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   TO:         I4235700 IBMMAIL   new address for ipc technet 25.6.96

   FROM:       DSTEWART EX2       D.Stewart        - Product Development Manager.

   DATE:       9 June 1997
   SUBJECT:    RE:BLIND AND BURIED VIA

    There are two methods of producing blind vias:
    1. Controlled depth drilling or "access blind vias", where the via
    goes from the outside layer and terminates on an inner layer. This
    can be mechanically drilled - hit or miss affair when the
    dielectric separations are small - or laser or plasma etched, where
    the layer you want to connect to, acts as a "stop pad". Of course
    these holes are open to the outside, and as such may or may not
    interfere with the component assembly. For via in pad technology,
    we bought a laser to manufacture access blind vias, because the
    hole size (100um 4mil) allows the via to be produced without
    interfering with the solder paste volume, and is ideal for BGAs.
    2. Sequential lamination. This technique produces "filled blind
    vias" where the surface of the board has only a witness of where
    the via lies beneath. The blind via "subpanel" is manufactured as a
    double sided or multilayer board, and then sequentially bonded to
    more layers or subpanels to produce the finished multilayer. In
    this process the resin from the prepreg used to bond the final
    package together, flows up through the vias and fills them to the
    top. When this finished package is subsequently plated, the copper
    covers the top of the resin and the blind via is then
    filled/hidden. The advantage of this technique is that the blind
    via aspect ratio can be much higher than is possible with access
    blind vias, as the blind holes are actually through holes at the
    time of plating. There is one word of warning though, do your sums
    on the resin expansion in the blind via, as the resin expands more
    than the glass reinforced resin of the subpanel, and this could
    lead to some issues during assembly. Having said that we are
    manufacturing in volume sequentially bonded blind via products with
    aspect ratios of 5:1 in 7 layer subpanels without any problems.


                     |????????????????|  outer layer plating
                     |????????????????|
                     |?????|??????|???|
    ??????????????????????||      ||??????????????????????
                          ||      ||
                          ||      ||
                          ||      ||
    ?????????????????????? |      | ??????????????????????
                |----------|      |------------|inner layer connection

    Dougal Stewart
    Exacta Circuits
    Scotland

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