THS3091EVM: What knuckle-head designed this PCB?

Hi David,

Sorry to hear that the THS3091EVM layout was not up to the thermal relief standard. Are you having any trouble using the THS3091EVM?

Best Regards,
Rohit

Thanks! The 3491 looks great.

Sorry for the rant. But encountering such goofs indicates that the layout engineer was open loop on this project.

Cheers,

David

Hi David,

yes, solder pads in a ground plane without heat traps are nasty...

I use hot air soldering when reworking SMD PCBs (low air flow and 350°C). Also, I use a cheap coocking plate with thermostat for the preheating. I do not put the PCB directly onto the cooking plate but have a huge aluminium heat sink between. This massively increases the thermal inertia of the cooking plate and by this drastically reduces temperature ripple. With standard FR4 I do only choose a preheating temperature of about 100°C. With this setup reworking is just fun...

Kai

Hi Kai,

Yes. Yes. and Yes!

I have the same setup!

I also have a dual thermocouple meter from Omega that I use to monitor the PCB and cooking plate temperatures.

I am currently looking for some silicone rubber pads at the kitchen supply store. I plan to cut them up and line the leading edge of the cooking plate. I find that when I am working over this setup with my microscope I sometimes touch my forearms to the hot edges of the cooking plate. Ouch!

I guess I could wear a silicone rubber jacket :-)

David

Hahaha!! I know exactly what you mean... :-)

Kai

I have been receiving help on another thread when I saw this. I run a professional PCB layout house so thought I might just comment on this.

Vias in flooded planes would normally have no thermal relief, this is to reduce inductance and resistance to an absolute minimum and normally nobody needs

to solder directly to a via. This also applys to vias connecting to inner planes.Pads that are used for external connections that may be hand soldered would normally

have thermal spokes to ease the soldering UNLESS these are high current points where again the thermal spokes would be removed. With a good professional soldering iron its not 

difficult to make joints like that.

Also IC's with central ground pads even if not required for thermal purposes, would normally have vias placed as again this reduces parasitics to an absolute minimum especially if there is an internal ground plane. 

All of this would be the norm for a professional PCB that would be built by a professional assembly outfit, these issues would not create any issues in a re-flow oven, however I do

understand that to the hobbyist it could create problems. The board is probably aimed at professional users.

Iain 

Iain,

Thank you for your thorough description of the motivation for using thermal spokes.

You are absolutely correct that the presence of thermal spokes impacts the parasitic inductance.

I'm sure you have measured S11 with your VNA to compare circuits with and without these spokes and found the difference to be of the same order as the uncertainty in your calibration kit and test fixture.

An evaluation PCB is a different beast from his mass produced cousins. An evaluation PCB expects to be modified (by hand) in order to rapidly determine suitability in the customer's application. (And hopefully this leads to increased sales for the chip vendor :-)

Please accept my apology if my use of the term "knuckle-head" upset you. I was expressing frustration while sitting at the electronics workbench at a customer site. Their solder rework tooling was not up to the task and I did not have the time to take the board back to my lab.

You implied that a "hobbyist" would be ill prepared to face these issues, while a "professional" would not. My impression is just the opposite. (I have a PhD in electrical engineering with 39 years of professional experience.) My "hobbyist" friends have impressive, well equipped labs. My "professional" friends must fight management for something as essential as a VNA.

Fortunately, I am both.

Cheers,

David

Hi David,

In no way did you upset me .... I was simply providing pretty basic info on this kind of thing. I do however agree that an eval board should be designed such that any modification or parts replacement should be considered and made manageable.

My term hobbyist was perhaps not the best .... I am a professional of 45+ years but am self employed thus no management to fight !!

Cheers

Iain

You and I live in the best of both worlds, hobbyist and professional.

What a blessing!

Cheers,

David