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| multiple vias | ||
| Author stgod Views 12 Posted at 2008/8/27 14:52 [View In Forum] | ||
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Hi,vpxfl vpxfl I'm a beginner in layout designing. I wanted to know why do we use multiple vias n contacts instead of a single via and contact.. |
| Reply No. | Replier | Total Replies 12 [View All Replies] [New Reply] | Replied at |
| 1 | dongyea | people use multiple vias to connect to different layers of a board foristance three layers may need to talk to one point but not at the same time via current limit resistors you can use a single via and contact if you like or use only top and bottom etc... |
2008/8/27 14:52 |
| 2 | htc2498 | usually a via will introduce "20ohm of resistance (90ns process), so if high current presents or you really care about the routing voltage drops, then multiple via will help. especially for cross layer power bus routing connection, you shall see tens of vias or even hundres of vias to reduce the resistance. | 2008/8/27 14:52 |
| 3 | ybyygu | Aditionally to the above answers, yield is also a factor for placing multiple vias or contacts. Especially contacts often cause yield problems in sectors where failures are practically unacceptable (eg. automotive). Some place&route tools are capable of doing post- routing via optimization in order to place double vias where possible to increase yield. Regards, C. |
2008/8/27 14:52 |
| 4 | ken7976 | Lets look at this query from a different perspective... Via is a connection between two metals. Imagine there is a metal 2 layer carrying 300uA of current which has to be connected to the source/drain of a transistor. Lets say, you put a single via and a single contact. This will usually have a size of around 0.2x0.2 ie 0.4um?.. So this actually means the 300uA of current has to reach the diffusion regions of the transistor through this small region only... This will lead to problems as this area gets heated up when compared to other areas due to high current density which will lead to a very short life for the device. Also, as long as the device works, it'll see a lot more resistance than expected which might lead to the failure of functionality as well... So one should not be happy to have put a wide metal at places carrying more current but also suitable increase the contacts and vias to enable easy flow of currents between the different layers. Hope this helps... I am a design guy. So layout guys, please correct me if I am wrong... |
2008/8/27 14:52 |
| 5 | hualeyan | thanks what is the advantage of using smaller vias than using one large via.. n contact. |
2008/8/27 14:52 |
| 6 | htc2498 | The contact and via sizes are usually fixed for a particular technology and fab... (Again, layout guys correct me if I am wrong)... Every via/contact will have some resistance.... By using many of them, the resistances will become parallel and hence effective resistance decreases... hope this helps... |
2008/8/27 14:52 |
| 7 | z315 | hi srieda... i agree with ur point but im working on BJT... n the foundry does not provide with fixed via or contact size. it just provides the minimum size.... i can go for a larger one... so i want to know which is preferable??? single large via/cont or multiple small vias?? |
2008/8/27 14:52 |
| 8 | htc2498 | hi all why isn`t anybody talking about reliability concerns with single vias and contacts??? Regards Brittoo |
2008/8/27 14:52 |
| 9 | mxg330 | How do we calculate the number of via required. | 2008/8/27 14:53 |
| 10 | ct731 | 2008/8/27 14:53 | |
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