5 Tips in BOM When Asking PCB Assembly Price Estimate

5 Tips in BOM When Asking PCB Assembly Price Estimate

July 15, 2019

5 Tips in BOM When Asking PCB Assembly Price Estimate

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

For most makers& new start-ups 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:
                                          BOM( bill of materials)

This BOM, together with the parts list, seems OK for a prototyping production. BUT actually, it is far from for a normal quotation, or cost estimation. There 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 necessary to specify the manufactures, 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're THT version& SMD version, and for SMD, sometimes QFN/SOIC/BGA, etc...
  3. For Electrolytic capacitor, the mechanical include height/diameter…

 
Tip 4. Parts Number
The parts number can be the final, while the most useful tools, that one part number (manufacture number/Digikey/Mouser) solve most of the communications problem. The factory can easily check the detailed info with part number, to avoid any misunderstanding.
There for this, We Makerfabs has 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:
“24F16KL402” on Digikey

So the sourcing engineer thinks these 2 items maybe 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…
PIC24F16KL402-I ML-ND 6x6 package
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 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:
Modified BOM List

As the factory get 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 OK for you and if you will proceed.

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