Thicker than 1.6mm PCB Stack-up

[guscrown]

Most (or most likely all) of my PCB stack-ups are 1.6mm (62 mils), but I am now in the need of doing a thicker PCB due to mechanical reasons (we need a thicker board so the customer doesn't break it easily when pressing on buttons).

I was thinking of going up to 2mm (78 mils), but I'm not sure if I should just increase the dielectric thickness in the middle of the board?

My board:

Layers: 4 (L1 = signal, L2 = ground plane, L3: power plane and some signals, L4: pads for buttons).

I have a pair of differential signals on L1 that I need to control their impedance to 90ohms, this is my original thinking:

Top Solder: 0.4mils
L1 - Copper: 1.4 mils
Dielectric 1: 6.7 mils
L2 - GND plane: 0.7 mils
Dielectic 2: 60.3 mils
L3 - Signal and VCC: 0.7 mils
Dielectric 3: 6.7 mils
L4 - Copper: 1.4 mils
Bottom Solder: 0.4 mils

Total: 78.7 mils or 2mm

I just simply added the extra thickness to the center of the board, but I'm not sure if that is the ideal solution  . All feedback is appreciated.

[tggzzz]

How certain are you that 2mm is sufficient where 1.6mm isn't? My gut feeling is that if someone is breaking boards by pressing buttons, then 25% thicker isn't guaranteed to solve the problem.

19" racks of military equipment traditionally had the bottom 6U left empty  - because that was the part the squaddies kicked in frustration!

[guscrown]

I can't be 100% certain, to be honest. Other than what the mechanical engineer required. I added additional mounting holes for additional support but they still want the board to be thicker.

I've seen PCBs go up to 1.8mm, but I don't really know where the extra stuffing goes, I am simply assuming that it goes in the core of the PCB

[SeanB]

Go for a 6 layer board, with the front and back being the added layers. Make the front a ground plane with large clearances around the switches and connectors, and the back layer the same. The copper will provide extra bracing against tension on the rear and at the front, which will make the whole board stiffer and also thicker. That will get you over the 2mm mark, and you can simply use vias to bond all the ground planes together, including the one inside, and the controlled impedance traces will benefit from the 2 layer ground plane both sides as well.

As well leave a row clear on the top and bottom of the switch rows that is about the spacing of the switches, and place a via there by each switch that has clearance in all layers of about 5mm, so that if needed you can drill it out and place a support brace direct by the switches without needing a board respin, just a simple extra drill operation.