John,
To the best of our knowledge....
The current carrying capacity of a blind micro-via is calculated just as you
would for any other via or flat conductor - by finding the minimum square
area of the conductor's cross-section, then using the IPC-2221 "Conductor
Thickness and Width" charts.  For a via, the exercise is simply to "unroll"
the barrel of the hole, and figure out the minimum amount of copper area at
the junction of the barrel and the pad layer.  This intersection can be the
weakest point in the circuit in through or blind vias, unless there are
thinner traces along the current path.  I recommend derating the final
numbers by 50% in the case of blind laser-drilled vias, because of process
variances from vendor to vendor in hole surface integrity and final plating.

min. dia.  x  pi (3.1414)  x  Copper thickness  =  sq. area

min. dia = via diameter (for a 'tapered' laser via, the smallest diameter;
for a drilled via, the smallest drill bit diameter likely to be used,
representing the outside diameter of the plated hole barrel)

copper thickness = minimum finished copper hole plating, per spec for the
board; we typically use IPC minimums for Class 2 or Class 3 boards for
through-hole vias (>700 micro-inches), and for blind vias we figure >500
micro-inches.

FYI, in board layout we typically design for a minimum heat rise (10degC) or
below, and also consider the duration of peak current draws, i.e. momentary
or steady.

Regards,

Dan Baumgartner
Quality Assurance & Training Manager
Praegitzer Industries Inc./Design Division
Office: 503-531-2050 x1822    Fax: 503-531-2051    Cellular: 503-780-3325
email:  [log in to unmask] <mailto:[log in to unmask]>    or   [log in to unmask]
<mailto:[log in to unmask]>


        -----Original Message-----
        From:   John Sabo [SMTP:[log in to unmask]]
        Sent:   Monday, November 15, 1999 12:21 PM
        To:     [log in to unmask]
        Subject:        [DC] bind vias / "micro" vias

        Can someone tell me how to calculate current carrying capabilities
of blind
        micro vias?

        Thanx,

        John Sabo