Tantalum Caps can cause you some hurt.

[uniserver]

This is some re-work that i am doing for a member.

It was sent to me already re-capped with tantalums.

I am trying to diagnose issues with it.

Removing some caps i saw this.

You can see one of the pads, and what over soldering can do… it creates a big blotch of solder.

to where it looked just fine from the top, the over sized tantalum cap had all this concealed.

Now if the Silkscreen is intact then its fine, not really a problem.

if you have exposed traces, this can cause you issues, shorts / solder bridges.

Sometimes traces are ran right next to pads, sometimes they are ran under the cap in-between the pads.

This is why i never take chances and will only install high quality OEM Panasonic lytics (in situations like this) instead because,

I dont have time to mess around.

 

[MacJunky]

This has nothing to do with the caps being tantalum. Thread title is incorrect.

 

[uniserver]

You are incorrect!

IT completely has to do with tantalum caps.

Tantalum caps are not the right cap for these electrolytic designed pads. Fact.

These pads are designed for SMT Electrolytic caps.(much thinner leads)

Tantalum caps are designed for much wider pads to accommodate the wider soldering lead surfaces on each end.

Even if you don't over solder, you can still bridge traces on ether side of the tantalum cap.

(if traces are exposed after your De-Cap and clean up)

I am not knocking the SOB's, I still have them in my inventory and will still install them as long as there are no exposed traces.

 

[MacJunky]

You can get them in different packages and package sizes you know. So to say that they are all going to be a problem is highly misleading.

On the other hand if you had specified a specific package or package size initially, instead of a sensationalized thread topic we would not be having this conversation.

It is fine if you want to warn about two certain specific things are are liable to clash.. just make sure you are specific.

 

[uniserver]

True, but show me a package that fits and ill buy them.

And dammit... i only said they would be a problem if you have exposed traces.

 

[James1095]

There is a product called Pebeo Vitrea that is intended as a glass paint to make faux stained glass and various artsy-fartsy stuff. It's the best thing I've found for creating and repairing solder mask though. You can buy it in bottles as well as pens where you simply draw it on. Let it dry for a while, then bake it and it cures rock hard and nearly impervious to solder. You can probably get away without baking it for this application as the hot solder will likely cure the area near it.

For creating a solder mask on a DIY PCB I toner transfer an inverse mask then airbrush slightly thinned Vitrea over the whole board. Bake it and the spots where toner is present bubble up slightly and can be scrubbed off and I'm left with a reasonable approximation to a real solder mask that does its job and really dresses up a home etched board. The stuff works well for coloring small light bulbs as well.

 

[genie_mac]

I have yet to come across a suitably packaged SMT tantalum cap to replace the aluminium electrolytic SMT caps. That's why I always use through-hole bead type tantalums in these situations. They are easy to solder and easy to visually inspect. If you equalise the forces they don't put any excess stress on the pads either (unless you're really clumsy with them). They are a bit more expensive but for the small numbers needed it doesn't make much of a difference (and compared to the amount of money spent on old Mac stuff it's pretty irrelevant ).

 

[James1095]

I haven't had any trouble installing SMT tantalums in place of the electrolytics. If you install them properly they fit just about perfectly, look good, work fine. I have seen plenty of pads ripped off various boards by people installing through-hole parts onto SMT pads.

 

[uniserver]

when in doubt install a high quality proper electrolytic.

 

[genie_mac]

I'm totally with you on this one James1095, I was just referring to Uniserver's original post where he was having problems with SMT tantalums and exposed tracks. I don't think SMT's are the most ideal choice for these situations, that's why I suggested through-hole types.

I guess it's as much personal preference as anything, the through-holes have worked well for me but obviously you have to be careful when handling the board.

 

[uniserver]

I wasent having problems.

The person that installed those tants was having problems.

A good hunk of my customers prefer the stock look anyways.

Hey, ya know. Cap technology has came along way since 1988! Electrolyte Mixtures, Sealing Technology.

If it wasn't for those goof balls ripping off that formula and selling bad caps around 6,7,8,9 years ago…

I am going to say that there is a reason apple engineers used electrolytic caps when they did.

And I will just leave it at that. I do not think it was a financial thing.

I'v first hand, seen in situations that electrolytic caps have to be reinstalled in-order for proper functionality.

 

[James1095]

In most cases it was absolutely a financial thing. I've had some involvement in consumer hardware design over the years and pennies count. When you're making 2 Million of something, the cost of goods difference between $2 worth of electrolytic capacitors and $5 worth of Tantalum capacitors is a very significant amount of money. There are some designs that do benefit from the additional ESR of the electrolytics, but it isn't terribly common. More often that's a case of the circuit being designed around parts that were deemed "good enough", and when you use a better part, it's possible for things to not work quite right since the original design took the lower performance of the originally chosen part into account. Tantalum caps of the era were not available in as many values, and were more expensive than they are now. Ceramic caps in higher values didn't exist at all.

 

[uniserver]

Well so apple decided to be a cheap ass with the 840av… shame shame shame….. should have been designed with solid caps like the q800 q700 q900/q950?

 

[James1095]

I'd say that's arguable. How long did Apple intend the machine to last? If it is estimated that 95% of units that will be produced will be obsolete and scrapped within 5 years, is it being a cheap ass to spec capacitors that will last "only" 10 years of normal use? No computer was ever designed to be a classic collectible, it was designed to be a functional tool with a finite lifespan.

Look at some of the boards used in satellites, missiles and other cost-is-no-object military hardware where absolute reliability is paramount. It looks almost nothing like consumer gear, nothing but the highest quality PCBs, gold plated pins, ceramic IC packages, usually no electrolytic capacitors anywhere. A computer built that way would last a very long time, but it might cost $100K while one designed to last 5-10 years might cost $5K. Engineering is a world of trade-offs. Price, reliability/longevity, performance, you can pick any two favorable traits. Stuff isn't designed deliberately to fail, it's designed to be good enough, while being as cheap to produce as possible.

 

[uniserver]

Oh .. no argument. I am making a statement:

Apple was a cheap ass for not using solid caps on the 840av.

 

[James1095]

It could be debated that the choice was a sound business decision. In any product cycle there will be a push from marketing to add features, a push from the bean counters to cut cost, and a push from the engineer(s) to make their baby as great as possible. All sides have to pick their battles and be prepared to compromise. In the end, the engineer may decide that it's acceptable to use a cheaper type of capacitor rather than cut other corners more detrimental to the final product in order to fall within the budget.

This is what makes clever engineers like Steve Wozniak so impressive. He had a real knack for cutting costs to the bare bones while still finding ways of packing in functionality above and beyond what anyone else was offering at anywhere close to a competitive price.

 

[uniserver]

Or maybe because the 840av was such a freaking beast. They needed the Punch of the electrolytic caps to make it run smoothly.

It was the fastest 68k Mac ever made