Working with SMD



The dreaded surface mount device has been around for ages, love it or hate it it’s here to stay. I actually learnt some tricks from a very large computer motherboard repair company which actually relied more on your skill with a soldering iron than expensive tools to work on these boards. Like all craftsmen, one can blame the tools but a good craftsman makes a plan. Where to start?

Coppermine CPU

Like most laymen I never got round to bothering about the intricacies of soldering – solder types, temperature, compounds, tip angle and the importance of flux.  Until a friend of mine did a soldering course – it was all top secret until he told me rather shamefacedly that it was part of his training. In fact it wasn’t any soldering course – it was to NASA standard. It was then that I acquired a book about soldering and I then rather ashamedly realised how arrogant I was – I really knew nothing about soldering techniques and more so, that what holds for us as mere consumer product repairmen does not hold the same for people developing and building a satellite or military spec radio receiver. Yes, we knew about the different skill-set required but we rarely if every worried about it. Solder was solder wasn’t it? There is an industry standard requirement for solder and soldering – it’s out of the scope of this article. This is a very generalised article only because the web has it’s own set of professional writers and more to the point (pun intended) expert “how to do” pages.


 

Tag Strip and PC Boards

If you have been fortunate enough to have gone through the transition from tag strips, to vero board to PC board, then to multi-layered boards and SMD you’ll note that there is always one common factor, when conducting a repair aften you need to remove a device to make measurements, most probably either just one pin as in a resistor or diode or in some cases just the input or output of a device such as a transistor or digital chip. The technique of fault-finding doesn’t change but in many ways with modern equipment it does become simpler. Yes, there will be disagreements to this but I’m following the rationale that more often than not it will be a power device that fails first or possibly a component that normally runs hot. Static electricity is also a big killer and in motherboards (computers) that flaky power supply can cause more damage than what it’s worth repairing. Always use good power supplies from a trusted source.

Where to start? What tools to use?  Your workshop has to have a very good soldering iron, these budget units don’t work. Get a heat gun with temperature control and fashion a set of tips for air concentration if you cannot buy.  Make a box where the heat gun can stand upright, the wooden pieces (yes, wood) on either side of the heat gun should be fairly narrow – in most cases 10 cm is sufficient – allow sufficient cooling for the heat gun. With the heat gun facing upwards and the faulty board resting on top you can heat up areas of the board very quickly, removing faulty chips or loose components with a pair of tweezers. Be careful not to remove or move components you don’t want moved. Be cautious of a double sided board where components can drop off – most computer motherboards don’t have components on both sides. This method described above works very well for very general repair but not BGA or Ball Grid Array repair work. BGA machines heat the top and bottom of the board equally, there is a top nozzle blowing hot air onto the component, possibly a chipset or processor.  The fitting has to be precise so the nozzle should be made to run as close as possible to the perimeter of the chip – standard masking tape is attached to the outside components to prevent these components from moving.  To get a good idea of this we have The Instructables website to the rescue:  http://www.instructables.com/id/BGA-rework/


 

There are quite a few companies supplying BGA Machines, the cheapest one I saw which I know worked very well cost in the vicinity of U$3500. This was the most basic machine. The next one up which also used hot air cost in the vicinity of 10 000 U$. The BIG difference between the two was that the more costly unit was totally automatic – the only real work you had to do was align the chip. With a magnified view this becomes a simple task. The cheaper units are for the more experienced or rather for those that feel comfortable in working quickly and not being shy of precise alignment work.  I am surprised to see that there are very few experimenters building their own BGA machines. Modern technology is also veering away from hot air to IR.

Building a BGA machine should not be difficult although I have not built one myself.  It is important to have a (preferably) digital temperature readout of top and bottom air flow temperatures. Being too impatient can cost you the job entirely – it’s not unheard of for operators to remove a piece of work too soon and components, including your replacement work shift. Worse still if a component drops off and you do not have the type or value.

Complications

Main difficulty here is working with RHOS which is another beast entirely. I started working with BGA reflow when RHOS started becoming popular and the biggest threat was the higher melting temperatures of RHOS solder over your conventional types. Why? Board damage. Even seasoned operators found themselves burning boards whist being as careful as possible. This is where the more advanced BGA machines showed their worth. Some chipsets can be extremely expensive – look no further than high end graphics cards. In companies conducting repairs of this type the graphics cards are normally repaired by the more seasoned operators – a chipset could cost in the vicinity of a 1 000 U$ and upwards so they do not take chances. A northbridge for a very popular older board cost about U$20.00.

Fortunately…

Fortunately for most experimenters and repair personnel the parts most prone to failure are the power components – usually FET voltage control circuits, the VCORE supply rail controllers and smoothing capacitors. On some motherboards your VCORE is controlled by a three phase supply so all the switching power components get changed – not just the one you deem to be faulty. Fortunately they normally sit on a very accessible part of the PCB – the copper acting as a heat shunt. BIOS or IO chips can be removed fairly easily – get an EPROM reader/programmer. More often than not the BIOS chip loses its instruction set – rewriting solves the problem. However it has to be done off the board. Not always, but mostly.


 

Solder baths

Here things can become tricky again – when removing connectors off a board the procedure is to run the connector over a bath of solder, with experience this becomes a very simple practice but agin here RHOS has it’s own behaviour because of it’s higher temperature. Inexperienced operators will find that RHOS is very stubborn and all the small components rather dislodge first, any rapid movement can cause the smaller components to dislodge and you end up with a heap of mangled components and solder. Patience is the key.

RHOS

The introduction to Restriction of Hazardous Substances Directive or (RoHS) soldering is nothing new – essentially lead free soldering. The higher than normal operating temperatures to melt solder can leave us confused, causing damage to adjacent components on your work. As mentioned previously, I have seen seasoned operators pulling their hair out when first working with RHOS. RHOS has changed over the last 10 years and there are lower melting point solders available. The main thing to watch out for is not to combine RHOS solder baths and stations with non-RHOS.  Mostly this has to do with compliance and ISO certified companies take this very seriously.

An article covering this can be found at Hardware Secrets.

Soldering tips

In order not to plagiarise a very common topic I’ll get straight to the point – patience, patience and more patience.  In my days doing repair work the two most common problems that stand out a mile is a) first and foremost, lack of patience. If you don’t have this virtue don’t do this kind of repair work. In fact don’t get involved in any form of repair because all that will happen is that you, the reapirman, will become more frustrated and cause more damage. It’s nice to learn everyhting through Google but there is nothing better than seeing someone actually walk the talk. This can be seen on You Tube time and time again. So not to beat this subject to death we move on to number two, (b) poor eyesight. Poor eyesight is not usually a hindrance, in fact I have seen some old – timers doing better work than our 20/20 vision workers, only because they apply a lot more patience to their work through of course, the best virtue of all ‘Passion”.  It may take a little bit longer but it will get you there.

General

For many people starting out in the electronics industry, possibly as a hobby or later on full-time engineering there is still nothing better than gaping at a well constructed piece of electronic equipment. I have seen some cheap computer power supplies from Asia which boggles the mind when it comes to quality – very good quality at a next to nothing price. There are expensive consumer products which look as if they were put together in someone’s backyard – yes, a NAD amplifier purchased in South Africa. I have seen television sets, the last of the CRT variety, with wire wrapped power supply components and a PCB for the main circuits. Marine and Aviation equipment is all but cheap and their construction shows it.  I haven’t seen NASA equipment, I have seen military and medical spec and again you pay for what you get. Soldering can be done by all ages but good soldering is only done by someone whom shows the ability and passion to want to do a good job.

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