Propeller 44-Pin QFN Chip

Humanoido

Is it possible to manually solder a few pieces of
"wire wrap" wire to some of the pin pads on the
chip?

Leon

You need a steady hand but it can be done; you need to glue it upside down, of course. I know someone who got an FPGA in a BGA package working like that.

Leon

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Amateur radio callsign: G1HSM

Miner_with_a_PIC

I found the image below on Wikipedia a few weeks back...added ease of soldering could be accomplished by bending alternating pins up prior to soldering wires.

Leon

That's a QFP part. QFN chips don't have leads.

Leon

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Amateur radio callsign: G1HSM

Miner_with_a_PIC

Leon>>Whoops should have read the title more carefully...still a cool picture.

I have had success soldering pad type QFN parts by taking wire strands from larger multi-stranded wire, some solder paste and a using a toaster oven to accomplish the soldering...This is relatively straight forward to do but requires a steady hand to place the solder paste and wires. The wires were then lead down to a through-hole board as this was for a prototype purposes. I have since reworked some of these wires with a soldering iron due to breaks although I have never used wire wrap wire as this may be a little on the thick side (solder bridges, solder joint stress issues) especially if the pitch is on the order of 0.65 mm.

Humanoido

Miner_with_a_PIC

How do you hold the chip, wire and position in the toaster oven?
Is there a web site showing pics of this process for the hobbyist?

WBA Consulting

With the right equipment you can solder wires to a QFN but it is a skilled art. I cannot do it, but one of my employees can. I'll see about having one of my Prop QFNs deadbugged this week and post pictures. She does amazing work. The picture attached is jumper wires on 0402 size caps because the board design had RX/TX lines crossed on 16 channels. The picture shows jumps for 4 of the channels (8 lines). The 0402 caps are only on one pad; the other end is crossed over to the opposite cap's pad.

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Andrew Williams
WBA Consulting
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Humanoido

WBA:

That's some very neat soldering!

One question always in my mind - in the past I always used
a heat sink, usually a flat ended alligator clip, on the component
leads to prevent overheating. I see these components are even
more tiny and must be much more sensitive to heat. So how can
one solder it without using a heat sink? In fact, it appears there
is no way to use a heat sink...

Leon

Modern chips can withstand soldering temperatures without any problems.

Leon

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Amateur radio callsign: G1HSM

Miner_with_a_PIC

Humanoido >> Unfortunately I don't think there are too many if any sites that discuss how to solder surface mount components in a non-PCB environment, so I had to get creative. Since I only had to solder a maximum of two of these parts and only about 8 connections each I had the liberty of taking many short-cuts...I'll explain further>>

First I tried hand soldering with a fine tipped soldering iron...the result was difficulty in heating up the tiny pads as the whole component acted like a "huge" (relatively speaking) heat sink...so scratch that idea

Second I considered placing the component on a hot plate such that it was below the melting point of solder but hot enough that the heat flux from a hand iron would allow for localized pad soldering...this was deemed non-feasible

Third...solder all at once but how?? The next day while heating up a slice of pizza in the toaster oven...Eureka!! So hear is what I did...first you need a substrate in order to hold the chip and to hold the wires parallel (see image2.jpg below). I used a piece of aluminum foil and some toothpicks (remember i took short-cuts), the toothpicks held the wires parallel to the QFN part as they were about the same thickness. The wires taken from a multi-strand wire (0.006" in diameter) were dipped in lead-free solder paste to coat one end. The coated end was then touched down on the pad using tweezers and held in place due to the solder paste's adhesion to the pad. Consistency of solder paste should be semi-liquid to aide with adhesion of the desired quantity to the wire and the pad. Of course all the wires are added while the part is on the aluminum foil jig shown in the image below. I used friction to hold the part in place as I was being lazy, this however caused the part to shift while placing the part in the oven...to fix this you could adhere the part to the aluminum foil using paste type flux or some wax, both of which could easily be removed later. The heating was done by bringing the oven to 400 F and letting the part set for a minute or so to heat up, then I hit the toast setting to illuminate the above heating element...the moment the solder melted the oven door was opened which switched the radiative heating element off and prevented overheating of the part. The part then is cleaned in your favorite solvent to remove the flux residues.

improvements>> if you are soldering many parts then perhaps take some clay press the QFN part into it and then make groves where the wires need to go...let the clay dry (it may contract some during the dry?) and then dry completely in the oven...use this as your jig...you might even be able to create a multipart jig for your multi-propeller systems.

Ultimately this whole method is really time intensive and the wires need to be carefully guided/insulated [noparse][[/noparse]epoxy?] post solder. There are almost an infinite ways to enhance the method I used, so use your best judgement for your specific application and tools.

Edit: The first image has a 30 gage wire (yellow insulation) placed for reference purposes

Post Edited (Miner_with_a_PIC) : 2/15/2010 4:12:32 PM GMT

Humanoido

Miner_with_a_PIC:

This is the first real description of soldering QFN chips that I've seen
for the hobbyist, and thank you for sharing. The results look perfect
in the photo. You are right, it would consume a lot of time to do this
process with many chips that require many positioned wires, but for
a few wires and a few chips it's a very workable low cost system. I'm
impressed with your fine skill and excellent results. This is the answer
and process I was looking for. Using this technique, what interesting
projects have you created?

humanoido

Leon

If you really want to use QFN chips why don't you use a PCB? It's much easier. I've even used them on home-made PCBs.

Leon

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Amateur radio callsign: G1HSM