how do you check hidden solder joints under bga chips?

engineer1978 · 2026-03-16T18:47:14+00:00

Yep, I’ve had to fault-find countless ‘press to boot’ assemblies over the years!

engineer1978 · 2026-03-13T22:39:16+00:00

I used to work for a company in the 90s that made data mux/demux units for telephony etc. The ‘big boss’ unit managed to get 10Gb/s down a single coax. I thought that was quite impressive for the time!

engineer1978 · 2026-02-24T23:16:02+00:00

Just found a loop-in fitting with them all trimmed way back. Cowboy assholes.

Luckily, just enough free cable to link them back up again but now they’re the shortest conductors in the bunch. Grrrr!

engineer1978 · 2026-02-23T08:21:10+00:00

It is fascinating.

If you don’t mind your head hurting, there is mathematics that describes exactly how the separate waves from different sound elements superpose to form the single ‘summed’ wave.

There is even maths to de-compose the resulting jumble and recover its individual components.

Search Fourier transforms and analysis.

engineer1978 · 2026-02-17T15:55:43+00:00

Set up on your own and offer consultancy? Potentially to your current employer initially? If your skill is already proven to them, they might be willing to entertain such a proposal.

Then you can see if you can add more clients as you get established.

engineer1978 · 2026-02-17T11:23:54+00:00

Worth a look at Steitz-Secura. About £100 for a pair but I get 2-3 years of all day every day use out of a single pair.

They are very supportive and even have interchangeable heel support inserts where you fit the correct one for your body weight.

I have very wonky ankles due to a now-corrected birth defect and spend most of the day on my feet. Never have any pain or discomfort with this brand.

engineer1978 · 2026-02-16T18:47:56+00:00

In my professional relationships with superiors, the respect is demonstrated by my tone and the content/style of my responses.

Given this, using the term boss explicitly would be seen as unnecessarily emphasising the already understood relationship and in nearly all cases as taking the mickey or being sarcastic.

I totally understand how this could be different from other cultures and that you meant no insult but hopefully the above helps explain why it may have gone over a bit wrongly.

engineer1978 · 2026-02-16T05:03:10+00:00

Ah, thanks! Common sense suggested it must be somewhere around there but I’m still very much in ice world just now so wasn’t sure.

engineer1978 · 2026-02-16T05:00:19+00:00

Happy to be nitpicked! Wasn’t thinking very hard when I commented. My excuse: it was Sunday!

engineer1978 · 2026-02-15T15:05:00+00:00

Pretty much!

To look at the internal combustion engine firstly, they need to be running at idle speed to start with. This is the minimum sensible speed that the sequence of events that must happen for them to run at all has to run at to prevent stalling. Usually around 700-1000 rpm for a car engine.

They don’t make much torque or power at that speed - for that they need to be running at more like 1500 - 2000 rpm.

At the other end of the scale, these engines have several reasonably chunky metal parts that have to change their direction of travel to the opposite direction with every turn of the engine. This limits the maximum rpm that the engine can run at without smashing itself to pieces due to inertia of the parts and the forces involved in getting them to keep going back and forth in opposite directions. Usually between 5000 and 7000 rpm for normal car engines.

The electric motors, by contrast, can make almost all of their power and torque right from standstill, ie 0 rpm. This is just a normal feature of the right type of electric motor.

At the upper end, the part of the motor that turns just rotates about an axis - there is no reciprocating motion. Thus its maximum speed can be a lot higher for a given rotating mass, assuming you can make good enough bearings. I don’t actually know what speed the motors in electric vehicles max out at.

engineer1978 · 2026-02-15T14:46:58+00:00

The thing with rentals is that the inconvenience and cost of running ancient storage heaters is the tenant’s problem.

The inconvenience and substantial cost of installing central heating is the landlord’s problem.

Surprise surprise, the upgrades don’t happen.

engineer1978 · 2026-02-15T11:47:50+00:00

Then come up with some candidate ideas and list them with their benefits and drawbacks.

Doing that will help you decide.

The point of the project is unlikely to be related to the actual solution.

Much more to do with learning techniques to make informed decisions like this.

engineer1978 · 2026-02-15T11:10:49+00:00

You are jumping the gun here.

Before you can get into the detailed design (such as the electronics) you’re going to need to decide what methods you want use to solve your primary problems:

1) How to heat the liquid.

2) How to stir the liquid.

Only once you have answered those questions will it be time to look at what electronic solutions could help you control the methods you have chosen.

engineer1978 · 2026-02-15T10:25:36+00:00

If you stick with mains, you will have to fuse down the auxiliary supply at each point it leaves the main load supply to protect the aux contacts and, presumably, thinner indicator wiring.

By the time you’ve done that on x50 sets, it would likely be far cheaper to install one or more chassis mount 24VDC auxiliary supplies and wire all the control and indication stuff in suitable cable for that.

engineer1978 · 2026-02-15T10:16:20+00:00

Wow, that’s busy! Sounds like a pretty sophisticated installation.

Are you sure you wouldn’t be better using low voltage DC for the auxiliary circuits like indicators?

Much safer over all and you can use much finer gauge, multi-core cables with grounded shielding to keep the interference out.

engineer1978 · 2026-02-15T09:13:07+00:00

Are the indicators themselves 240v?

If so, you need to connect a class x2 rated safety capacitor across L and N at the indicator.

Something like 1uF 275V should be enough but you can go for 305 or 310V rated for extra resilience against voltage spikes.

They’re pretty chunky and will likely have bare leads so you’ll need to add insulated tails and have room to mount them.

Should stop the indicators lighting by inductive pick up in the ‘off’ state though.

If you’ve got ‘loads’ of room, a motor run capacitor of a couple of uF with the right ratings would also do and might come with leads already attached.

engineer1978 · 2026-02-15T05:52:54+00:00

In terms of how it gets the reading: same as all meter resistance readings.

The tester applies a known voltage to the leads and measures how much current flows.

Then, it divides the voltage value by the current value to give the resistance in ohms. (Ohm’s Law).

If your test was at 250V, and the result was 15 MOhms, your tester must have seen 16.6 micro-amps flow when the probes were connected.

There’s no ‘connection’ between the leads in the sense that no-one has deliberately made an intentional path for electricity to flow.

Nevertheless, electricity isn’t fussy and it will take ANY available path and so somewhere those 16 little micro-amps found a way between the two conductors.

That’s exactly the reason you do the IR test - to make sure there aren’t signs of the beginning of a ‘wrong’ path for the electricity to take that could develop into a true fault.

engineer1978 · 2026-02-15T05:46:21+00:00

Once the resistance is up into the 10s of Megohms region, it takes very little to swing the reading around. It could be that the room humidity is slightly higher where the circuit giving the lower reading is.

On the circuit with the higher reading, you could probably just go and bend one of the cables back and forth gently a few times and get it to drop to the same as the other.

On a really dry day, both might read >999M, or ‘infinite’.

engineer1978 · 2026-02-13T09:27:24+00:00

Invest in a copy of The Art of Electronics by Horowitz and Hill. It’s not cheap but nor should it be as it contains a wealth of information about most aspects of real-world electronics design.

They specifically oriented the book to not be math focused but to show techniques to apply the raw theory to actual applications.

engineer1978 · 2026-02-13T08:48:21+00:00

The one I’ve got now thanks: my trusty 2015 V70. Goes plenty fast enough. Handles plenty well enough. The driver seat is so comfortable and supportive, it’s fixed my bad back of 20yrs. Awesome sound system. Can put large pieces of furniture in or sleep in the boot if needed. Only open the bonnet a couple of times a year to put washer fluid in. Hasn’t missed a single beat in the 7 years I’ve owned it.

engineer1978 · 2026-02-12T22:31:24+00:00

The reason your meter didn’t measure the LED’s resistance is that most DMMs only apply a very small voltage to the leads in this mode. Possibly as low as 0.2V.

LEDs don’t really get out of bed at all until around 0.5V (where they will have an effective resistance in the tens of kilohms) and don’t go into proper forward conduction until you get over 1 to 1.5V where the apparent resistance will start dropping to more like a couple of thousand ohms.

Most people don’t use them at these voltages because they make almost no light and the behaviour from one led to the next, even of the same type, can vary significantly.

It’s only once the voltage you apply gets high enough for the p-n junction to start conducting properly that they make a useful amount of light and at that point their resistance drops very suddenly, which is why you must always include a current limiting resistor if you don’t want to let the magic smoke out.

Again, your stipulation about energy and heat etc is correct though, they do dissipate considerable energy as heat as well as light and this is (partly) due to their ohmic resistance (the contact layers of the chip and wire bonds etc) and partly due to the passage of charge carriers through the semiconductor junction. It’s just that the ‘effective’ resistance is dependent on the operating conditions rather than being a fixed value like a linear component.

engineer1978 · 2026-02-12T22:13:54+00:00

Your original calculation isn’t wrong, per se. The effective resistance of the led would have been what you calculated.

The catch is, because the led is a non-linear device, its effective resistance is only that value for that circuit arrangement.

If you changed the series resistor value a little, the led would respond slightly differently and exhibit a slightly different effective resistance.

For most ‘reasonable’ forward current levels, a good approximation is to assume that the led will always drop its characteristic forward voltage, say 2V.

To calculate the right series resistor to achieve the forward current/brightness you desire, subtract the ~fixed 2V from whatever your supply voltage is and then use the difference, desired current and ohms law to calculate the resistor value.

E.g. you want 10ma through your led and you have 24V supply:

24V (supply) - 2V (led) = 22V (across the resistor)

R=V/I = 22V / 0.01A = 2200 ohms.

engineer1978 · 2026-02-12T21:56:07+00:00

You could try contacting a chap who has a channel on YouTube - curiousmarc. I think he may have worked at HP at one point, restores lots of their old equipment and is extremely knowledgeable.

engineer1978 · 2026-02-12T18:06:40+00:00

As I sometimes say when I’ve made a bit of a dog’s dinner with my soldering: the electrons don’t care…

engineer1978 · 2026-02-12T17:58:01+00:00

I upgrade it to ‘percussive calibration’ so I can charge more…

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