you want or parts manufactured by a specific manufacturer. A BOM list can well solve this problem.
    6
    If the parts can't be used in PCB, you can discuss and immediately provide other options according to the specification in the BOM list.

    Requirements for making BOM list of PCB

    The BOM list is significant because the quality of the BOM list decides the operation quality of the products. So BOM list-making is critical. Its making requirements are incredibly harsh. The requirements are as below: 
    • Rate of coverage: All PCBA products under production will need to make a BOM list, so the coverage rate needs to reach more than 99%. So without the BOM list, it is impossible to calculate the purchase demand plan and manufacturing plan for the PCBA assembly, and it is also impossible to do the component purchasing quantity control.
    • Timeliness rate: BOM production and engineering changes need to react very timely, any change will affect the purchasing plan, so the BOM list must be finished before MRP, and engineering changes must be completed before releasing components the production department. It has two meanings: A. Making in time. B. Update in time.
    • Accuracy Rate: The accuracy of the BOM list should reach more than 98%, the best is to achieve 100%. The evaluation requirements are: Compared with the bill of materials, the actual assembly parts disassembled randomly are counted by single-layer structure. If there is one discrepancy, the accuracy of this layer structure is 0.
    • From the three requirements above, you can know that strict requirements for producing the bill of materials (BOM) are immediately apparent on the paper. However, it is not only a common framework. The establishment of the BOM directly affects the operation of the ERP system. There is no good ERP without a good BOM.

    Factors to be considered when making a BOM list

    What should you do when you receive an order for 50 items of PCB assembly from a consumer? Besides arranging PCB production, you also need to consider the following points when making a BOM list: It is not a good idea to consider the total quantity required (considering 50 items of PCB assembly at a time). Instead, consider one PCB assembly item at a time, find out the type of PCB and the required components, list the part details of only one PCB assembly, and do the same for the following item. Ask your engineering team to divide the work and determine all required components. Send the list to your customer for verification after sort out.

    Determine the BOM list for each item after a final discussion with your team and customers.

    The BOM must show the “when”, “what”, and “how” questions related to the project.

    So, never make a BOM list hastily because that will be easy to miss some components or get the wrong quantity. That will cost a lot of round-trip mail and waste much production time. But most companies will provide the BOM list format, so it is convenient to fill in. But, except BOM list, your PCB components must be accurate and functional, so cooperating with reliable PCB assembly manufacturers and service providers are crucial.

    Steps in PCB BOM reverse engineering

    Some customers have PCBA samples, not have the PCB Gerber file and BOM list, but he wants to make that PCBA so that the customer can send the sample to the manufacturer, they can copy the Gerber file and BOM list, the BOM production steps as below:
     
    1
    Check whether the components of the sample provided by the customer are damaged, or whether there are polished chips. If there are polished chips, don't hurry to disassemble the components on the PCB first. Instead, take a close look at the circuit of the PCB to analyze the chip principle.
    2
    Remove the components on the PCB, and use the electric bridge to test the value for each component.
    3
    Some components will have a silkscreen to show their part number. You can copy them to put in the BOM. Some components, like triode, have code and should search the manufacturer and part number according to the code for later commissioning; If there is an uncertain part, you can analyze its circuit and principle.
    4
    Sort and file all components. And make it into EXCEL form for later production.

    Tips

    The difficulty of the PCBA can be judged roughly according to the number of materials in the BOM list.

    When you get the BOM list, you should check if the BOM are complete or not. Typically, the BOM list should have a part number, brand, tag number, quantity, packaging and description of components. What is indispensable are part number, amount and tag number. Without a part number, you don’t know what part it is. You don’t know where the part should be soldered in the PCB without a tag number. Without quantity, you don’t know how many parts you should solder in the PCB.

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