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Blog Soldering Surface Mount Exposed Die Pads with an Iron
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Engagement
  • Author Author: shabaz
  • Date Created: 28 Oct 2017 2:58 PM Date Created
  • Views 1867 views
  • Likes 6 likes
  • Comments 4 comments
  • smd
  • diy
  • surface mount soldering
  • smd soldering
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Soldering Surface Mount Exposed Die Pads with an Iron

shabaz
shabaz
28 Oct 2017

Introduction

Many integrated circuits nowadays have a pad on the underside that needs to be soldered down. It can (for instance) serve as a ground connection or it might be needed to be soldered for better heat dissipation. Since the pad is not exposed on the edges of the device package, it can be awkward for prototyping. Ordinarily the solution is to use a reflow oven, a hot plate or a hot air tool. But, it is also possible to just use a soldering iron. I'm not sure how common this technique is, but it works well for me so I thought I'd do a brief write-up. I don't always use this technique, but is still a useful thing to know.

 

Here is the part to be soldered:

image

The photo above shows the underside of the integrated circuit, and it can be seen that it needs to be soldered on the pad marked U1 on the PCB.

 

Solder Paste and Tape Technique

The technique I use involves first applying some solder paste to the center pad on the PCB. The pads for the pins do not have solder paste applied for this method to work, just the center pad.

Next, take a short length of Kapton tape, and pick up the integrated circuit with it. Then, it is an easy matter to hover it over the PCB at the correct landing position, and tape it into position! If it goes on wonky no problem, just peel the tape and reattempt; the tape is sticky enough that you can re-position several times. The photo below shows one device taped down. To the left of it, you can see two more center pads which have been pasted but I have not taped parts down yet.

image

The paste was applied using a splint from a matchstick, since the area to paste is so tiny,

 

Once you're happy with the positioning of the taped parts (use a magnifier to check it is perfectly square and aligned), flip the board onto the other side. You may need to prop it up slightly (for example with some scrap PCB) to keep it flat and horizontal. Get a soldering iron ready; something like a 3mm ellipse shaped tip is ideal, because a lot of heat is needed. Also, regular solder wire is needed.

image

 

Soldering the Pad

 

Apply the solder and the iron to the PCB, so that the copper heats up and heat travels down the vias in the pad. For my 3mm soldering tip, about 10 seconds is needed with a 50W iron set to 330 degrees C; adjust accordingly to suit your iron and tip size. If the time is too short there really is no issue, because you can lift the tape from one end and see if the integrated circuit lifts up with it or not. If it lifts, then just put the tape down again and reapply the heat. Kapton tape is not stretchy and therefore it acts as a hinge when you lift it from one end, and when the tape end is reapplied the part should go down onto the PCB in the same precise position as before, but you can confirm with the magnifier again.

 

The photo below shows what the pads look like from the underside after the heat has been applied. Once you're happy, flip the board over and remove the tape. Now the device is firmly attached by the center pad!

image

 

Finishing Off

The remainder pins can be soldered by hand using thin solder and a smaller tipped soldering iron.

image

After you're done, clean the board and check the pins really are soldered down, and examine with a magnifier to confirm there are no shorts. If there are shorts, desoldering braid can be used to correct it.

 

I hope the (small bit of) information was useful!

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Top Comments

  • DAB
    DAB over 6 years ago +3
    Nice result. DAB
  • johnbeetem
    johnbeetem over 6 years ago +2
    I learned about resoldering a heat dissipation pad when we got the first prototypes of a new board. It had a dual switching voltage regulator with a ground pad, which was the only ground. When the voltages…
  • rachaelp
    rachaelp over 6 years ago +2
    Hi Shabaz, Thanks for the look into how you go about soldering down parts like this. I use a similar method but I also sometimes use a makeshift PCB pre-heater so it's easier to get the joint to flow which…
  • mcb1
    mcb1 over 6 years ago

    Nice trick.

    Must add some tape to my next purchase.

     

    I have been informed that if anyone has a hot air smd rework station, using a preheater underneath really does make it much easier for this sort of work.

     

    Mark

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  • rachaelp
    rachaelp over 6 years ago

    Hi Shabaz,

     

    Thanks for the look into how you go about soldering down parts like this. I use a similar method but I also sometimes use a makeshift PCB pre-heater so it's easier to get the joint to flow which can be tricky in there are a lot of thermal structures built into the board to conduct away and dissipate heat when the board is in operation.

     

    I converted a spare heated bed PCB from an 3D printer design I did for somebody into a makeshift PCB pre-heater. I used a spare metal tray for my Rayburn oven and covered it in Kapton and then attached the heated bed PCB below it. I then put the PCB on top of this and I have another Kapton covered thermal insulated sheet which I place over the top to contain the heat so it warms up to temperature quickly. This can then heat up to 100+ DegC relatively quickly (not too quickly) and the board gets some heat soaked into it prior to trying to solder so it then takes less added heat from the iron to get the paste on the thermal pad to flow and give a good joint.

     

    Best Regards,


    Rachael

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  • DAB
    DAB over 6 years ago

    Nice result.

     

    DAB

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  • johnbeetem
    johnbeetem over 6 years ago

    I learned about resoldering a heat dissipation pad when we got the first prototypes of a new board.  It had a dual switching voltage regulator with a ground pad, which was the only ground.  When the voltages didn't regulate, I quickly found they forgot to solder the ground pad.  Grumble grumble.  The pad didn't have vias, so Shabaz' trick wouldn't work.  However, the ground pad did connect to copper next to the chip, just like Shabaz' board.  I cleared the solder mask from that copper with an X-acto knife, tinned it, and heated it up with a good iron.  The heat conducted to the pad under the chip and reflowed the solder nicely.  This lets you work from one side of the board.

     

    Since then, I always made sure to leave some copper exposed next to the ground pad, if possible.

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