Hi

I am working on design with a GSM module, which will require 50 ohms impedance to the RF connector.

Do I necessary need to ask the PCB manufacture to create the PCB using Impedance control or can I simply use there standard process by adjust my design to achieve 50 ohms impedance?

 

Many PCB houses, if you do not specify a stack up, you get what they want to make. They have a default stack up. When I say I want to impedance match, then pick the default they will definitely make the default and the price does not go up. I talked to one of the engineers about controlled impedance and he subjected I push the controlled impedance button to remind them that I care about the stack up. (even if I want the default)

I am using a program "PCB Toolkit V8.05". It helps with stack up impedance. I can see what trace width works with this stack up. I just ask for 0.1mm or 0.2mm or (what ever) insulation between the top layers.

 

Thank You for your reply

I am using a company called PCB way, they have stack up as shown below.




Based on my calculations 50 ohms impedance can be achieved with stack up by keep the trace to 0.50mm, which is ideal as my pads sizes are also 0.5mm. Spacing the the trace from GND on the top layer is not an issue.

So my question is does the PCB house, do anything other then arrange a stack up when you request impedance matching?

 

So my question is does the PCB house, do anything other then arrange a stack up when you request impedance matching?

I think they do nothing. I think they arrange the insulation and copper. They tell you the dielectric constant.

 

As the designer, you tell them what to do. You pick the board material and thicknesses for the layers, the prepreg thicknesses, etc. usually, you get the parameters from them for their standard materials and adjust your design to get the performance you want. but if you can't get the results, you direct them to different materials / thicknesses / whatever.

Not all board shops have decent in-house impedance test equipment. For the ones that do, what you can do is add a "coupon" to the pc board layout. This is a long, narrow separable strip down one side of the board, with contact pads at each end and a trace between them, forming a track impedance test area. Since the plane thicknesses and track etching are the same as for traces in the main board area, the impedance of the test coupon is accepted as verification of the impedance of the other traces. In some high-reliability (MIL) projects, we had to have matching serial numbers on the boards and their coupons, with a written test report for each board, and keep the coupons available for inspection for xx years.

ak

 

The board house gets 5 tons of 0.20mm sheets of material that has a dielectric constant=____. They cannot go back and sand off 0.01 off of a sheet. They cannot adjust the constant. They have what they have +/- some amount. They are captive to their suppliers. I cannot see a way for a board house to adjust anything.

 

Its a good idea to have a sample test traces, maybe i will added single trace and a differential traces on the side.

Yes, I agree they have standard stackups, it just we got quoted a higher rate for impedance control. But one of the standard stackups can also achieve the impedance we require.

 

I cannot see a way for a board house to adjust anything.

There is a way - a board house with better suppliers. Over the years we had boards made with five different materials. Standard FR-4, an FR-4 variant with a mold/fungus inhibitor additive (for MIL projects), blue phthalate (?) to match a customer's "look", and two different materials for very high speed; one for 10GBASE-KX4 (10 Gb Ethernet in 4 pairs), and one for 10GBASE-KR, where the whole 10 Gbps is in one signal pair. The shops we used in New Hampshire stocked both FR-4 variants in many thicknesses. I vaguely remember some bragging about 28 layers in a 0.125" backplane.

ak