This page is discussing of my experienced defects facing in
operation at Surface Mount processes and experience methods to solve
the issues. Some of these defects are due to inefficient reflow
profiles - examples of these defects such as:
1)
Cold Solder Joints
Root Cause :
1)
Insufficient heat during reflow
2)
May also be due to the flux's inability to accomplish the soldering
task.
3)
May be not enough clean of the component and PCB pads prior to
soldering, or it may simply be due to excessive impurities in the
solder solution.
Possible solutions:
1) Raising the maximum reflow temperature high enough to reflow the
material thoroughly.
2) Preventing vibration of assembly during and immediately after
reflow.
3) Speed up the cool down rate after reflow and
4) Checking the alloy analysis for high levels of contaminants.
2)
Non-wetting
Root Cause :
1)
It could be due to the base metal being visible, and since this is
typically more difficult to solder to.
2)
It might also be due to too long of a reflow time in using up the
flux prior to soldering and flux being used ineffective from an
activity standpoint.
3)
It could possibly be due to insufficient heat during the reflow
process where the flux doesn't see the correct activation
temperature.
4)
Or, it could be due to component layout and weigh.
Propose Solutions
:
1)
Rectifying the situation with the PCB manufacturer if the base metal
is present.
2)
Reducing the total profiling time prior to the reflow stage,
3)
Or as following;
a)
Solder paste maker recommender reflow profile, such as temperature,
soak time, or to increase solder and flux activation,
b)
Fine tune customer recommender reflow profile,
4)
Reviewing board component layout and location, there is some cases
facing non-wetting issue due to layout or component weight; somehow
may be need to re-rotate board reflow direction to get proper
wetting or adding weight on top the component.
3)
Solder Ball
Root Cause :
1)
One possible cause of solder balling may be moisture contaminated
solder paste.
2)
An improper reflow profile can also cause solder balling.
3)
Reflow temperature ramp rate is commonly too high which increases
the probability of paste splattering.
4)
Possibly be due to excessive oxides on the solder powder in the
solder paste which inhibits solder coalescence during reflow.
Propose Solutions
:
1)
Replace the contaminated solder paste and try with new solder paste.
2)
Fine tune a reflow process which best fits the paste selected (by
refer to solder paste temperature characteristic)
3)
Minimize solder paste exposure to high temperatures and humilities
whenever possible.
4)
Replace the oxides solder paste and try with new solder paste along
with same profile.
4) Tomb Stoning
Root Cause
:
1)
Can be caused by uneven heating causing a differential across the
component terminals. The solder melts at different rates and one
side reflows before the other forcing the other lead to stand
upright.
2)
The solder ability between two terminations of the component or PCB
pads can also be blamed.
3)
Uneven paste deposition on the two solder pads has also yielded tomb
stoning defects.
4)
Insufficient tack force of the solder paste to hold the component in
place during reflow can also be a factor,
5)
It could due to temperature and humidity effects on the solder
paste.
6)
Excess movement during the reflow operation can cause component
misalignment which results in tomb stoning, and inadequate placement
force to make intimate contact between the paste and
Propose Solution
:
Increasing the preheating temperature (following the recommended
guidelines from Solder Supplier) so that the temperature
differential between the two terminations is low at the time of
reflow.
1)
Selecting components and PCBs with consistently solder able leads
and pads.
2)
Ensuring consistent solder paste deposit heights between pads via a
vision system designed to measure solder paste deposition height.
3)
Avoiding inefficient tack force by avoiding extreme environmental
conditions.
4)
Review solder paste storage environment include screen printer
humidity reading. The humidity should under control as per
solder paste recommender characteristic.
5)
Minimizing the amount of movement the assembly sees during reflow,
increasing component placement force to ensure contact of the
component terminal to the solder paste deposit (not too much because
bridging may occur if it is too high). Such as reflow fan speed,
oven exhaust fan speed and can cause movement during reflow.
I do hope the above information shared would be able to help when
you are facing the same defected symptoms.
Adopted from Reference :
http://www.smtinfo.net/docs/Smtrecs.html
Author : Kurt Rajewski
Assistant Manager - Market Technology Kester Solder Co
:: Quoted by Neck-Skin,