Get more benefit from COB technology in PCB Assembly fields
This article try to bring some ideas for the PCB designer who want to launch COB (Chip On Board) technology on the boards.
Firstly, we got more pressure from the end customer who need more competitive products and we might be seeking a new way to meet quickly change demand. COB technology is really make a lower cost on the boards, it's cheaper way to use bare DIE's assembly boards compared the traditional surface mounted technology (SMT). We know that Die's are more cheaper than a SOP pacakge parts. Secondly, we can design a smaller PCBs. That means we save PCB material cost at beginning. Thirdly, COB manufacturing process is very common in China. And thus we can get a cheap assembly cost.
How to make our own boards with COB technology? It's a better way to get some advice from PCB manufacturer and assembler. This can ensure our design achieve a very satisfied result.
Low cost COB technology is usually use FR4 base material and finish way on PCB is Ni plated, Gold plated, immersion gold. Note that H.A.S.L is not working here. We have to keep this in our mind. When we choose COB on our boards, a mixed SMT and COB process is recommended. Efficiency would be so perfect that we use COB and SMT process. PCB assembly manuacturer dislike COB and Through holes mixed process, cost will be higher than the formal way.
Some rules on COB PCB design
1, Gold wire's diameter is 20um~30um, so we have to make ball pad at least 2.5 times larger than gold wires. Making pad like a finger shape.
2, Power ring and ground ring is usually between Die's pin and Pads and make power ring and ground ring is coated with solder mask. Short circuit is big issue on process if we did not make good insulate on solder mask on this.
3, Dimention of finger's pad design
| A | distance between finger pad |
| B | clearance of pad |
| C | width of finger pad |
| D | distance between finger and soler mask |
| E | minimum clearnce between finger pad and wire |
| F | minimum solder mask on Via |
| G | minimum distance between solder mask |
4, Required dimension size for wire and bond width
5, Clearance distance ( B on figure4) between two finger pad should be big enough to keep short circuit issue. Usually the length of bond wire should be less than 5mm, and the recommended value is less than 100time diamter of wire( 100D).
6, Glue requirement on the chip when cover the Die.
7, Longest wirebond calculation
I hope information above can give you some idea when you design a COB boards. If you want to COB borads to be assembled, please contact me at info@onlitex.com . I am Mr.Allen Shieh from ONLITEX Electronic Co.
More information about ONLITEX please visit http://www.onlitex.com
I am planning to write more articles refer to PCB Assembly fields in the following days. Give me some time! ^_^
2009-12-06
Allen Shieh - ONLITEX
2 comments:
COB provides high packing density, quick turnaround, enhanced thermal characteristics, can be adapted to high frequencies, can mix standard assembly technologies and is applicable to most substrates. Thanks a lot.
These kind of post are always inspiring and I prefer to read quality content so I happy to find many good point here in the post, writing is simply great, thank you for the post.pcb suppliers
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