BOM and Stuckup Layer

[diegoalves]

Hello, I'm designing a linux board, and came across some issues:

1 - When is the moment to contact a manufacture house to provide the stackup layer? When the schematic is finished? or layout?

2- In assembly house, is it a good practice provide the BOM to see if they have all the components? or modify my schematic to the components they already have? I want to reduce cost.


Thanks!

[mairomaster]

1. Once you have a good idea about how many layers you will need, what difference impedance controlled signals you have and on which layers you can request a stack. There are different advantages to get it earlier rather than later. You will know what track widths/gaps you have to use and won't need to fix them later. If you've than most of the routing when you get the layer stack, you will need to change lots of widths. That could create different problems, such as not enough clearance if track's width/gap goes up. Also you will need to re-route all differential pairs since normally you don't have an easy way to change the gap. If you get the stack too early and realise it needs modifications later on, it's not a huge problem since the manufacturer would be OK with modifying it for you.

2. I always provide the BOM to the assembler. I think this is a must since it is the most basic and standard document describing the parts in your assembly. I have a column stating which parts can be replaced by their own components - normally that would be all chip resistors/capacitors and some other parts if appropriate. If you establish a good relationship with an assembler and use it all the time for a particular design, you can eventually switch some of your components according to what they have in stock, to make it easier for them.

[robertferanec]

1) The easiest is to re-use an existing stackup If you need a new stackup, depends on a project, but it can easily take 2-3 weeks to communicate with PCB manufacturer. Normally, if we need a new stackup we ask for it when everything is connected and I know we still need at least 3 weeks completely finish the PCB. Otherwise, as mairomaster mentioned, the best is to ask "once you have a good idea about how many layers you will need". (PS: We normally route PCBs with track width and clearance bigger than we are expecting to have them and make the tracks thinner by the end of the layout to meet impedance requirements. However, if you do not have final stackup before you start layout, differential pairs usually needs to be re-routed).

2) We always provide BOM to assembly house - together with the components they should fit.