Blind/buried vias vs using a second board with castellated mounting holes

[gperoni]

On the subject of designing the smallest possible electronics device I would like to have a board with tight components on both sides. One relatively expensive option is to go with blind/buried vias. A second option is to threat the other side as a different board, use two cheap multilayer board stackups and solder them together manually after single-sided assembly.

This would be super inexpensive to prototype and cheaper to manufacture at small (and maybe medium) scale.

Is there anything oblivious I'm missing? I don't think I've never seen this done before.

[elecman14]

Labor cost of the PCB module would probably be higher than a HDI pcb. You also have the cost of at least two PCBs to deal with. You may be able to make the HDI pcb smaller so it may actually be cheaper or about the same cost as spinning the PCB stack up you suggest.

[asmi]

Don't be afraid of advanced multilayer boards. In volume even advanced PCBs's price is usually insignificant compared to parts cost. They are only expensive for the first time due to large tooling cost, but this payment is paid only once for the specific set of Gerbers (and this usually happens when you build last pre-production batch), after that you only pay for actual boards and finish. To give you an idea, my last order for 10 impedance controlled 6 layer PCBs (10x10 cm each) was $148.95, and of those $86.51 was tooling cost (58% of the price!), and $30 for ENIG 2um finish (flat rate), so actual PCBs were only $32.44 for 10, or $3.24 each. And this is for only 10 boards, if you order more boards - the price per board will go down significantly.

[TimCambridge]

I know you didn't say it but is $86 the street price for HDI tooling for a 10cmx10cm board? By HDI I mean laser microvias and 0.1mm (4 thou) tracks. Nothing super fancy, just support for 0.5mm BGAs.

And any recommended HDI PCB house?

[SiliconWizard]

Depending on the PCB manufacturer and the pitch of the castellations, getting metallized castellations could cost you just as much.

[asmi]

I know you didn't say it but is $86 the street price for HDI tooling for a 10cmx10cm board? By HDI I mean laser microvias and 0.1mm (4 thou) tracks. Nothing super fancy, just support for 0.5mm BGAs.

And any recommended HDI PCB house?

No, my board only had 0.2 mm vias and 0.1 mm trace/spacing, though the fab can go down to 0.15 mm vias and 3/3 mil trace/spacing. You can send them an email and ask if they can do microvias.

[asmi]

Also - I think most of my tooling cost was for impedance control as their production engineer had to perform simulations to ensure the stackup they are going to use to make boards is going to produce impedances I've requested.