It certainly looks that way, but if you are going to change your stencils
you might wish to take the opportunity to step back and review overall
process from first principals using best practice rules. This way you will
give yourself the best chance of having an optimum material and process
combination. At a minimum this will identify where your present process
deviates from optimum.



Regards

Mike 


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Rex Waygood
Sent: Friday, August 17, 2012 8:22 AM
To: [log in to unmask]
Subject: Re: [TN] CSP BGAs

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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