Guys, guys, guys, Entering a pad at 90 degrees is not considered an acid trap, never was. Not even a 90 degree trace corner, it is any acute angle entry below 90 degrees. For the 90 degree unmitered corners, it does effect to some degree impedance controlled traces simply because of the change in trace width area at the 90 degree corner, effectively changing the impedance of your trace at each 90 degree corner. A 45 degree miter has a much smaller change in trace width area at the corner and therefore a much smaller change in impedance at that corner.
There is an old wife's tale in the industry about electrons flying off the trace when it runs into the 90 degree corner,or that it radiates wildly (not in normal circuits below several Ghz). Sort of like a pile of wrecked cars at the bottom of a cliff where there is a 90 degree corner at the top of a cliff. Possibly the previous designer offering advice was referring to this or other variations of this myth over the years.
As Gary said, teardrops are actually for two mechanical reasons. They decrease the probability that a drill mislocated towards the trace entering the pad/via will result in a minimum annular ring violation severely reducing the traces connectivity to the via/pad. It also reduces the chance of mechanical cracking of the trace at the point it enters the pad/via due to thermal shock, solder joint stresses and board loading in high stress applications. I disagree that teardrops ever had anything to do with acid traps because angles of 90 degrees were never considered an acid trap, acute angles less than 90 degrees was an acid trap.
As for Gary's comments on the state of technological affairs, I also agree that it is not so much of an issue with most fabricators these days. However, it has not gone completely the way of the Dodo bird. Cleaning techniques are much improved these days and acid traps are nearly a thing of the past with most fabricators. At least until you get down to acute angles less than 45 - 30 degrees.
--
Sincerely,
Brad Velander
"Gary Crowell / VCP" wrote in message
news:8udn31dcji25fob6kd69991be2jjbbf0q4@4ax.com...
>
>
> Ah, at trace-pad intersections, what you are referring to is
usually
> called \'teardropping\', that is, using added traces at the
trace-pad
> joint to \'feather\' the trace into the pad. Done for three
reasons
> that I can think of.
>
> 1. to eliminate the acid trap. (and, as I mentioned, not
usually
> important anymore.)
> 2. to relieve the stress concentration and cracking potential
at the
> joint.
> 3. to reduce the potential for cutting the trace by a wandering
drill.
>
> You\'re right, this would be extremely tedious to do by hand,
though
> many CAD/CAM systems will do it automatically. You could even
ask the
> board fab house to do it for you, and it should be no charge in
most
> cases.
>
> Two notes: In some pin layouts, interstitial connector pin
patterns,
> in particular, teardropping can cause DRC errors due to reduced
> clearance. Teardropping with arcs can reduce this problem, but
I\'ve
> not seen a CAD system that does this.
>
> An alternate to teardropping is a \'snowman\'; just a smaller
round pad,
> off-center from the pad, that overlaps the trace-pad joint.
Probably
> easier to do if you have to do it by hand.
>
> Gary Crowell CID
> Micron Technology