A frustrating problem, a solution to another problem and some news

[charliecoutas]

Hi Guys

Although I have been looking at this forum now and then, I realise that I haven't posted anything recently. Putting that right:

I spent about two days on the following: I designed a PCB (made by Elecrow - China) and populated it. When I tried to program the 18LF26K22 my PicKit2 said there was no device. Damn, I had made two (sloppy) mistakes on the board. I put those right and.... still no device found. It must be the chip. I cut all 28 legs (SMD) and soldered my last 18LF26K22 on the board. You've guessed it, no device found. This took quite a while. In desperation I just sat and looked at what else I could do. Change the programming voltage, which PicKit2 said was 3.6 volts. I dropped it to 3.4V and bingo, it worked. It also worked at 3.5V (on the PK2 screen display) but not 3.6V. On checking with a DVM my PK2 puts out 3.7V when it says 3.6. Has anybody else had this?

On several of my projects I use the SH1106 display. Some (at the museum mainly) have now been in use for a few years. There is a noticable "moth eating" of the displayed characters. Some segments are fading. This has made me develop a screen saver. If anyone would like the code then no problem.

In 1949 Cambridge University, under the leadership of Prof Maurice Wilkes, built the first large scale stored-program computer EDSAC. We have a rebuild of EDSAC at our museum. Our group at The National Museum of Computing at Bletchley Park have been asked to design and build a paper tape reader for the rebuild, along the lines of what they had in 1949. We feel very honoured and I am doing most of the electronics. "No PICs" was a request from our group because most of our exhibits make use of these lovely devices. But I managed to sneak one into The Engineers Control and Test Panel. The main reader electronics use valves - EF91's.

Regards to everybody
Charlie

[trastikata]

I cut all 28 legs (SMD) and soldered my last 18LF26K22 on the board.

For cases like that I "developed" low melting point non-eutectic solder. It's very easy to make:

1 part standard 60/40 or 63/37 Sn-Pb solder
1 part Bismuth - Bismuth is easily obtainable from I-net (Aliexpress, EBay)

Melt both together in a can on the stove and pour it as ingot or similar.

That solder melts around 105-110 degrees and stays semi-liquid down to around 95 degrees C. and it is not toxic like Cadmium containing low melting point solders (I consider Lead completely safe as long as you don't eat its salts  ) 

So, when I have to de-solder a chip from a board I take a big drop from the previously mentioned low-melting point solder and place it over the chip pins and go around with the soldering iron heating the drop right over the pins.

When that drop of solder mixes with the standard or lead-free solder from the chip pins, the melting point of the newly formed alloy drops significantly (around 120-130 degrees depending on ratio new/old solder) and the new mix of solder is still non eutectic which gives you more than enough time to remove the chip without overheating anything. Then clean everything with a soldering wick and put the new chip.

When prototyping for test purposes I usually solder chips and modules with that low-temp solder and the rest with normal solder which gives the possibility for easy replacement of important or tested parts without risk of overheating or displacement of the rest of the components.

 

[TimB]

Sorry only picking up on the one element in your post and that is removing the SMD chip.

I use a hot air gun. Very cheap but works great. I just recently removed a small SMT chip as I swore it was faulty. What else could it be? Very easy to do. Turned out though chip was not faulty it was the probe wiring Doooow

[charliecoutas]

Trastikata: I'd forgotten about low melting point solder, thanks for the info, I'll try it.

But, as Tim said, it HAD to be the chip. I am nervous of hot air guns but so many people seem to use them.

Charlie

[top204]

See if you can afford an infrared rework station Charlie, or see if the Museum will help out with purchasing one for them to use as well. :-)

I have used one in the past and they make life so much easier because they warm up the main board, then the infrared is moved over the component and it just lifts off after a few seconds. Even large multi-pin devices just lift off all in one go because the heat is evenly distributed around it and the board is pre-warmed. Also placing components is easy with one because the board is pre-warmed, then solder paste, or use the solder that was left on the board and a small amount of flux added. then just place the component where it should be, and it just plops back into place with evenly laid solder all around it when the infrared beam is turned on!

A piece of tinfoil is sometimes required with a hole in it so that the infrared does not loosen the surrounding components too much as well if a large components is being removed or placed, but they are great to use and so quick!

They are also excellent for re-using components from scrap boards, because they are so efficient. They also work with lots of wired component types on PCBs, and not just SMD types.

There are lots of them around, and here is one I found with a quick search of Ebay:

SMD Welder IR Rework Station

As soon as I get my workshop up and running, I will be saving up for one because components are not cheap, so re-using them is always a good idea.

Good to have you back with us Charlie, and your email message was so warm and charming to read, it lifted my day up. Many thanks. We cannot help worry about friends. :-)

[TimB]

I have an expensive hot air rework unit (~£150) but most of the time I use my cheep unit like this one 858D that I see can be had for £31

You only need a decent set of tweezers and some good flux past to easily pull comps off and put new ones back

https://www.ebay.co.uk/itm/314310879314?hash=item492e636852:g:4KwAAOSwt-JjAwFn

[top204]

I use a 858D as well Tim, and you are right, it works great.

When I bought it from China for about £28.00 (free postage), I could not believe it was so cheap for what it does, so I was a bit wary of it, but it does work excellently, and I have had mine now for about 7 years!

It is a bit of a bugger on sockets and plugs and castellated PCBs etc, because they require so much even heat, but it does work with a bit of time and patience, sometimes! LOL. That's were the infrared station comes into its own, because it gives very even, pre-set, heat around a component or castellated PCB.

[charliecoutas]

Les, Tim

Thanks for the info, when I get back from next week down in Dorset with Lynda and Rollo (doggie), I'll get myself a unit.

Charlie

[John Lawton]

Snap! I use the 858D too. Tim, what flux do you use as a matter of interest. I have a flux pen but it is too runny for my liking?

[rick.curl]

The first time I needed to remove a SMD chip I tried some "Chipquik" low melt solder and I was immediately hooked! I could easily remove a 44-pin chip with only the low melt solder and my usual soldering iron.
I found other brands of low melt wire solder solder that seem to work equally well for hand-soldered boards- but for reflowed boards, nothing beats ChipQuik TS391LT low melt paste solder.  I use that on ALL of my reflowed boards now. I reset the temperatures on my reflow oven to be much lower.  In my opinion this reduces stress on the components during the reflow process. It has the added benefit of making the boards easier to repair. I still have to add a gob of low-melt solder to remove a chip, but the overall process is much easier.

-Rick 

[charliecoutas]

Well draw the curtains and call me Gladys! I never thought of using low-melt solder for making a board. Thanks Rick!

Charlie

[top204]

what flux do you use as a matter of interest.

I used to buy the liquid flux in a small bottle, but it cost a bloody fortune for what it is, so I then started making my own based on what dad used to make when I was a young-un and we could not afford rosin filled solder for our electronics.

I bought some rosin chunks and mixed a small amount of them in with ethanol or isopropyl alchohol, and it works a treat once the rosin has dissolved. Because it has so little rosin in it, it does not make a mess of the PCB, and it is easily cleaned off with isopropyl alchohol. Even a small amount of rosin in the alchohol will make the surface tension good for soldering.

Keep away from the plumbers flux, because this has metal compounds in it that will make the board conductive.

Here is one of many videos about DIY Rosin Flux:

https://www.google.com/search?client=firefox-b-d&q=DIY+rosin+flux#kpvalbx=_IQqLZJPbIc_dgQbXhYvYBA_35

[John Drew]

Wow, I learned a lot from this thread. Thanks everyone.
Good to see you back Charlie.
John

[charliecoutas]

Thanks John.

[Fanie]

If you make yourself a hot plate, they are the easiest and safest to desolder any component.
If you don't remember the hot plate I posted on the old forum. it's a 1000W 220V element sitting in a 12mm ~300 x 240mm aluminum, the element is pressed in the milled slot.  You have to do temperature control of course.

We solder our boards at 180oC.  To desolder you use a piece of aluminum that will fit under the target component, and place it on the hot plate to heat up.  The board is then simply pressed onto the aluminum, the component heat up and you can remove the component.  You can then use the same to place the new component.  The idea is to heat the board up instead of risking burning the component.

I have found the hot air is excellent for smaller components, but more timely and difficult sometimes with larger components.

[Fanie]

If I may add.  To strip scrapped boards can be easy if you use a solder bath.  Use a salad tong with slots that will clamp the side of the board, place the board on the solder and simply lift the components off.

[See_Mos]

The IR workstation looks very interesting. I have four well packed SMD boards which each need 30 'lytics replacing.

Any advice on suitability would be welcome.

[Peter Truman]

I couldn't manage without my hot air gun   - I have one with a reed switch in the holder - so when the handset is lifted from the holder the air flow starts immediately. But... I'm paranoid about the handset being knocked off accidently when I'm just leaving the workshop. So, I built a small PIR detector (with a small PIC for timing and switching), located in the grey box above my work position, that disables the power supply 30 seconds after the last movement detected. Helps me sleep at night!

I also have infrared and an oven - my preference though is for the hot air gun overall, probably because it is convenient.

[Peter Truman]

Apologies - I added the photo twice - not sure how to remove it?

[JonW]

I always use low melting point solder (Loctite) for hand soldering and clear no clean flux.  The flux is much cheaper now and can be bought for less than £10, its great for rework and PCB cleaning.

Be careful with hot air pencils and removing devices > 350C with lead-free solder as the IC can delaminate extremely easily.  We recently had some IC's fail in the field and ended up having to get them analysed.  When we removed them for analysis, we used a hot air pencil (<350C), and even at this temperature it caused delamination of the die from the lead frame and masked the real issue.  For hobby use, I can't see it being an issue, but anything we build using the hot air pencil doesn't ever go out and is only used for development.  This is why a reflow profile for inline ovens is ramped in zones. If you struggle to remove the device then re-tin the legs using an iron with low melting point solder and they come off at a much lower temperature.