Soldering a QFN type package on a PCB?

hello,

do you know how i can desolder a power 33 package fet off a pcb and solder another one on in its place?

it is the FDMC7570S FET...


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Normally you would use a hot air soldering station for smt rework.
Unfortunately, decent ones such as Leister, Weller, OKI are not cheap.

By using a hot air tool with carefully adjusted temperature. Preheating the board from the bottom would be advantageous. Porofessional repair stations have the feature.

I use an alloy with a very low melting temperature to form a low-melting alloy with the solder. One commercial product for that is called ChipQuick. I believe it is mostly bismuth; although, I think almost any low melting alloy will work. If there is gold plating, I have been advised by Indium America not to use an indium-containing solder, as it will remove the gold. That may not be an issue for you. Mercury will do the same thing, but I hope you do not even think of using it.

The process is simple. First, I use desoldering braid to remove excess solder. Then I add an excess of the low melting solder to form an alloy at the joints. Finally, I remove the chip with low heat, like a regular hot air gun focused on the back of the board. That method worked quite well with a QFN accelerometer I needed to remove a few weeks ago. I also add a little paste flux to help the metals flow together better.

John

I believe it is mostly bismuth

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According to the specified melting point of 138 °C, it's most likely the well-known Bi(58)Sn(42) eutecticum.

As the previous posters have mentioned the use of a Hot Air Rework/Reflow Station combined is the preferred method.

A good set of conventional or vacuum tweezers are helpful as well.

The following tutorials demonstrate various techniques and maybe of help.


Soldering:

Soldering a QFN (Quad Flat No-Lead) Package by Hand

QFN reflow without hot air

Hot-air magic tricks!

How does hot air rework actually work?

Hot-Air Soldering QFN packages



Desoldering:

The follow actually demonstrate a soldering techique, however it maybe used as a desoldering method when combined with Topside Hot Air.

Reflow Skillet

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BigDog

Get hold of IPC-7093

thanks i'll look through these ways.

-At the moment, i am soldering the Power-33 package onto the board by first soldering the big pad on..........then i hold it with tweezers and literally incinerate the power-33 package with a hot air gun from above.

(first of all, i had tinned the pads on the pcb)

...this heats the package up so much that it melts the solder beneath it and you kind of feel it "settle" squarely onto the pads, and you kind of guide it with the tweezers, and then its done.

But it seems awfully crude, incinerating it like this.

We cannot very well heat the board from underneath.....or at least we can, but its an 18 layer board and the chances of heating the pads isnt that great, even though there are thermal vias going down to the bottom layer.

There must be some bona fide way of doing this...and i will check your links above......i have heard of infra-red techniques which somehow manage to "laser beam" solder it.

We cannot very well heat the board from underneath.....or at least we can, but its an 18 layer board and the chances of heating the pads isnt that great, even though there are thermal vias going down to the bottom layer.

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If you think about suitable prototyping methods, it's good to understand the regular production solder methods. In reflow solder process (either IR, hot air or vapour phase) the board is uniformly heated up, applying heat from both sides.

The 18 layer argument is more less pointless in my opinion. Technical parameters are heat capacity and thermal conductivity. They are not so much different for a 18 layer board. Preheating to a reasonable temperature, e.g. 100 °C is always possible, even solder/desolder on a hot plate with solely bottom heating if no other methods are available.

Hot air is likely to overheat parts as well as blowing away small ones. You need good tools and a lot of skill to apply it appropriately.

Study the available professional repair stations available e.g. from Weller, ask for a demonstration in your company or visit respective trade shows. Ask qualified assembly service providers how they are doing these things.

we are re-working boards, and they are populated top and bottom, so we cannot put on hot plate etc.

I will ask around, but it seems to me that the ONLY genuine way forward for us is to spend £30000 on infra-red solder pick-and-place machine?

we are re-working boards, and they are populated top and bottom, so we cannot put on hot plate etc.

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I was talking about the more simple case. The usual rework stations with infrared or hot air preheating will also work for top/bottom populated PCB.