Changing copper weight is not as simple as changing the database setting or drawing.
On power-intensive designs, PCB designers feel like Scotty from Star Trek: caught between competing demands. The engineer needs more copper to pull more amperage, but the designer has to be aware of the board fabricator’s capabilities. Applying increased copper weight to power boards is not an uncommon approach, but designs requiring a great deal of power on high-speed digital boards are becoming common. Designers now juggle the needs of high-speed design and power in one board. Increasing layers is one way to increase copper coverage, but the additional layers will increase the thickness of the board more than increased copper weight. If the finished product can handle the increased thickness, and smaller vias are not required, an option is to increase the layer count. If the vias are dictated by the component (BGA fanout or limited routing channels) or routing parameters, then increasing copper weight is an option.
Changing copper weight is not as simple as changing the database setting or fab drawing. Close communications with the fabricator are crucial in the success of the final product. Changing the copper weight increases the board thickness. The increased board thickness changes the aspect ratio of the vias.
A 62 mil board with 8 mil drills is acceptable for the majority of fabricators. Once the copper weight increases and causes the overall board thickness to increase to 80-plus mils, however, the via aspect ratio becomes a concern for some fab shops. The increased copper and increased drill affect the anti-pad for internal layers. The majority of fabricators used for the latest high amperage board wanted a minimum of 22 mils over drill for 3 oz. The increased anti-pad permits less copper to flow through via fields. If the copper is flowing through a BGA, the change in copper weight could be robbing Peter to pay Paul (Figure 1). The increase to 3 oz. reduces the copper channel between the BGA fanout. Gaining 1 oz. of copper loses 3 mils through the channel. If there are several channels through the BGA, then the decision becomes easier.
Figure 1. Increases in copper weight can distort the via aspect ratio.
There are other concerns when using higher copper weights in the design. Material availability becomes an issue as copper weight increases. While 3 oz. may be a common size in the US, it might not be in another market. Once product ramps to volume, 3 oz. for production runs may not be readily available. Verify with the expected fabricator the availability and their process limits. If the end-product requires UL approval, ask the fab for the highest UL-approved copper weight. Shops are able to get higher copper weights but may not be UL-approved for higher weights.
Close communication with the fabricator will ensure an on-time, robust design. Keeping communications open and honest with the fab vendor is the shortest route to a successful build.
Noah Fenley, CID, is design manager at ACD (acdusa.com); This email address is being protected from spambots. You need JavaScript enabled to view it..
