Hi Ivan,
Your point about the residues being isolated sounds fair, and I will
make sure that we use no-clean flux just to make sure.
Thanks very much for you help - I feel a lot more comfortable about this
now.
Regards,
Jeff
-----Original Message-----
From: Ivanoe Pedruzzi [mailto:[log in to unmask]]
Sent: Wednesday, 23 November 2005 7:15 PM
To: Jeff Brown
Subject: Re: [TN] Unusual soldering challenge
Well, I don't have an answer, I can only speculate on it and see if
it all makes sense.
To one extent, I can immagine that flux might get entrapped
underneath a pretty large area of soldering, it is almost unreal,
thinking that it it gets completely pushed out by the tension
strenght of the solder in its liquid state.
Now, let's see what it represents in the real world. I've reflowed
SMD power mos transistors, BGA, PGA, as well as pin-in-paste. Almost
in all case I've see flux extrapped but never paid any concern just
because the flux has been whether REL0 or ROL0 type (no clean,
halides free), in that, it may sit on board without representing a
risk. Supposing it very active, I would assume it is not an issue
either, just because it is in a complete isolation from the outer
moisture.
There are some pictures of BGA spheres cross sectioned with voids
inside, are they empty? Presumably not. The case, might be how large
or hom many of those voids are in the solder joints (any solder joint).
My 2 cents.
Ivan
Il giorno 23/nov/05, alle ore 03:48, Jeff Brown ha scritto:
> Thanks Ivan,
>
> Sounds like I'm just being overly pessimistic then...
>
> For future reference, is there an approximate size limit at which
> stage
> flux entrapment would start to become a problem?
>
> To extrapolate to an unlikely extreme, if we tried to solder two
> 100mm x
> 100mm ground planes together face to face using a grid pattern of
> solder
> paste, it is hard to imagine that all flux from the centre of the
> ground
> planes would be able to find its way out to the edge of the planes
> during the soldering process.
>
> Or am I just not giving the flux enough credit? :-)
>
> Thanks again,
> Jeff
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Ivanoe Pedruzzi
> Sent: Tuesday, 22 November 2005 7:56 PM
> To: [log in to unmask]
> Subject: Re: [TN] Unusual soldering challenge
>
> Jeff,
> a grid in the aperture of a stencil of that sort is, in my
> experience, a good advice.
>
> This is the only way to keep the part seated in its position when
> reflowing.
>
> If not divided in smaller parts, the surface tension of the liquid in
> that area is strong enough to re-align the part on the center of the
> stenciled paste.
> Besides shifting, you can't even prevent the component from rotating.
>
> I haven't ever experienced any problem with flux entrapped nor with
> boiling.
> A sort of "tic-tac-toe" grid is what I was used to have in the
> aperture.
>
> have a good one,
> Ivan
>
> Il giorno 22/nov/05, alle ore 06:46, Jeff Brown ha scritto:
>
>> Hi Everyone,
>>
>> We have a problem with a fairly unique component that requires
>> soldering
>> to a PCB. I'm really hoping that someone has seen this before as it
>> looking really awkward.
>>
>> The component is mounted on a piece of fiberglass about 25mm x
>> 16mm. The
>> footprint consists of a ground pad which takes up 95% of the area,
>> and
>> four signal pads with 0.5mm clearance from the ground pad.
>>
>> The manufacturer recommends a grid of stenciled paste for the
>> ground pad
>> due to the large area, but we are concerned that if the paste melts
>> from
>> the outside edges in, then it will at a minimum result in flux
>> entrapment, and at worst, the trapped flux will boil and lift the
>> component, or move the molten solder causing shorts to the signal
>> pads.
>>
>> I imagine that even if we used solid paste pattern that this would
>> probably only make the problem worse.
>>
>> Or are we just being overly pessimistic?
>>
>> Any thoughts on options would be gratefully received... Currently
>> wondering about the possibility of applying BGA balls to the
>> package so
>> that there is some clearance between the board and the component.
>>
>> If anyone is interested, I can send the component datasheet to them
>> directly.
>>
>> Thank you all in advance
>>
>> Jeff Brown and Josh Vear
>> EOS Space Systems
>>
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