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Resistor Arrays, Concave or Convex? Ease of Soldering? — Parallax Forums

Resistor Arrays, Concave or Convex? Ease of Soldering?

Miner_with_a_PICMiner_with_a_PIC Posts: 123
edited 2010-02-21 06:56 in Propeller 1
I am transitioning to using the Propeller SMT form factor and am in the process of filling an order for some resistor arrays that have a pitch tight enough to match the prop. I have my sights on 0.5mm pitch arrays but have the choice of concave or convex pins and would like to hear opinions based on ease of soldering, self alignment and so on. So which is more advantageous or does it make very little difference either way?

Tool-wise I have access to solder paste, regular wire solder, an oven (toaster oven), soldering iron and hot air rework capability. Words of wisdom and cautions are welcome.

Thanks in advance...

Comments

  • Cluso99Cluso99 Posts: 18,069
    edited 2010-02-20 17:01
    Mine are convex on the RamBlade and they are a PITA to hand solder any way. They are worse than the SOT-323-6 ICs.

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  • Toby SeckshundToby Seckshund Posts: 2,027
    edited 2010-02-20 17:15
    I would guess that the convex ones would make it easier to get heat onto the joint (hand soldering), but the convcave ones would be better for sucking in the solder to itself, and prevent any bridging, once the joint was up to temp.

    I have enough trouble with 0.8mm stuff, so I have stripped off a load of 0.5 resistor strips (concave) just to have a practice.




    Edit second convex should have been concave. That was a repitition, and therefore one point away.

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    Style and grace : Nil point

    Post Edited (Toby Seckshund) : 2/20/2010 8:43:44 PM GMT
  • Miner_with_a_PICMiner_with_a_PIC Posts: 123
    edited 2010-02-20 19:01
    Cluso99 and Toby >> Thank you both for your inputs. My plan is not to hand solder these unless I had to rework due to a fry, design change or poor initial solder alignment. My takeaway so far is to adopt a 0.8mm pitch over the 0.5mm to reduce the PITA factor exponentially. Since I am not hand soldering with an iron I am in agreement with Toby that the concave pin shape will likely wick the solder toward the pin and reduce the chances of bridging. The good news is that I get a price-savings/increased selection for the concave so seems win-win so far.

    Additional inputs/advice are welcome especially if anyone has used a non-soldering iron method (i.e. oven and/or hot air rework station).
  • Tracy AllenTracy Allen Posts: 6,666
    edited 2010-02-21 00:36
    Much better results and assured sanity can be had if you apply the solder paste through a stencil, such as the ones available from www.ohararp.com. With emphasis. It is painstaking otherwise to apply just the right amount of solder to the pads. For rework, a hot air pencil is the best bet, so all the connections can be melted at once. The worst thing is extra solder paste under the chip that bridges the connections.
    I've used the CTS series with concave pads...
    743C 2 or 4 0805 resistors on 50 mil (1.27mm) spacing.
    742C 2, 4 or 8 0603 resistors on 32 mil (0.8mm) spacing.
    741C 2, 4 or 8 0402 resistors on 20 mil (0.5mm) spacing.
    740C don't even think of it!

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    Tracy Allen
    www.emesystems.com
  • Miner_with_a_PICMiner_with_a_PIC Posts: 123
    edited 2010-02-21 03:14
    Tracy, thanks...I had not considered using stencils for application of the solder paste as I figured with enough effort it could be placed manually using a syringe. Have you tried the homemade stenciling method similar to the one in the product shown below? Can the solder paste used in most syringes be used for stenciling or does it need to be thicker (higher viscosity than the syringe injectable type)?

    www.renaelectronics.com/product_stencil.htm
  • Tracy AllenTracy Allen Posts: 6,666
    edited 2010-02-21 06:56
    I use Zephyrtronics solder syringes either in an air pressure dispensing tool or for stencils. There's no reason paste can't be placed by hand from a syringe, given good eyes and a steady hand, and patience. Figure a stencil is quicker and intrinsically more uniform from pad to pad.

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    Tracy Allen
    www.emesystems.com
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