5 Tips in BOM When Asking PCB Assembly Price Estimate

BOM (Bill of Materials)  is the basic for Print Circuit Board assembly. Makerfabs will tell 5 tips in BOM List when asking for a PCB assembly price estimate.

For most makers& new start-up electronic projects, it is important to get a project cost estimate before the final production or even the prototyping, a quick path forward is outsourcing Shenzhen electronic factories. But it is not Schematic plus cash for a final product, a clear parts list, all called BOM, is the very basic step for quotation, and good PCBA production.

Basically, the BOM is fundamental for components sourcing and soldering. Every single component needs to be spelled out unambiguously, for a third company to understand. Any missing or incomplete words may lead to troubles such as delays or cost overruns. It can’t be too careful to make a complete & accurate BOM for a quotation.

Let’s begin with a BOM,  that seems good and sufficient, for an electronic engineer:
                                          

This BOM, together with the parts list, seems OK for a prototyping production. BUT, actually, it is far from a normal quotation, or cost estimation. There is some important info missed:
1. Tolerance.
2. Approved Manufacturer
3. Package
4. Parts number
5. Other accessories, such as package/programming..

Tip 1. Tolerance
For passive components such as resistor/capacitor, some key parameters:
1. For resistors, besides the resistance/package(0805/0603/0402..), the accuracy& power is indispensable.  A 4.7k, 1% 1/8W resistor would be cheap and OK as a pull-up resistor, but a 1 ohm, 0.05%, 1/2 w resistor would be very expensive.
2. For capacitors, at least the tolerance, voltage rating, and dielectric type should be specified. If special parameters such as ESR, specify them there.
3. For inductors, unless a simple 0603/0402 inductor for isolation, detailed give the info of parts number.. no substitute inductor suggested.
 

Tip 2. Manufacture

For components especially analog items, such as Diodes/ Triodes, and some important connectors with big current, it is necessary to specify the manufacturers, as the same diode (1N4007) from different producers may be different, this also happens for many connectors/switch/LEDs.
 

Tip 3. Package

It’s important to fully specify the form factor, or "package type", of a component. Poorly specified or under-specified package parameters can lead to assembly errors. Beyond the basic parameters such as the EIA or JEDEC package code (0402, 0805, TSSOP, etc.):
  1. For capacitor/resistor, the package 1206/0805/0603/0402/02021...
  2. For IC, normally there THT version& SMD version, and for SMD, sometimes QFN/SOIC/BGA, etc...
  3. For Electrolytic capacitors, the mechanical include height/diameter…
 

Tip 4. Parts Number

The parts number can be the final, while the most useful tools, that one parts number (manufacture number/Digikey/Mouser) solve most of the communications problem. the factory can easily check the detailed info with part numbers, to avoid any misunderstanding.
There for this, We Makerfabs has had really serious problems before. A customer Rond send us the BOM list, with a PIC chip: 24F16KL402, totally quantity 2000 pcs.

Checked the Digikey, with “24F16KL402”, it shows 2 items, almost all the same:


So the sourcing engineer thinks these 2 items may be the same….Only package difference, as personal experience.

But finally found that they are different, with the PIC24F16KL402-I/ML-ND 6x6 package, while the  PIC24F16KL402-I/MQ-ND 5x5 package…


Although Makerfabs bears the loss for this accident finally, this indeed a serious communication error because of bad parts number, but not only a single "24F16KL402".
 

Tip 5. Other accessories, such as packaging/programming/Testing

Normally you can ask the PCBA factory for some other service such as packaging, you need to prepare them in the BOM list, so there they can estimate the availability/ lead time/cost.
 

Revised BOM

After the modification, the BOM comes out:


As the factory gets this BOM,  they will fulfill them with price, depends on the quantity, the unit price varies from 5 pcs PCBA prototype to 1000 pcs PCBA production, which we will discuss some later. Then you can consider if there the quotation is OK for you and if you will proceed.

Finally, to ensure all the components are exactly what you want especially for PCBA mass production, as all components are prepared OK, ask your vendor to solder one sample circuit board to you for confirmation, and also to ensure the components in the sample board are same as that in mass production. 

Hope those 5 tips in the PCBA BOM list will be helpful when you are asking PCB assembly cost estimate.

This article is first published by Makerfabs.