What it is PCBA?
To assembly components on bare PCB, it is PCBA.
PCBA=PCB（bare PCB/ blank PCB）+ Components+ Assembly (SMT and DIP)
That is to say, the finished PCBA is the PCB with components assembled.
What makes the assembly cost different between PCBAs
1) Both sided assembled PCBAs will be expensive than single sided ones.
As we can see, some PCBAs with single sided components assembled, such as many LED lights, which just need SMT on solder mask side, and it just need one piece stencil.
For some PCBAs with both sided components, they have to start up twice, one side one time. And they need twice stencil for SMT, one side one piece. Except they need more production time, but also more production cost, such as solder paste and stencil etc.
2) The more artwork of the PCBA have, the more cost it will be.
Some PCBA need SMT only, some need hand soldering only. But some PCBA need SMT and hand soldering both. Usually we choose wave soldering or hand soldering, it depends on the PCBA order qty, components features in the bom and the cost that customers can afford etc. Compared with hand soldering, wave soldering is more suitable, faster delivery and well quality control for bulk order;
1) The type of components packages also effect their cost; The more types of packages , the higher of assembly cost. And the more points for assembly of PCBA, the higher of the cost;
Some components package, such as 0402,0603 and 0805 are the most common packages. They are cheaper than packages such as 01005, QFN, SOP and BGA etc per component. The smaller of components package dimension, the more difficult to assembly. Because they need more accuracy. And QFN,SOP and BGA etc with more pins.
Here attached some components for reference:
The Package of Components on the SMD Surface
Dip before Wave Soldering
|1||Chip||Resistor, capacitor, inductor||0201,0402,0603,0805 etc|
|2||MLD： Molded Body||Tantalum capacitor, diode||A,B,C,D|
|3||CA：Aluminum Electrolytic Capacitor||Polar Aluminum Electrolytic Capacitor|
|4||Melf:Metal Electrode Face|
|5||SOT:Small Outline Transistor||IC||SOT323, SOT353,SOT363,SOT23,SOT143,SOT223, SOT143 etc|
|8||SOJ：Small Outline J-Lead||IC||SOJ|
|9||QFP:Quad Flat Package||IC||QFP|
|10||QFN： Quad Flat No-lead||IC||QFN|
|11||SON： Small Outline No-Lead||IC||SON|
|12||BGA： Ball Grid Array||IC||BGA|
If you would like to know more, pls feel free to check on Digikey or Mouser.
To the point, the assembly cost mainly take them for reference, but far more than those. Anyway ,if you would like to know the detailed price of your project, pls feel free to share your PCBA information with your PCBA vendors , such as gerber file, bom list and function test etc. Then they can offer us the best and detailed price for your PCBA project. Or you can contact our company( SHENZHEN KINGFORD TECHNOLOGY CO.,LTD),which is a one-stop PCB&PCBA Turnkey Manufacturer in Shenzhen, China.