Intermittent Potterton? Flashing red LED? PCB Fault finding

[LeaUK]

Hi all

Potteron Suprima 100
PCB 5102160

Firstly many thanks to those offering solutions to the infamous Potterton PCB failure - mine is now fixed, although I will confirm after a week or so. However I wanted to contribute in return and show a few interesting pictures I took recently.

BTW - my background:

I'm a Electronic Design Engineer with over 15 years experience including full product life-cycle. I'm experienced in CE marking regulation and a variety of safety standards and thought some other maybe interested in my findings too - especially the close up of solder joints.

I have concerns not only with the poor quality solder joints exhibited by elder PCBs, but also the safety implications of this coupled with the lack of sufficient earthing found on the right hand panel, however I'll comment on my findings from a design point of view first.


1. Poor quality solder joints

These develop over the first three years and on my PCB look like this:

Fig 1.1 - example 1

Fig 1.2 - example 2

Fig 1.3 - example 3

A sample of poor joints were found predominately in these areas:

Fig 1.4 - general area of poor joints


1.1 Defective joint areas

After careful inspection I discovered around 15 defective joints on predominately the larger plated through hole (PTH) components, ie. connectors, relays and the larger black diodes. The evidence points to the failure mode being a combination of poor wave soldering and significant thermal cycling. Thermal cycling would explain why some see exacerbated problems when the PCB is cold.

Fig 1.5 - defective joint areas


2. Carbon deposit

Watch out for carbon deposits on the PCB's top surface caused by local ionisation. This can bee seen as very fine black powder - see right hand arrow of fig 1.5. Carbon power will also be evident on the HT lead connected to the igniter.


3. Excessive power dissipation of diodes

Diodes exeeding their maximum power dissipation can be clearly seen Not only are their leads discoloured but scorch marks are evident on the PCB surface - this maybe contributed to poor design.

Fig 3.1 - excessive power dissipation of two diodes


4. Safety implications


4.1 Solder joints

Due to the sheer volume of poor joints it may be possible for the PCB to enter a state where byt the gas valve is opened in an undesirable condition - although I would hope that this isn't the case.


4.2 Earth protection

Typical safety standards (EN 60950, EN 601010 & EN60601-1 and alike) mandate that LIVE parts be separated from unearthed accessible metal parts by at least DOUBLE insulation or SINGLE insulation where the ACCESSIBLE part is sufficiently earthed - neither of which the Suprima 100 complies with!

To ensure the outer panel is earthed the boiler relies on one screw head and 'crinkle washer' to make sufficient contact with the front panel. The paint hasn't even been removed beneath.

Therefore for the purposes of safety, the fron ACCESSIBLE panel should be considered unearthed.

Fig 4.1 - poor earth contact of screw in centre of panel


As the boiler has only SINGLE insulation protecting its mains wires AND these can easily touch the metal outer casing and the outer casing should be considered unearthed LIVE parts being able to contact the front panel should be at least DOUBLE insulated - and they're not, they're only single!

Please don't panic as the safety standards applied by Baxi may not be as strict and the unit has of course been CE marked, however any company can declare their product CE compliant but it's only when someone challenges it do they have to prove it!

With that closing note I hope this has been of interest to some and maybe I'll take the issue up with Baxi - see what they have to say

And remember - don't attempt any repair for yourself, if you haven't solid experience and skill the simplest and best practise would be to purchase a new 5111603 PCB and ask a qualified Corgi registered plumber to fit it.

Lea

PS - still nice and warm and I've spent 2 hours writing this and had dinner that would normally mean at least two resets...

 

[lor]

thank you Lea uk ,v interesting . have replaced 100,s of these boards .

 

[Agile]

From an electronics point of view I found what you said interesting although nothing you said was new to me.

There are a few comments I would add though!

Firstly manufacturers make most of their profit from the replacement parts rather than the initial sale, this is true of boilers as well as cars. Reliability is therefore not a commercial interest to them as long as initial sales hold up.

Some of the discoloration on PCBs is caused by heat dissapation when the varnish is new and not properly cured and arises during the manufacturers initial burn test.

I have never ever seen a Suprima fail due to a problem from carbon deposits like you claim to have found. Certainly electrostatic attraction to the HT lead area does occur but I dont know if the particles concerned are particularly carbon in composition.

You mention the overheating of the diodes with extended mounting leads. Most boilers are fitted without adequate ventilation in accordance with the manufacturers instructions. If fitted on an open wall at 21*C ambient I doubt there would be much problem. Unfortunately people love to box in boilers and reduce the free air ventilation.

A gas boiler is a harsh environment for electronics with a gas burner at say 600*C within 70 mm of electronic components intended to operate below 85*C. Typically thermal cycling 30-300 times every day is not helping! Certainly an insulating layer would help but see Para 2 above.

Tony

 

[ChrisR]

Some of the holes are just plain too big!

Lets not lose sight of the point of Baxi-Potterton being in business. It isn't to make us warm, it's to make them money. How they do it is up to them. I have worked on/managed design & development & QC of pcb's through to manufacture and distribution. There aren't any secrets - haven't been for decades. If B-P had wanted to make the board reliable (MTBF 10 years, say) it wouldn't have been remotely difficult. Just consider defence, aerospace, automobile requirements - standard stuff.

I'm going back a bit now but looking at the board there's nothing which I would expect to cost near £1. Whole pcb maybe £10.

After careful inspection I discovered around 15 defective joints on predominately the larger plated through hole (PTH) components

That's interesting. None of the holes on the one I have in front of me are plated-through - they aren't made like that! If they were they would be much less prone to solder failures.

 

[LeaUK]

HI Guys

Thanks for your comments. Unfortunately my report was made late last night and upon re-read I would have adjusted a picture's position and corrected the obvious spelling mistakes but I hope you forgive me - is there an edit button somewhere?

Agile

Some of the discolouration on PCBs is caused by heat dissipation when the varnish is new and not properly cured and arises during the manufacturers initial burn test.

You mention the overheating of the diodes with extended mounting leads. Most boilers are fitted without adequate ventilation in accordance with the manufacturers instructions. If fitted on an open wall at 21*C ambient I doubt there would be much problem. Unfortunately people love to box in boilers and reduce the free air ventilation.

I can see no evidence of varnish on my PCB and in my opinion the scorching mark is too localised to have been caused by the heat from the boiler. It is concentrated on the PCB surface right beneath the two orange coloured diodes within an area measuring around 10mm squared. The piccy isn't particularly clear.

Look at the severe colour discolouration of the diode legs - this is the clue and points directly to excessive power dissipation of the diodes.

I have never ever seen a Suprima fail due to a problem from carbon deposits like you claim to have found. Certainly electrostatic attraction to the HT lead area does occur but I don't know if the particles concerned are particularly carbon in composition.

Please don't get me wrong, I didn't attribute this level of deposit to PCB failure but to be 'careful' ie., to consider dust removal while the PCB is out of the unit. Deposits of this nature are routinely found within products containing HT.

ChrisR

That's interesting. None of the holes on the one I have in front of me are plated-through - they aren't made like that! If they were they would be much less prone to solder failures.

You have hit the nail on the head Chris and of course you are quite right. My familiarity with PCBs meeting military, medial and industrial high reliability regs led me to mistakenly assume this inexpensive PCBs used PTH holes (just because all my PCBs do!!). BUT, if a PCB is single sided (as in this case) it doesn't!

Couple typical single sided lands (pads when the component solders) with rapid and large temperature cycling and possible wave solder bath contamination and you may have problems!

Also be aware that during repair one of the lands lifted, this is also attributed to non-PTH lands. As Chris mentions the mechanical strength of PTH is far superior.

 

[Agile]

"""in my opinion the scorching mark is too localised to have been caused by the heat from the boiler""".

I think you have missed my point!

If boilers were installed with adequate ventilation then it is unlikely the heat dissapated from the components would cause any problems. The discolouration is caused by heat from the components which is not removed by natural convection.

Most consumer electronics are expected to operate in an ambient of a maximum of 35*C. Put your boiler in a cupboard and the ambient can rise to 60*C plus due to the lack of proper air flow, and thats just on the OUTSIDE of the case containing the PCB.

Thats on the outside! Inside the boiler there is a combustion chamber with flames at 600*C very close to the PCB.

I agree from an electronics point of view the situation is far from ideal. But this is a mass produced consumer product and not an industrial device.

Tony

 

[LeaUK]

Hi Tony

Product design specification *should* encompass the ambient temperatures found within boilers however I'm sure this isn't always the case - consumer or not.

If boilers were installed with adequate ventilation then it is unlikely the heat dissipated from the components would cause any problems. The discolouration is caused by heat from the components which is not removed by natural convection.

My boiler has plenty of external cooling as it's not in a cupboard so I think we'll have to agree to disagree here as in my opinion the discolouration of the diode legs (which is heavy and may not be so visible on the photograph) has been caused by the excessive temperature rise of both diodes. This colour temperature indicates significant (much greater than the ambient + normal working component temperature) localised leg temperature and has subsequently caused the scorching evident on the PCB top surface.


Leaving this issue aside - whatever the reason for the poor solder joints and excessive heating, what I can say is that after 2 days no resets have been required, so thanks to all those for pointing me to the PCB.

Personal cost saving of £100 (new PCB off ebay)

And a few hours typing here...

One last thing, what typical safety standard are boilers CE marked against, as I'm interested in how these products have actually attained a CE mark! - for the reasons of poor earth bonding mentioned previously.


Cheers
Lea

 

[Hagbard]

Good pics LeaUK, and exactly how the joints looked on my Suprima 50 board. I've had 2 goes at re-soldering it. The first was to cure a fault whereby the pump would switch off with the boiler running. This rapidly made the boiler shoot steam and boiling water out the vent pipe (which is not over the F+E but discharges above a path) before the boiler (correctly) locked out. Poor joints on the output pins to the pump were the cause of this. Second go was last October and cured the common intermittent lockout problem, in my case about once every 3 days. The culprit this time was poor joints near the diodes at the centre of the board.
My board is varnished and also exhibited the scorch marks near the diodes. Is it possible the overheating of the diodes is caused by the high resistance or arcing at the faulty joints?
The boiler has now run faultlessly for 4 months. Had it not done so I would not have 'upgraded' the PCB but bought a new boiler, any model so long as its not made by Baxi/Potterton, who are apparently unaware of any problem with Suprima PCB's although they continuously redesign them.
Why is it that cars with dangerous faults are recalled by the manufacturer but boilers aren't?

 

[Agile]

Hag, you have identified another of the problems of the Suprima, poor temperature control.

I dont know quite why but there is a very slow response to a rapid temperature rise should the pump fail resulting in steam being produced before the boiler finally realises that its passed its set temperature and turn the gas off.

The temp control is on the flow pipe where an NTC is set in heat transfer paste and I would have thought would respond quite quickly. Part of the problem is understandably caused by the latent heat absorbed by the C.I. heat exchanger which continues to boil the water after the gas goes off.

The diodes are presumably zeners and as they dont fail I can only assume that they can operate at the elevated temperatures they seem to reach on that PCB. If I was designing it I would be worried about that but presumably the commercial pressures to keep the cost to a minimum mean that if they dont fail then thats OK.

As far as CE marking is concerned, Lea has probably already explained it above when he referred to "self certification"!

To be honest boilers operate in a multiple earthed situation where there is usually a multiple earth path via pipes and electrical connections. Its interesting that one manufacturer, Ideal, do seem to take electrical certification seriously and fitted TWO RFI filters on their Response model as well as very seriously effective earthing.

Tony

 

[LeaUK]

Some interesting points to watch out for regards the NTC - cheers.

Interestingly enough both zenner joints *looked* ok and so I'd imagine the resistance of the joints low - although maybe there is poor conductivity beneath the surface. On my PCB it was the black relays and larger mains headers that suffered the highest poor joint count. In fact under a little light hand pressure I could physically see the solder joint opening and closing as I 'rocked' the relay!

CE marking is a hot topic for non-consumer companies as many firms won't rely on self-certification and send there products to external test houses (TUV, ITS, UL etc) after in-house compliance checking.

We typically review products to appropriate safety standards first (used to comply with the LVD) then EMC check them in our chamber. We could of course self-certify at this point however we still insist on a test-house rubber stamp for safety - guess that's the rules of good Medical and Industrial business - but a costly one at that!

Sorry, I've slid off-topic......

If I was designing it I would be worried about that but presumably the commercial pressures to keep the cost to a minimum mean that if they don't fail then thats OK.

Quite right!! I actually think they knew about the significant temperature rise as the zenners are mounted purposely off the PCB intending to use the legs as cooling. I don't think they're high voltage so can't see many other reasons to mount these components in this manor.

Today I'll locate what safety standards boilers fall under and see what the earthing requirements are.... I'll let you know what I find if anyone's interested

Cheers
Lea

 

[LeaUK]

Hagbard

BTW - I was rather chuffed with the pictures too.... so simply had to make a post....

cheers
Lea

 

[Agile]

Yes, nice pictures!

CE marking has no interest at all to me! There is however a reference, I think, to only fitting CE marked gas appliances probably with the GSUR 1998 Regulations to which all CORGI people have to work!

I am interested in electronics design and reliability though.

The zeners have been selected and fitted to do the job and as they dont fail I have to admit that they are suitable. I agree the lead discoloration is disconcerting to anyone not familiar with this PCB. To get to that colour I would expect the leads must exceed 230*C. I am surprised the reverse voltage does not drift at that temperature!

Tony

 

[LeaUK]

As you so rightly say the goals of mass produced consumer products are at opposite ends of the spectrum to higher quality markets - it's more important to squeeze every ounce of price/performance from one's components than worry about MTBF!

So I guess it's time to close now (especially as I'm still nice and warm ). Thanks to the forum members and especially to all those who have contributed to this thread.

I'm off to research a little on boiler safety standards and may make contact with Baxi to find their point of view. If anything interesting turns up I'll make a post..


Cheers
Lea

 

[LeaUK]

Just to let others know, 6 months later and the PCB/boiler is still functioning fine

Lea

 

[Agile]

Well that proves that the zenners are adequate for the job even if they create some localised overheating.

Tony Glazier