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choosing a soldering station

Discussion in 'Electronic Design' started by Michael (Micksa) Slade, May 3, 2004.

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  1. I'm about to get a nice soldering station and I thought I'd check with you
    guys before I buy.

    I've been led to the hakko 936. Lots of positive comments about hakko
    irons in general. Price is right, don't see any other reason not to get
    one but if someone has a good reason, let me know :)

    Also, I have a choice between the 907 handle and the 900S handle. This is
    where I'm stuck. Which one is better for what? Could/should I just get
    both?

    Mick.
     
  2. Bob

    Bob Guest

    http://www.hakkousa.com/products.asp?PID=936-9&Page=1

    The 900S is the small handle, while the 907 is the medium one.

    I have the 936 with the 907 handle. These are what we use at my work, too.
    It's a very good station, it's very reliable, and the tips are easy to find.

    Bob
     
  3. Yeah, I know that. I was wondering if anyone had any info on either
    handle, or both, and could tell me whether it was worth getting a 900S
    instead of a standard 907.

    I have plans to go down to .5mm pitch SMS stuff, but I want to be able to
    solder RCA plugs etc too, if that is relevant.

    Mick.
     
  4. Does this mean the tips you buy come with temp sensors in them, at least
    for the 941?

    One thing where opinion seems to be divided is on the subject of thermal
    response (I presume that's how quickly the tip temp recovers after losing
    heat to a joint?). I've read that having an iron with a smaller tip that
    loses temp is better for SMT because its low heat capacity is less likely
    to wreck tiny components from uneven heating or whatever. Is there any
    truth to this?

    Mick.
     
  5. Leon Heller

    Leon Heller Guest

    [deleted]

    Metcal STSS units often come up cheaply on eBay, I've got two of them. They
    aren't the latest model, but work just as well and have some advantages over
    the later unit.

    Leon
     
  6. Terry Given

    Terry Given Guest

    I have a horribly expensive Metcal rework station, which is amazing, as it
    should be - cost about NZ$3500. At the place I contract to, we bought a
    whole bunch of Hakko 941's (NZ$400). They are nearly as good as the matcal,
    and about 1/10 the price. Thermal response is fantastic - this is what
    governs how good your soldering iron really is. The distance between the
    temperature sensor and the tip of the iron is extremely small, so the
    temperature controller can detect applied loads and wind up the power. The
    pluggable tips (about NZ$40 each) are great too, tip changing is so fast. We
    use them for hand-soldering (rework) TQFP144's, 0.5mm leadpitch. There are
    lots of different tips available.

    http://www.hakko.com/itnl/product.html?catID=1&prodID=3

    Cheers
    Terry
     
  7. Terry Given

    Terry Given Guest

    There is lots of subjective argument about soldering irons, and everybody
    has their favourite iron. Some people get quite passionate about it!
    Ultimately though thermal engineering analysis will tell you what works
    best. I will ignore smt devices in particular at first.

    Your basic soldering iron consists of a heating element followed by a
    thermal conductor (the tip). The heater dissipates some power P (the iron
    power rating). This causes the tip to heat up. Heat then flows from the tip
    to the surroundings, through the surface of the tip. Heat loss is
    proportional to temperature, so as the tip gets hotter, it loses more heat
    to the surroundings. Eventually a steady-state temperature is reached,
    whereby the heat lost to the surroundings is equal to the power dissipated
    in the heater. (the actual heat flow calculations are pretty nasty but
    do-able).

    When the iron is used to solder a device, the tip is applied to the device,
    solder is applied etc. When this occurs, because the device and the solder
    are at fairly low temperatures (compared to the iron - around 300 deg C) and
    have mass (lumps of metal really) heat flows from the tip of the iron
    (wherever its touching) into the device and solder. This heatflow through
    the tip will cause a temperature drop across the tip. Long, super-skinny
    tips are TERRIBLE, I'm sure most people have experienced them sticking to
    moderate-sized components/pads etc.

    cheap crap soldering irons have no temperature control at all, and instead
    rely on the temperature drop across the long tips (40mm or more). Slightly
    less crappy soldering irons have a "temperature control" thats usually like
    a light dimmer in the input, ie allows you to set the steady-state
    temperature by varying heater voltage and thus power P.

    Try an experiment: get your soldering iron, and try to solder a 1/4W
    resistor leg onto a 4"x4" square of copper-clad pcb. Try on the edges
    first - in the middle of one edge, then at a corner. Then try in the centre
    of the pcb - where is where you are likely to find your iron tip sticking to
    the copper, and not melting the solder. The problem here is the large
    temperature drop across the tip, as it tries to heat up the big copper plane
    (my metcal will solder a resistor to 1mm Cu sheeting using a tip I also use
    for smt work :).

    A monstrously fat tip is a much better conductor of heat (less T drop from
    element) and also stores a sizeable amount of heat in its mass, further
    adding to the ability to supply heat quickly. Problem is, that sort of tip
    can do a lot of damage (ie heat stuff up real fast)

    In a temperature-controlled Iron, the temperature sensor notices the drop in
    temperature, and winds up the heater power P, to try and keep the tip
    temperature constant. If the temperature sensor is a long way from the tip
    (my Ersa soldering station tips are about 25mm long, and 2-3mm in diameter,
    compared to the 5mm long, 5mm diameter conical tip on the Metcal. The Ersa
    wont solder onto 1mm Cu sheet without a tip thats almost 1cm in diameter -
    its 3mm chisel tip sticks to the Cu.

    Some types of temperature controlled soldering irons (eg weller, metcal) use
    fancy materials/properties (eg curie temperature) to precisely regulate
    temperature. These can be really expensive - tips for my Metcal are
    $hundreds.....

    In addition to having a thermal "resistance" (relating temperature drop to
    heat flow and conductor dimensions, analagous to current flow), metals also
    have thermal "capacitance" - they will absorb heat at a certain rate,
    causing their temperature to rise (specific heat capacity, J/kg/K)

    So think about applying a heat source to one end of a rod of metal, which is
    at a uniform, lower temperature. Initially the "hot" end of the rod slowly
    heats up. as it does, the next bit of the road slowly heats up.....each bit
    taking time. This is of course what you observe when you stick a poker in a
    fire - heat travels up the poker slowly, and depending on the length of the
    poker, might not make it to the end - heat is also being lost through the
    surfaces.

    the themal model of a tip is therefore a current source (power loss) in
    parallel with a thermal capacitance, followed by a series thermal resistor.
    When tips get beefier, R goes down but C goes up, so although the
    temperature drop is lessened, the slow RC response time can somewhat (or
    even completely) negate the advantage of bigger size.

    For fast thermal response, you therefore want very little thermal
    capacitance (not much material) and very low thermal resistance (low
    length/area of ratio of tip, ie short fat tip)



    Soldering SMT components is a different matter altogether. Any component
    sees this when soldered: Pad temperature rises during soldering. Heat flows
    up into lead, and along lead (usually small leads, so temperature drop is
    high) before entering device, and heating the device up. As everything heats
    up, it expands, due the the Coefficient of Thermal Expansion (CTE) of the
    various materials involved. Unfortunately all devices are made from a
    variety of different materials, with differing CTE's so some bits expand
    more than others.


    Leaded devices can tolerate quite large changes in dimension, as the leads
    can flex. many smt devices (R, especially C etc) have metallised endcaps on
    a ceramic substrate - no leads. Therefore no temperature drop across leads.
    Therefore smt device endcaps get stinking hot, ie maximum expansion. But
    there are no leads to flex to accommodate the dimensional changes, so the
    endcaps crack - perhaps immediate failure, perhaps later on.

    This "thermal shock" problem of course occurs in leaded parts too, just to a
    lesser extent. A wave-soldering machine has a preheater specifically to
    reduce thermal shock. smt soldering machines are precision dynamic
    temperature controllers, and have a pre-programmed temp profile that slowly
    ramps the temp up to say 100C or so, lets it sit for a while, then cranks up
    to 200C or so, and back down - this is in most smt component manufacturers
    data, say www.avx.com

    High-voltage smt caps in particular are susceptible to this - micro-cracks
    develop due to overheating, which (IIRC) the electrostatic forces inside the
    cap can make propagate, leading to eventual failure - I have personally
    witnessed this with a flyback smps. Looking at the charred remains, we found
    a 15nF 1000V X7R cap had burst, causing the problem. Two of these were in
    series across a 600V bus (which could go up to 950V), so the caps should
    NEVER have failed. The prototype was hand-assembled with a soldering iron.


    Ideally, use an smt placement machine & reflow soldering machine. This is a
    bit too rich for most hobbyists! Hot-air rework stations are the next best
    thing - companies which re-work smt R's, C's with soldering irons are just
    asking for field failures - and are not too expensive. If you have to do it
    by hand, use a good temprature controlled iron, and keep the temperature
    low. IC's (eg SOIC) are usually ok, as are any other leaded smt part. Its
    just the metallised pads you have to watch. A heat gun (hair dryer) can be
    useful as a pre-heater, too.

    Or, be prepared to check bits after you solder them.....

    cheers
    Terry
     
  8. I am still happy with the Voltcraft LS50, in fact is looks so good, I treat
    it with extra care, unlike the old Weller that got thrown around.
    www.conrad.nl
    or past the spalsh screen
    http://www1.nl.conrad.com/scripts/w...er_js=Y&shop=A_B2C_NL&p_init_ipc=X&~cookies=1
    Type ls50 in the search window for a picture, type details for a bigger
    picture.
    Now 70 Euro!
    JP
     
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