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July 2009

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Subject:
From:
Bev Christian <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Bev Christian <[log in to unmask]>
Date:
Wed, 29 Jul 2009 17:09:46 -0400
Content-Type:
text/plain
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Okay.  Some things are new. Some things people did not like. I have a
new version (version 3) below. I have made changes in CAPITALS, since
colors won't show up in TechNet.  Let me know what you think.  I can't
say I'm thrilled with some of the things I have written here, but hey,
it should lead tot some healthy discussion if nothing else.  There is
also a section at the end on testing, which I can't remember if I added
before or not. Too lazy to go back and look.

 

Bev

RIM

 

 

Workmanship Standards for BGA and CSP Underfills

 

For general inspection of all circuit packs coming down a production
line, apply only the portions of this document that can be performed
visually to underfilled, uncovered CSPs and BGAs or those same types of
components under fence and lid RF shields. The visual inspection rules
do not apply to CSPs and BGAs for general production under single piece
RF cans. This would require destruction of all product.

 

However, all the rules apply for process development/product
qualification where one is willing to sacrifice finished circuit packs
to gain knowledge of the acceptability of the overall quality.

 

 

Target - Class 1,2,3

*         Component requiring underfill completely underfilled with a
good fillet all the way around, the fillet just approaching the maximum
component body height, completely cured, no encroachment onto other
components and no voiding whatsoever

 

Acceptable - Class 3

*         At least 50% fillet height up the side of the component body

 

*         Small pin holes and voids are permissible provided they do not
exceed 5% of fillet surface area and have a maximum diameter of 0.5 mm
(0.020 inches).

 

Acceptable - Class 1,2

*         Component requiring underfill underfilled such that all balls
are covered, but no evidence of an exterior fillet, no encroachment onto
other components and no voiding whatsoever.

 

*         Component requiring underfill underfilled such that all balls
are covered, with or without an exterior fillet, no encroachment onto
other components and only small voids, not touching any solder joints.

 

*         Component requiring underfill underfilled such that all balls
are covered, with or without an exterior fillet, no encroachment onto
other components and even large voiding, but not touching any solder
joints.

 

*         Total void area, irrespective of location, as a percentage of
component area shall be equal to or less than 10%.

*         A void or voids less than or equal to 50% around the perimeter
of one or more solder joints, including in an outer row.

 

*         Voids greater than 50% around inner row solder joints.

 

*         Solder joints with voids completely surrounding individual
solder joints and/or voids connecting two solder joints make up less
than 10% of the total number of solder joints of the component.  This
rule only applies to inner row solder joints (E).

 

Defect - Class 3

*         Less than 50% fillet height up the side of the component body

 

*         Small pin holes and voids exceeding 5% of fillet surface area
and/or have a maximum diameter of greater than 0.5 mm (0.020 inches).

 

Defect - Class 1,2,3

*         Underfill completely missing from components where it is
required

 

*         Underfill not completely covering all the peripheral solder
joints

 

*         Voiding such that a void traverses the distance between two or
more OUTER ROW solder joints (DELETE under the array device requiring
underfill)

 

*         A void or voids more than 50% around the perimeter of one or
more OUTER ROW solder joints

 

*         Material extending on to the top of the BGA or CSP

 

*         Uncured material

 

*         Underfill interfering with proper placement of a heatsink or
other mounted or mechanical devices or would interfere with the assembly

 

*         There should be no evidence of underfill flowing out through
holes under the device.

 

*         After curing there should be no visual evidence of cracks
between the device surface, underfill or board.

 

*         There should be no evidence of damage to the board or device
caused by contact with the dispensing needle or height sensor.

 

*         Underfill filler shows evidence of separation

 

Special Conditions - Class 1,2

*         Encroachment onto other components, but meeting all other
criteria above for target or acceptable conditions will be dealt with as
a pass/fail on a case-by-case basis

 

 

 

Methods of Test

Unfortunately the presence of RF cans make it impossible to do the
testing by acoustic microscopy for components under cans.

 

Other possibilities and their advantages and disadvantages:

 

Method

Advantages

Disadvantages

Prying

Fast, easy, can be done by anyone, checks whole area under component at
once

Misses embedded voids that don't reach the underfill/component interface

Vertical Cross-sectioning

Finds buried voids

Requires lab staff, only finds voids in the particular plane sectioned,
unless grind further (very time consuming)

Horizontal cross-sectioning

Finds buried voids, checks whole area under component

Requires lab staff, very time consuming

 

 


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