[sdiy] SMD comming your way

[Magnus Danielson]

From: Sten Björnebro <sten at ebs.homedns.org>
Subject: Re: [sdiy] SMD comming your way
Date: Sun, 29 May 2005 09:47:01 +0200
Message-ID: <20050529094701.17050b30 at saturnus>

Hi Sten,

So you've come over here too... lovely!

> On Sat, 28 May 2005 23:33:36 +0200 (CEST)
> Magnus Danielson <cfmd at bredband.net> wrote:
> 
> > Hi SMD-lovers!
> Hi i lowe them.

Yes, it's a known fact! ;O)

> > Also, all of a sudden electrostatics comes into play. It doesn't help just to
> > keep an ESD environment, but we might need to keep tools grounded as well.
> 
> I felt it was the resistor that had been charged whith static.

Mmm, yes, now that you say it I recall that you said that.

> My two twezer was antistatic and my desk an the shoes and even the chair.

Not to mention the floor in that whole area. I'd say your at least 10 meters
walking distance from non-ESD floorspace where you sit.

> So it has to bee the resistor that had to bee taken out from the tape on the
> wheel.

Indeed. Paper-tape with a plastic-tape cover to keep them in place. It is
probably not designed with hard ESD in mind since resistors isn't ESD
sensitive.

> The resistor whas from koa.
> 
> Magnus cold you calculate the force on the resistor compared to columb or voltage?
> 1000pcs weight 0.03g "gram"  !..
> Here is the reference.
> http://www.koaproducts.com/english/catalogue/012-013-RK73B.htm

One should recall that since space-capacitance is 1/r and weigth is 1/r³ as we
shrink the size, the space-capacitance take over and becomes big eventually.
Let's see, we know that for a sphere of radius R we've got

C = 4*pi*epsilon*R

Let's assume that the 01005 has the equalent size as a sphere of radius 100 um
(the diameter is equalent to the width of 200 um and the length and thickness
average out some). Then we get that C = 11,1 fF. Consider that we have
succseeded in charging it with 100 V, then we have a charge of 1,11 nC.
The surrounding (being the lab-bench, tools etc) thus have -1,11 nC in charge.
Let's find out at what distance the electric force is equalent to the gravity
force on the poor component. The gravity force is 30 mg times 9,82, so it is
294,6 uN. We end up having this condition at 6,1 um. This distance grows
linearly with voltage.

I'd say that the effect could indeed be there, but it seems a little week to
me, but maybe I am not considering that this is when it is equalent to the
gravity force and alot happends before we get THAT close to it. 100V is also
quite conservative.

Anyway, when you come down to these sizes, electrostatics sure comes into play
and care needs to be taken to ensure that resistors and caps is not charged
relative to surroundings. Until now we wouldn't care as much.

So, time to pull that static rug out of the lab guys!

> > Also, the tweezers not only need to be grounded, they need to be really clean.
> > Any residue solder resin make the 01005 stick with the tweezer, so they have to
> > be cleaned with suitable alcohol to be bare metal, but that works. However, in
> > any real lab that stuff is just a short stretch away anyway.
> > 
> > OK, so with some minor exercise will the 01005 be handsoldered, or at least by
> > someone already doing 0402 on a daily basis, which is also possible I might
> > add.
> In standard mobile telefone there are 0201 to day so when you to repair yor
> celluar  in future ther whold bee 01005 in it.

They will most certainly.

However, if we look at the maximum voltages of the lower end, 0201 and 01005
may not be generically used in the standard synthesizer voltage-span, but very
well in certain parts of the design. The usual +/- 15V is all of a sudden
high voltage!!!

> > Cheers,
> > Magnus - keeping the notes behind the masters back
> 
> //Sten Björnebro. -=|***----------------------------------------
> // Soldering addicts.

The master has spoken...

Yes, he *is* a soldering addict! ;O)

Cheers,
Magnus