YES I have seen PWB failures apparently attributed to corrosion, (visible green / white crud), in plated holes that were tented with a dry film solder mask on product less than 1 year old. At a large computer mfg. corp. I was a pwb process development, manufacturing, and product application engineer, specializing in pwb photolithography, Laser drilling of blind vias and pwb technology definition for specific product applications. Over 25 years ago I worked on reliability testing of PWB designs in "Hostile Environments" and office (which is often contain corrosive (hostile) gasses. I believe that today's user enviroments are more varied and are as corrosive as in the 70's. In my definition of any PWB technology, I personnally would not tent any via / hole without thoroughly examining the type of hole (aspect ratio, blind, through, plating thickness, plating void criteria) the processes and chemicals that the via / hole sees from initial drilling through final product (box) ship, the type of tenting material, the processes and chemicals the tented holes sees after tenting. Smaller holes, higher aspect ratio holes and blind holes (by design, by blockage, by partial solder fill) are all suject to corrosion failure. Holes of .016" and less in diameter are the concern. I am (and am not) surprised when a PWB manufacturer says that they have seen no corrosion failures related to entraped contaminates in PWB holes. The least that any PWB manufacturer do is design a state of the art pwb reliability test vehicle to test various hole sizes, tenting materials, ... in addition to other pwb design features, (line widths, Cu to Cu spacings, SIR, etc. , etc., etc.) I don't doubt that this has been done and that someone reading this note has done it. The question that the pwb design engineer has to ask, "Do I feel lucky today" If not, do more homework. All pwb mfgs are not equal. All pwb usages are not equal, End of life for a PWB varies upon application. Reliability requirments vary. So what if the holes fail after product end of life. Reference notes below: ========================================================== > From [log in to unmask] Wed Apr 3 14:04:01 1996 > Resent-Date: Wed, 3 Apr 1996 12:48:42 -0800 > Resent-Sender: [log in to unmask] > Old-Return-Path: <[log in to unmask]> > Date: 03 Apr 96 10:59:38 EST > From: Tony King <[log in to unmask]> > To: <[log in to unmask]> > Subject: fwd: FAB: Tenting Au/Ni vias with LPI? > Resent-Message-Id: <"QqBtO2.0.Vq7.IFgOn"@ipc> > Resent-From: [log in to unmask] > X-Mailing-List: <[log in to unmask]> archive/latest/3285 > X-Loop: [log in to unmask] > Resent-Sender: [log in to unmask] > Content-Length: 3230 > > ======== Original Message ======== > Sender: [log in to unmask] > Received: from simon.ipc.org (IPC.ORG [168.113.24.64]) by > arl-img-7.compuserve.com (8.6.10/5.950515) > id IAA00673; Wed, 3 Apr 1996 08:14:54 -0500 > Received: from ipc.org by simon.ipc.org via SMTP > (940816.SGI.8.6.9/940406.SGI) > id HAA29355; Wed, 3 Apr 1996 07:14:47 -0800 > Resent-Date: Wed, 3 Apr 1996 07:14:47 -0800 > Received: by ipc.org (Smail3.1.28.1 #2) > id m0u4S0K-00006mC; Wed, 3 Apr 96 06:49 CST > Resent-Sender: [log in to unmask] > Old-Return-Path: <[log in to unmask]> > From: "Dill, Norm J" <[log in to unmask]> > To: "'IPC Technet Input'" <[log in to unmask]> > Subject: FAB: Tenting Au/Ni vias with LPI? > Date: Wed, 03 Apr 96 07:44:00 EST > Message-ID: <[log in to unmask]> > Encoding: 12 TEXT > X-Mailer: Microsoft Mail V3.0 > Resent-Message-ID: <"jnwkP2.0.P7F.ZFdOn"@ipc> > Resent-From: [log in to unmask] > X-Mailing-List: <[log in to unmask]> archive/latest/3278 > X-Loop: [log in to unmask] > Precedence: list > Resent-Sender: [log in to unmask] > > > Typically when LPI is used to tent vias, one side of the board will seal > > and the other may have pin holes or openings in the mask that covers the > via. This creates a copper lined cup capable of trapping any subsequent > chemistry liquids that it may be exposed to (fluxes, cleaners, etc.). > Is this concern reduced when nickel/gold are plated over the entire > copper board? > Please share your via tenting experiences. Thanks. > > Norm Dill > RF Communications > [log in to unmask] > ======== Fwd by: Tony King / N ======== > I hear this concern sometimes from customers or designers, yet as a board > manufacturer have never seen a failure in a printed circuit board attributed > to copper attack in a hole partially filled with mask. Further indication of > entrapped solutions in the hole should be detected at ionic cotamination > testing, yet again has not been seen. > > There are several options to consider when designing product for plugging > holes with mask, in reference to the solder mask artwork design, a hole > should be plugged (exposed to light) from both sides of the hole or neither > side (not plugged). The LPI process outcome is very unpredictable when > exposing a mask filled hole from one side only at image.(dependent on the > amount of mask in the holes, the degree of developing, light intensity etc.) > If the design requires the holes to be plugged completely, this can be done > prior to (or after) primary lpi coat using a screened dot pattern. The dot > pattern plugging operation can also be performed after HAL to allow solder > coating of the copper prior to plugging if corrosion is a concern. > > Some customers absolutely require holes to be plugged from one side while > leaving the other side open for testing reasons. This dot pattern screening > operation must be done after HAL to insure the partially plugged hole is > adequately cleaned after soler coat. > > Though I have not seen copper fail, I would expect nickel/gold to present > better protection against possible corrosion, though the gold would be too > porous if immersion coated, electroplated gold would be better. > > > Tony King > Elexsys International Inc > 603-886-0066 > [log in to unmask] > >