In the third in a short series on modern moving light service and repair, particularly LED based fixtures, On Stage Lighting looks at a LED problem and the difficulties and a NSFW solution to changing an individual RGBW LED chip on the Chauvet Rogue R2 Wash.
In previous articles in this series, we looked at some common themes when it comes to keeping LED stage lighting gear on the road, and at some specifics of a particular fixture model, the R2 LED Moving Wash light. We briefly considered the common practice of replacing whole component assemblies, such as PCBs or power supply boards. But what about component level repair? And what about LED replacement in particular?
When is component level repair feasible?
There are number of considerations to take into account when it comes to deciding if component level repair of your fixtures is viable. To start with, this is often down to the time required to find and verify a faulty component rather than a whole assembly. Dealing with such a level of detail is always gonna take time, not to mention expertise and equipment. For many users, identifying that you need a new screen PCB and fitting one is much more easily achieved than hunting down a particular issue on that PCB – and even if you find the issue, can you even get and fit the part you need?
A lot of the time, with a stage lighting inventory that contains many modern fixtures including LED based light sources, component level diagnosis and repair needs the luxury of time that may be in short supply. One common approach is to initially replace a whole assembly and place the faulty one on a ‘donor or fix’ pile, ready to be tackled during the last half hour of the working day. If it the board gets fixed, great we have a spare for next time, if not then at least nobody is waiting on a dead fixture to come back into use.
The main situation where component level repair of a complex assembly is attractive is when the cost of the whole assembly is high and easily offsets the time required and the opportunity cost of a fixture that is AWOL.
I’m going to describe such a situation today.
Replacing an LED chip in the R2 Wash
In our example case today, one of our R2 Wash fixtures featured in the previous article has a dead LED chip. Just one, of nineteen. In fact, the chip is actually not completely dead but one of the four Red, Green, Blue and White (RGBW) LEDs in the chip doesn’t work. We have already verified that it’s not the driver circuit, the loom or anything else – it’s the LED and that’s that. At this point we may have even got to verifying the chip itself which on the R2, like many other LED based stage lighting fixtures, involves stripping the unit right down to the middle of the fixture to actually get at the LEDs themselves.
Is this a reasonable case for component level repair? Yes, because the only other option (and the one usually suggested by Chauvet) is to replace the LED PCB. Replace the whole board, nineteen LEDs and associated circuitry. Just like when you take your car to your mechanic these days, the standard practice is to replace a whole assembly – not just a part of it. There are good reasons for this practice and don’t let me encourage to hack repair your hard earned pride and joy WobblyLEDs against the manufacturers advice…. It’s just… look, the whole LED PCB assembly is expensive. Like, more than your fixture is worth expensive. I can’t remember how much exactly from Chauvet right now but I see it listed on a 3rd party parts seller site for around $1500 USD. Yeah. Er, no.
So, I have a faulty fixture with a missing LED or a huge parts bill that isn’t gonna fly. I need to find a way to swap that LED out.
The problem with LED replacement on the R2 Wash
I can get a single LED chip from Chauvet for a reasonable price. I can also probably pick it up from other suppliers. I don’t know for certain because I haven’t really looked into to but it’s probably the OSRAM Ostar Stage or something similar. Anyway, that’s not the tricky part. I’ve replaced faulty LEDs with brand new ones from Chauvet and also with old LEDs taken from donor boards.
The LED comes in a type of package like a DFN Dual Flat No-Lead chip, it’s a surface mount on the PCB with eight terminals in two rows of four and a large ground/heatsink pad in the middle. Being a LED, the bits and pieces sit underneath a clear plastic window/lens on the top. This mean, unlike many SMD processing chips in a similar package, you can’t apply heat to the top of it in order to solder/desolder it from the PCB. This is only some of the problem when it comes to replacing the LED chip. The main one is the fact that the chip is soldered onto a Metal Core PCB – the whole board itself is designed to conduct heat away from the LEDs and onto the large heatsink that it’s mounted on in the middle of the fixture itself.
So, we have SMD and a large thermal mass. Obviously, I can forget using the usual soldering iron. Also forget using something like Chip Quik, this package has no legs even if I could get enough heat through it. Due to the nature of the entire, heat-sinking PCB, you can also forget a hot air tool providing local heat from any direction. When first tackling this repair, I tried any number of ways to get localised heat to remove the faulty LED and none of them provided enough heat before the PCB wicked it all away again. I needed to get creative and consider how the PCB was made in the first place.
(t’s worth just clarifying at this point, that all this work happened it was with the LED PCB completely removed from the fixture.)
Disclaimer: The following information does not constitute not advice. It simply documents an experimental process that I have undertaken at my own risk.
Reflow of the Metal PCB. With a gas stove.
The LED PCB is a chunk of metal, with tracks, LEDs and some other surface mounted components on it that include 000 ohm ‘resistors’ (jumpers) and multipin headers to connect the LEDs to the driver board. When it was manufactured, the components were positioned by machine on the PCB board along with solder paste and the whole thing was heated up in order to make the solder/flux combination melt and make connections. The board was then cooled to finish off and probably cleaned and all of this was probably highly technical and computer controlled.
I just needed to find a way to do this without all the gear, a hotplate style of heating from below this board. The components sit on top, the board is plain metal underneath. If I could just heat up the board well enough from underneath to melt the solder/flux, then the faulty LED could be removed and a new one put in its place. Once cooled, the LED would be reconnected via the solder joints and with luck we’ll have a working board.
Enter a camping gas stove. The kind of gas hob used for camping is fairly well suited to the size of the PCB in question. While the central flame may have some concentration of heat, the board is not that much larger than the key heating areas of the hob and the metal is sturdy and thermally conductive enough to transmit heat around the place.
In order to keep stuff clean/safe, I scraped off all the thermal paste on the back of the PCB and give it a good clean with Isopropyl Alcohol so you are heating up clean metal.
Risks of gas hob reflow.
Because this is a single sided board with bare metal on the underside, I don’t to worry too much about the flame and components as they are protected from the flame by the metal – it’s like a skillet. I do however need to be concerned about myself and the flame, and also that there are several large holes in the board that allow heat to come through to where I am. Don’t forget I am removing and replacing components manually with tweezers during this process.
With reflow work like this, I risk distorting the PCB with poor timing, temperature control or uneven heating. While this is still a risk, the chunk of metal that is the base of the PCB is more robust than a normal composite board. I just need to be a bit careful.
While replacing components, I am working with every solder joint on the board in a molten state. This means that every single component is ‘loose’ and free to move. It’s very easy to knock something out of position or accidentally swipe it off the board completely while concentrating on the task of replacing another component.
On researching manufacturers reflow techniques, there seems to be some good practice in terms of the use of temperature to get the best results. This is usually controlled by machines, but I attempted a stab at a reflow temperature ‘profile’ in order to make a nice job of it. Profiles come in a few stages:
- Initial Ramp Up – heating the workpiece up to Soak temperature
- Soak – Keeping the workpiece at a steady temperature, lower than the melting point of solder.
- Ramp to Reflow – Temperature rises to reflow melting point.
- Cool Down
So my plan was to gently heat up up the board to a base level, leave it to stabilise at a soak temperature for a short while, then step up a gear in order to reflow, then cool down. The great thing about a gas hob is that there is no lag in control of the heat of the flame itself – only the workpiece metal.
The first time I attempted this, I didn’t use a thermocouple or any other kind of temperature sensor. During a later repair, a colleague lent me an Infra Red Thermometer which was useful for interest but didn’t really change how I did things.
To recap, I was going to attempt to slowly heat up the PCB by setting the hob on its lowest flame setting, let it ’soak’, then bump up the flame to reflow. Then shut the flame off (and probably use a desk fan to help with the cool down).
During experiments, the first thing that I noticed was that even the lowest flame on the gas hob created quite a bit of heat (turns out the board got to 160 degrees on the IR thermometer, even on the lowest gas setting).
The adjustment was to hold the board in a set of long nose pliers and introduce it to the lowest heat gradually.
The second thing was that the metal of the PCB held the heat pretty well after gas shutoff. This observation led me to decide that I would do all the replacement work with the gas off, after the reflow temperature had been achieved. This was not only safer for me but the solder remained workable and the cool down didn’t rush me too much , as long as I was organised and swapped the LED without too much messing about. (On boards where I have swapped a number of LEDs out, I sometimes give the gas a quick blast again to keep the reflow going.)
Solder Paste, Flux, or not?
When PCBs like this are made, a solder/flux combination in the form of a paste is usually stencilled onto the board and this is what makes the connections during the reflow process. I had some solder paste to hand, but pre-repair experiments with this suggested that it was too easy to use an excess of paste and, because the PCBs themselves already had solder on them, there was a risk of too much solder on the pads. This had the potential to result in bridged connections and short circuits. This is very much the case when using LEDs from a donor board, as adding more solder to a PCB with ‘old’ solder on it that is accepting a LED, also with ’old’ solder on it, equals too much solder on the board. In early trials, I did use solder wick to clean the pads as much as possible but even then it was easy to overdo the paste. Similarly, this also could lead to adding too much flux and problems resulting from this – particularly underneath the LED itself. If I was going to use extra flux, I would opt for some of the No-Clean type as there is no telling what is happening under the chip once it’s been reflowed.
In the end, I was satisfied with the results of not adding additional paste or flux and just using what was already on the board. Bear in mind that these chips aren’t that big, the pad connections are pretty small and close together, and so aligning stuff and not creating shorts isn’t that easy. I felt that it was gonna be made even harder by adding too much additional ‘product’.
Reflections on the process of R2 LED replacement.
I should once again stress that the above procedure is not what Chauvet tech support suggest you do to your prize R2 Wash. I mean, if I told them I had some form of properly controlled hotplate reflow equipment then perhaps they might be OK with it as a method of LED replacement and reflow. I’m pretty sure that they don’t advise you cook up a $1500 PCB on a camping stove.
First thing to note was that I was able to try some stuff out on an already retired board due to an issue with track corrosion. Now, PCB track repair is perfectly possible and on another day I’d have sorted that too but on this occasion the fixture was to be a donor anyway. This meant that the first times I tried this relflow process out, and the various other failures on the way, I didn’t have to do it on a board I was trying to fix.
Another reflection on even attempting this kind of ‘off-piste’ repair process is that it is not gonna be appropriate in many other situations and with other fixture models. If a fixture is of higher value, or newer, or the process itself is impossible due to the design, then plenty of times this hack isn’t gonna be the answer. There still could be issues with the method that may only manifest over time – although in this particular case, the units repaired didn’t actually have a lot of prime time left and were no longer A or even B stock.
What we have highlighted today is the issue of equipment longevity and repairability, including that of LEDs themselves, is not a discussion that is over in the world of stage lighting. In fact, when it comes to LED based fixtures, it’s only really just starting. We’ll look at that next time.
See you then.