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August 2012

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Subject:
From:
Rex Waygood <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Rex Waygood <[log in to unmask]>
Date:
Fri, 17 Aug 2012 08:22:22 +0100
Content-Type:
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text/plain (131 lines)
I think you should be using Type 4.
We now buy type 4 at the price we used to buy type 3 and now only use
type 4. This has removed problems of having to relife paste or throw it
away (then buy more to throw away!).
The transfer of all type 3 usage to type 4 has been virtually
transparent other than some small quality improvements.
Rex


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Amol Kane
(Asteelflash,US)
Sent: 16 August 2012 18:41
To: [log in to unmask]
Subject: Re: [TN] CSP BGAs

Yes. You are correct. we are currently using type three solderpaste with
a 4 mil stencil stepped to 3 mils for this part. I want to see if the
problem alleviates by going to a square aperture vs. the round one I
currently have to increase the solder deposit volume.

Thank you,

Amol



-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Mike Fenner
Sent: Thursday, August 16, 2012 1:24 PM
To: [log in to unmask]
Subject: Re: [TN] CSP BGAs

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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