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August 2012

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Subject:
From:
Mike Fenner <[log in to unmask]>
Reply To:
Date:
Thu, 16 Aug 2012 18:24:20 +0100
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I think I see where you are coming from. I think your intuited concern is
about relatively few big balls sitting on a pad with not enough flux.
All stencil grade solder pastes are about 50% solder/50% flux by volume and
this doesn't change on or by being printed [on different pad sizes]. 
Smaller apertures might require smaller balls and thinner stencils, but a
type 4 powder paste is still around 50% v/v. 
Specified weight per cents will vary according to alloy to maintain the
50/50 volume relationship; and slightly with powder size to maintain
rheology. This latter is to accommodate the change in packing densities of
different powder sizes. 
Powder size is determined by aperture size. As a rule of thumb select powder
to have 6 or more (largest size) balls to line up across minimum pad. 
T4 powder has a higher surface area than T3, but if you are buying your
paste from a recognized supplier this will not translate into higher oxide
content.
If going to finer powder/smaller apertures then you will need to also reduce
stencil thickness to maintain wall height to aperture ratio.

Hope this helps.


Regards


Mike 


 
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Amol Kane
(Asteelflash,US)
Sent: Thursday, August 16, 2012 8:34 AM
To: [log in to unmask]
Subject: [TN] CSP BGAs

Dear Technetters,

We are going to be assembling some lead-free  1.6x0.8 mm micro BGAs with
0.5mm pitch and 0.17mm dia board pads. Anybody else have experience with
this small a package?....I am concerned that there won't be sufficient flux
in the tiny solder deposit to clean the oxides during reflow, leading to
graping and/or head in pillow situation. Do I need a dip fluxer for the part
to do a flux only attachment, or to augment the amount of flux in the
solderpaste-solder bump system? I am told I have no time to do DOEs to
determine stencil and reflow parameters, so I am turning to technet for
help.....any thoughts?

 

 

Regards,

Amol

 


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