Positive Etchback

[davepcb]

I was recently asked to explain what the phrase "There shall be evidence of positive etchback on the inner layers of the printed wiring board".

I found a definition for it:

Etchback .
During etchback, in addition to smear removal, the glass fibers themselves are etched back from the hole wall. The goal is to remove about 0.5 mil from the top and bottom of the innerlayer copper so that it will protrude out from the hole wall. This creates three surfaces (also known as a three-point connection) for the copper to bond to during the making holes conductive step. Glass etchants include hydrochloric acid, ammonium bifluoride, and hydrofluoric acid (rarely used). Etchback with plasma can be achieved by varying the type and amount of reactive gases.

It seems that the part "remove about 0.5 mil from the top and bottom of the innerlayer copper so that it will protrude out from the hole wall" should have been "remove about 0.5 mil from the top and bottom of the innerlayer FR4 so that the copper can protrude from the via. I get lost here.

Does this mean that the purpose of POSITIVE etchback is to improve the connection from a plated thru-hole to an internal trace?

Thanks,
Dave[B}FR4[/B]

[cadpro2k]

Hi Dave,

Yes, been there, done that. Etchback gives you exactly what you're talking about; "a better connection of the through hole wall to the inner layer etch."

I'm not sure where the POSITIVE vs. Negative connotations came from, and it's got me in touble in the past. I'd suggest not using either, but spelling out that you want the etchback per IPC reqmts.

This etchback of the FR4 material gives you 'three points of contact' vs. what might possibly be only a single point (i.e. the hole wall surface to just the front edge of the copper etch - or trace)

Good day.

[Tom]

Dave,

You're on the right track.

Etchback is the controlled removal by a chemical process, to a specific depth, of nonmetallic materials from the sidewalls of holes in order to remove resin smear* and to expose additional interal conductor surfaces.

*Resin Smear is base material (FR4 Core or Prepreg) that covers the exposed edge of conductive material in the wall of a drilled hole. (This resin transfer is usually caused by the drilling operation.).

Smear Removal Notes:

a) Smear removal is the removal of resin debris which results from the formation of the hole.

b) Smear removal shall be sufficient to meet the accepability criteria for plating separation.

c) Smear removal shall not be etched back greater than 25um (0.00098"); random tears or drill gouges which produce small areas where 25um (0.00098") depth is exceeded shall not be evaluated as smear removal.

d) Smear removal is not required of 2 layer boards.

Here are some Etchback notes:

a) When specified on the fabricatiom drawing, the printed circuit boards shall be etched back for the lateral removal of resin and/or glass fibers from the drilled hole walls prior to plating.

b) The etchback shall be between 5um (0.0002") and 80um (0.00315") with a preferred depth of 13um (0.00051")

c) Shadowing is permitted on one side of each land.

d) When the board manufacturer elects to use etchback, the manufacturer shall be qualified to perform etchback through test coupons or boards.

[davepcb]

Thank you both very much. I can finally visuallize what etchback does.

So is etchback a standard part of the fabrication process? It seems to be a necessary step in the hole cleaning process. There is a brief mention of it in IPC-2222 and based on that I don't think I should have to add a separate note to the fab drawing. I'm trying to reduce the notes and focus on referencing IPC specs which cover this.

Dave

[Tom]

Dave,

Etchback is a standard procedure in the manufacturing process.

[cadpro2k]

Oops, there we go with the rounding issue again...

"b) The etchback shall be between 5um (0.0002") and 80um (0.00315") with a preferred depth of 13um 90.00051") '

Tom, did you do this on purpose? :)

Mitch

[cadpro2k]

90.0015 inches???????????

[Tom]

The 9 and the ( are on the same key. I forgot to select the Shift Button and I got 90" Yikes!