Nowadays, we meet new challenge in loadboard design for PHY Chip: 112Gbps high speed, up to 8 lanes TX-RX loopbacks with 0.1uf cap, 0.5mm BGA pitch. Usually, it will need 3-5 layers to route the loopback, do length match and add the isolation via. As a result, the board fabrication risk is getting much higher, and also,the tight SI spec is getting much harder to meet.

In this paper, two traditional solutions have been verified. The first one is PTH via backdrill solution, it can be used when BGA is 0.8mm pitch or larger, it can help easily pass the simulation spec, however, it is not suitable for pitch with 0.5mm or smaller, because there is no enough space to do signal optimization, which leads to bad return loss and crosstalk. The second one is Blind via solution, as blind via has no stub, it can help to get good simulation result, however, it will need more than 3 types of blind via, which will increases 80% board fabrication risk and the board yield will be very low.

To overcome this dilemma, a new solution combining the advantage of above two solutions is proposed, it is laservia+PTH backdrill solution. With the help of laser via, it allow us to route the loopback in outer ring of BGA in 1 layer, so that to other loopback in inner ring, we can easily do fanout use PTH via, and then backdrill the via stub. In this case, there will be larger space to do signal optimization, which will guarantee the impedance of whole signal path to be controlled within 10% tolerance.

As BGA fanout vias should be filled with epoxy, to be more serious, in this paper, the effect of air over stub and epoxy over stub after backdrill is also discussed. It is verified that air over stub hardly leaks the energy away, it will have better insertion and return loss, which means we should apply epoxy filling first, back-drill last.

The final result shows we get good signal performance (0-28Ghz, Insertion loss=-4.27dB, Return Loss=-12.14dB, Power-Sum crosstalk=-40dB) and low board fabrication risk, and also the board lead time can be reduced by 40%.