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SMD Oven soldering without stencils — Parallax Forums

SMD Oven soldering without stencils

william chanwilliam chan Posts: 1,326
edited 2010-07-02 17:07 in General Discussion
Since stencils are costly, why can they put solder paste into a syringe and needle so we can squeeze out tiny amounts onto each smd pad?
Wouldn't this way be easier and cheaper?

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Comments

  • hover1hover1 Posts: 1,929
    edited 2010-07-02 01:43
    If you are doing a one off board with ~100 or so pads, that's a viable alterntive. If you are doing 100 boards in a production run, a stencil makes more sense. Time = money.

    Jim
    william chan said...
    Since stencils are costly, why can they put solder paste into a syringe and needle so we can squeeze out tiny amounts onto each smd pad?
    Wouldn't this way be easier and cheaper?

  • Erik FriesenErik Friesen Posts: 1,071
    edited 2010-07-02 01:57
    Solder paste tends to compact in a needle and syringe.

    Why do you say it is expensive? You can get plastic for 35+, small metal for 100, and fairly large stretched stencils for 175+

    Once you have done the stretched stencil, you will fall in love with it.
  • william chanwilliam chan Posts: 1,326
    edited 2010-07-02 02:44
    Why can't the PCB fabricators pre-coat all pads with a slightly thicker layer of solder so we can
    use it right away without any solder paste? (maybe just add a bit of flux before putting into oven )

    Wouldn't it be much faster and cheaper this way?

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    www.fd.com.my
    www.mercedes.com.my
  • LeonLeon Posts: 7,620
    edited 2010-07-02 09:58
    Not many PCB suppliers tin pads any more, they are usually gold or silver plated. What you are suggesting won't work, for lots of reasons.

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    Leon Heller
    Amateur radio callsign: G1HSM
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2010-07-02 16:57
    One function the sticky solder paste performs is to hold the components in position between placement and reflow. Otherwise, any small amount of vibration would cause them to slip off of their pads.

    -Phil
  • WBA ConsultingWBA Consulting Posts: 2,935
    edited 2010-07-02 17:07
    The method of applying solder and flux to the pads on a bare fab so that no "process applied" solderpaste needs to occur is a process that is readily available, but not cheap. The process is called Solid Solder Deposit and is performed by a company called SIPAD. This allows you to take the bare fab, place parts, and process through reflow to achieve a built board. No solderpaste application is necessary. I have a few of their sample boards as I was looking at the process a few years ago. It really only works for single sided boards (double sided boards require a flux application process to the second side after the first side is reflowed) and due to its cost, is not typically used for products that can be built using normal processes.

    In regards to the syringe method, it does work in high volume applications using dispensing equipment, but when using a small needle, the pressure used to force the paste through the needle opening does produce friction which leads to heat and can alter the performance of the flux in the paste. When doing it by hand, the application does get exhausting really quick and the accuracy of application may limit you to passive components or maybe some larger QFPs or SOICs unless you don't mind doing solderwick touch-up.

    In regards to using HASL to solder, the thickness of HASL is not sufficient for this method. HASL is still a common method of plating and is still performed by almost all FAB houses, but with the lead-free hogwash*, it is easier to use an ENIG or Gold Flash plating process as opposed to a SAC305 HASL (I do build boards at work that use SAC305 HASL by the way, so don't think it doesn't get used).

    A common rework method for QFNs and BGAs is to tin the pads on the board with a good amount of solder, apply a tacky rework flux, place the part onto the pads, then reflow using IR, hot air, or a reflow oven.

    *start.rant
    yes, I truly believe the lead free initiative is hogwash because of it's poorly handled implementation and, overall, is more damaging to the environment than tin/lead when you factor in things like the extra energy consumed by reflow ovens for the higher temps, increase chemical usages when manufacturing lead free solders, increased usage of dry-packs due to higher MSL parts, etc. It was originally intended to battle heavy lead products like TVs, but applying it across the board for everything has really caused more damage then good. I like the US Army's stance on lead free myself: don't use it
    stop.rant

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    Andrew Williams
    WBA Consulting
    PowerTwig Dual Output Power Supply Module
    My Prop projects: Reverse Geo-Cache Box, Custom Metronome, Micro Plunge Logger

    Post Edited (WBA Consulting) : 7/2/2010 5:13:28 PM GMT
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