Soldering a Multilayer PCB

Discussion in 'Computer Information' started by Jeff Strickland, Dec 15, 2009.

  1. I have a notebook with a broken power plug. I've never soldered a multi
    layer board before, and I'm wondering if the odds of success are good or

    As far as I can tell, the power plug extends through all of the layers on
    each of the taps, so I _think_ my only concern is that the middle layers get
    hot enough to melt the solder if there is a trace on the layer that needs to
    be connected.

    My soldering iron is old and small, I forget the wattage, so I'll probably
    buy a new one. What size should I buy to solder a 5-layer board?
    Jeff Strickland, Dec 15, 2009
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  2. Jeff Strickland

    Paul Guest

    Holes in motherboards are plated. That means there is a continuous metal
    surface in there, which will wet by capillary action when the solder
    is hot. The plating touches the inner metal layers exposed by the drill,
    so any inner track intended to be connected, is connected by the plating.

    Plating gets harder to do, depending on the aspect ratio. That is the
    ratio of height to diameter, of the thru-hole. That places a limit
    on drill size and donut, for things like vias (holes used solely as a
    means to connect a track from one layer to another). What that means
    is, if you maintain a fixed aspect ratio, then decide to make a PCB
    thicker, the hole has to have a bigger diameter, and it means vias become larger.
    And then you get room for fewer vias per square inch. Vias are important
    in designs where a large number of signals need to be routed.

    If you make the vias super-tiny in diameter, then you need more
    specialized plating techniques, costing you more money.

    The holes for the vias are the ones with the aspect ratio problem.
    The holes used for the legs of thru-hole components (like your power
    plug legs), should have good quality plating, as the aspect ratio
    is lower. It is the via that is constrained by the highest practical
    aspect ratio for the plating operation.

    In terms of soldering, what you have to be concerned about, is whether
    the designer knows what an isothermal is, and whether one has been
    used to make soldering easy. If the holes join to copper planes,
    the copper planes operate as a heat sink. And then you need your 80 watt
    iron, instead of the 25W-35W you might normally have used. I picked
    the 80 watt number, because I bought a cheap one from Radio Shack.
    I use that for situations where there is a lot of heatsinking capacity
    on the board.

    If you apply too much heat, to a poorly fabricated board, you can
    lift a copper pad right off the board. I find motherboards are
    not very good in that respect. The PCBs my company used to make,
    cost more than the retail price of an ATX motherboard. They were
    rock hard and difficult to damage. ATX motherboards, on the other
    hand, have the copper peel off them, without too much abuse. I've done
    minor damage to a couple of motherboards already. Fortunately, the
    connections in question didn't affect board operation, so the
    boards I damaged, survived.

    Paul, Dec 15, 2009
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  3. Jeff Strickland

    baron Guest

    Jeff Strickland Inscribed thus:
    Hi Jeff,
    Adding to what others have said...
    Replacing a damaged power socket on a laptop/Notebook whatever is not a
    job to be taken lightly !

    First make sure that you can get the correct replacement socket.
    Depending upon the manufacturer this is not always possible. Having
    said that some machines have sockets that are supplied as a spare part,
    usually these are the ones that are on a daughter board or a flexible
    cable. These are usually easy to change because no soldering is

    The ones that are secured to the mainboard are the worst possible to
    play with ! You are right to worry about the capabilities of the
    soldering iron. Someone mentioned 80W. Barely adequate ! These
    sockets are soldered with lead free solder and require much higher
    temperatures to melt it. In addition there are multiple fastening
    points all of which have to have the solder removed or you will have to
    simultaneously heat up all the attachment points at the same time so
    that the socket can be removed.

    This also assumes that there are no mechanical retainers that also may
    be soldered as well.

    Now if I haven't succeeded in putting you off doing this replacement
    job, I assure you that it can be done, but it takes skill, care and
    patience, along with the right tools. I've done quite a few !

    Good Luck.
    baron, Dec 16, 2009
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