IPC-7095C-2013.pdf - 第158页

9.4.3 Evidence of Dewetting 9.4.4 Mottled Condition 9.4.5 Tin/Lead Solder Ball Evaluation 9.4.6 SAC Alloy Possible Cause Dewetting of the solder at the contact interface may be due to excessive oxidation on the land, org…

9.3.2 Solder Joint Opens Due to Interposer Warp

9.4 Solder Joint Conditions

The following sections deal

with the conditions of the solder ball in relationship to the

mounting structure and the component interposer. In each

instance, an explanation is provided as to the reason for the

occurrence of the condition.

9.4.1 Target Solder Condition

9.4.2 Solder Balls with Excessive Oxide

Possible Cause

– The failure appears to be due to insufficient paste release

from the stencil or insufficient ball size.

– Warped package (smiling BGA) with corner balls lifted up.

Potential Solution

– Check balls at incoming prior to assembly.

– Return package to supplier.

– Implement incoming inspection and/or source audit.

– Apply extra solder paste on corner lands.

Possible Cause

Solder balls exhibit a uniform shape and texture, and they are

symmetrically aligned to the attachment sites, showing ball col-

lapse on both top and bottom of BGA balls.

Potential Solution

Document process parameters and maintain a stable process.

Possible Cause

Solder ball oxides will form due to exposure to multiple reflow

solder cycles (topside or bottom side).

Potential Solution

This can be reduced significantly by correct flux selection,

reducing the number of production reflows to two and not

cleaning the PWB between reflows.

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9.4.3 Evidence of Dewetting

9.4.4 Mottled Condition

9.4.5 Tin/Lead Solder Ball Evaluation

9.4.6 SAC Alloy

Possible Cause

Dewetting of the solder at the contact interface may be due to

excessive oxidation on the land, organic contamination. Poor

plating can contribute to this condition as well. ENIG comes in

a number of systems, i.e., hi P and low P systems. Dewetting

of ENIG is due to oxidation of Ni barrier not excess P on sur-

face.

Potential Solution

Develop process effect controlled experiment to determine

which attribute contributes most to dewetting.

Possible Cause

The surface condition of the solder ball is likely the result of

overheating during the reflow solder process or excessive/

repeated exposure to temperatures above liquidus.

Potential Solution

Evaluate reflow process in order to determine proper ball char-

acteristics.

Possible Cause

Reflow of solder paste should form a proper ball geometry.

Potential Solution

Maintain process effects standardization.

Possible Cause

Slightly mottled appearance of the tin-silver-copper alloy.

Potential Solution

Determine paste and ball characteristics through experimenta-

tion.

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9.4.7 Cold Solder Joint

9.4.8 Incomplete Joining Due to Land Contamination

9.4.9 Deformed Solder Ball Contamination

9.4.10 Deformed Solder Ball

Possible Cause

The ball contacts have not completed wetting at the attach-

ment sites. Condition may be due to poor solder paste printing

or contamination on the attachment site that cannot accept

wetting.

Potential Solution

Check stencil and printing process to determine sufficient sol-

der paste deposit.

Possible Cause

Organic contamination or oxidation of the PCB land can nega-

tively compromise the uniform and complete joining of the sol-

der ball and PCB land surface. PCB land was not sufficiently

clean to promote proper wetting prior to application of solder

paste.

Potential Solution

Test for cleanliness as part of process. Implement proper PCB

storage and handling procedures to prevent contamination and

exposure to conditions which promote oxidation.

Possible Cause

Solder ball deformation can occur due to the component mov-

ing during the reflow soldering process (package warp or board

warp) and/or when the land pattern geometry is not correct.

Potential Solution

Avoid overheating which causes quality disparities of the inter-

poser.

Possible Cause

Ball deformation (column shaped) can result from temporary

warping of the substrate during reflow soldering. The package

substrate corner, in this case, moved upward at the higher

temperature, away from the board surface. While in this condi-

tion, the solder alloy cooled to a solidus condition resulting in a

column shape. This is known as dynamic warping because the

warping increases along with the reflow temperature. As it

cools, the package would return to its original shape if the

solidified solder was not preventing it from doing so.

Potential Solution

This is caused by CTE mismatch of the materials in the pack-

age or board. Though elimination of this condition is outside the

control of the SMT process, the effect can be minimized by

avoiding overheating of the interposer and the board.

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Provided by IHS under license with IPC

Not for Resale

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