While checking a layout for a client I saw this for the umpteenth time: Trace connections to a plane via thermal relief, not in a solder area where it would thermally matter.
Question: Was it taught once upon a time that thermals must be used everywhere? Or is there some other reason why it's done so often?
Joerg, I would have to believe that it is the old problem of a little knowledge being misapplied by the uninitiated. A general comment on a rule or practice then being taken to further extremes and actually misapplied. Sometimes it may be software initiated, some packages actually have default rules that start out your design with thermals on 'all' plane connections. i.e. Protel P99SE (quite possibly the current AD software also), the default plane connection style rule on a new PCB is for "Board" (everything, vias and pads) to be thermally relieved. So the less knowledgable and experienced get introduced to the silly practice by their acceptance of software package defaults.
Certainly seems silly doesn't it? Then there is the problem of all the discontinuities caused in the planes by these unnecessary thermal reliefs. It is fairly common to see required thermal reliefs that are actually starved connections because of all the unecessary reliefs impeding a good connections to those required points.
It is easier to have a consistent style of trace to plane (or to polygon) connection throughout the board. For many types of board, thermal reliefs are just as good as direct connections - they are only a problem if the relief wires are very thin, or if you are working at very high frequency, high currents, or with high accuracy analogue signals. Personally, I use thermals everywhere if I am doing a simple card, and direct connections everywhere if it is higher speed or has smaller geometries.
mvh.,
David
Makes it easier to wire in a fix later.
I would guess that the layout person is inexperienced. In Orcad Layout, you need to specifically check off a box to get rid of thermal connections for a particular pad stack.
Yes, I've had one case where a supply plane split open into four parts because the fab house took the liberty to "improve" the clearances. They ended up doing another free blitz run for us.
Not really anymore. Most boards have this all covered and vias are so small anyway. Plus, don't we all design perfectly so no rework is ever needed ... ?
My CAD software automatically has an underlap from the power plane to the pads. SO the only connection to the pwoerplane is via a normal track.
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