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Subject: Re: [geda-user] Thermals: size problem, SMD
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References: <xny3eg42eu DOT fsf AT envy DOT delorie DOT com>
From: "Richard Rasker (rasker AT linetec DOT nl) [via geda-user AT delorie DOT com]" <geda-user AT delorie DOT com>
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Date: Fri, 13 Jul 2018 11:52:41 +0200
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Hello DJ,

Op 12-07-18 om 22:36 schreef DJ Delorie:
> "Richard Rasker (rasker AT linetec DOT nl) [via geda-user AT delorie DOT com]"
> <geda-user AT delorie DOT com> writes:
>> 1: When I create small vias with a thermal, e.g. 0.3 mm drill width,
>> 0.15 mm annulus width and 0.15 mm clearing, the resulting four thermal
>> connections to the surrounding copper plane get so narrow (just below
>> 0.125 mm), that my PCB manufacturer flags a warning.
> The Thermal[] entry in your *.pcb sets the thermal ratio; the default is
> 0.5.
>
> http://pcb.geda-project.org/pcb-4.1.2/pcb.html#Thermal-syntax

OK, this goes some way to explain things.
And I see that the actual thermal finger width is always very slightly 
less than the ration mentioned, so a clearance gap of 0.13 mm (not 0.15 
as mentioned above) results in a finger width of 0.124 mm. Which is why 
my PCB house complains...

>> 2: SMD thermals, or the lack thereof. If I really need thermals, I
>> create tiny rectangles without clearance (S key) to connect pads to the
>> surrounding copper planes.
> Lack thereof.  Technically possible, but difficult to guess what kinds
> of thermals work for the wide range of SMD pad shapes and groupings.

Yes, I gathered this already from earlier information here.

One important question still is if (and when) those SMD thermals are 
necessary, recommended, or merely a matter of preference.

I see the use in case I want to be able to take certain components off 
with soldering tweezers, but otherwise I never had any structural 
problems with component pads fully connected to the surrounding copper 
plane.

Best regards,

Richard Rasker