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How the heck do you solder SMD's that have pads underneath?

How the heck do you solder SMD's that have pads underneath?

(OP)
Such as devices like the one below.

Link

If you check out Page [5 of 10] you can see a diagram of the device.
Pins 6-8 and 9-11 are all mounted underneath.

I had a small board made up using the recommended pattern in this document.

I have tried using solder paste, by placing very small dabs on the pads then heating,
but I am always getting shorts between the inside and outside pins.

I really don't like the clearance between the inside pins and outside pins on the recommended
layout, but i figured it was fine.

Whats worse is I can't get in there to fix mistakes as the device is now glued down.

Is there a technique i am un-aware of or something? This is quite frustrating.

RE: How the heck do you solder SMD's that have pads underneath?

I would always avoid a lamiod part like that. That given, it looks like the part has holes in the hidden pads?? Why not try soldering thru them with fine solder?

Otherwise, I'd layout those hidden pads under the display so they projected towards the part center a little to allow too much solder somewhere to flow two rather than just beading up and overflowing onto the outer pads.

Also, "then heating" isn't a prescription for success. Are you running a proper drying, pre-heating, reflow, cool down, temp controlled cycle or not?

Keith Cress
kcress - http://www.flaminsystems.com

RE: How the heck do you solder SMD's that have pads underneath?

Those are easily done with a fine-tip iron... touch the outside edge. Solder paste should not be "dabbed on"... it should be screened on for a proper amount. It sounds like you were dabbing too much on and flowed across every pad.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: How the heck do you solder SMD's that have pads underneath?

If you don't have access to a screen printer then try a syringe with a medium / heavy needle.

RE: How the heck do you solder SMD's that have pads underneath?

Hot air gun, reflow oven, hot plate

If your "very small dabs" are causing shorts then they aren't "small enough"

RE: How the heck do you solder SMD's that have pads underneath?

(OP)
It's pretty crazy. I could have sworn they were pretty small dabs of solder paste. I only have access to several different tips for my soldering iron and a heat gun.
The gun will melt the plastic on the device, but I have a tip that is fine enough to fit in the hole on those inside pins.

I was thinking about using a fine syringe to place some in the hole on the device then heating the contact pad on the top, but I am concerned that
if it isn't pressed firmly down that the PCB won't conduct the heat properly.

RE: How the heck do you solder SMD's that have pads underneath?

That might work. Give it a shot.

Alternatively take it to an assembler and pay them $20 to put them on.

Keith Cress
kcress - http://www.flaminsystems.com

RE: How the heck do you solder SMD's that have pads underneath?

Try adding a bunch of liquid flux and reheating until the solder reflows. That stuff works wonders

RE: How the heck do you solder SMD's that have pads underneath?

If the holes have metallized walls, the solder will wick down into the holes.

TTFN
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Of course I can. I can do anything. I can do absolutely anything. I'm an expert!
There is a homework forum hosted by engineering.com: http://www.engineering.com/AskForum/aff/32.aspx

RE: How the heck do you solder SMD's that have pads underneath?

(OP)
Yeah, the inside of the holes are metal. I tried the above technique and it is 5 / 6. For the pads that don't conduct I add a little more solder and try again and it works.

This way I haven't had any more shorts which is the good thing at least. But another annoying thing is I have tiny burn marks on the sides of the device.

RE: How the heck do you solder SMD's that have pads underneath?

Solder paste can be difficult to manage using manual dispensing and a soldering iron or heat gun. If heated too fast the flux can cause the solder to move or scatter. If heated too slow the pad wetting on one side can be poor. For a part like this you need to get the PCB pad and solder mask designs just right. The solder paste needs to be applied using a stencil to the correct thickness. Finally the heating profile needs to be controlled. A heat gun alone often makes a mess out of SMT boards. You could try placing the PCB on a hot plate let it warm (flux just starts to activate) then use hot air to bring the solder all the way to temp. If your solder paste is old or separating, replace it. It's temping to use expired paste but it can add to the challenge. You can buy a kapton stencil for $10 buck so that might help too.

RE: How the heck do you solder SMD's that have pads underneath?

(OP)
Well, we have 1 dud board now with three LED slots left. I could try a hot plate method to compare the results to. I just don't have any
immediate good options to control paste thickness and placement. The best I can do is use the fine tip syringe.

RE: How the heck do you solder SMD's that have pads underneath?

For the LT3755HMSE-2#PBF Linear Tech LED controller
A similar issue GND under the IC was solved by creating TWO holes !

The BIG hole allow solder and the small hole allow to check with an Ohm Meter
if the solder attach the ground of the IC to the common circuit GND

As good Engineering practice avoid using IC's that have conductors under the IC

Any question please-daveross100@gmail .com

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