Aug.4, 2003
Voiding
Most probable factors which may cau voids are ;
1) Solder paste
2) Component
1) Solder paste
Most of the flux medium of a solder paste is in general compod of some volatile components such as solvents, rosins, activators and thixotropic agents.
Depending on what lection and blend of the components is adopted, may have some influence over the degree of voiding.
A void is caud as the result of entrapment of “flux gas” in the joint, and there are basically three
different modes which generate gas, which are ;
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1. Solvents
Solvent is suppod to totally evaporate during the pre-heating stage. If some voids are obrved in BGA bumps, attention needs to be paid to pre-heat temperatures and time.
2. Rosins & thixotronpic agents
Some rosins and thixotropic agents have some volatility and become gaous when they are expod to a high temperature environment.
Since this is the nature of such chemical constituents available for flux mediums, careful lection of each constituent will help reduce the degree of flux gas.
3. Chemical reaction
Soldering requires the removal of the oxide film from the surface of the solder particles and probably also from the solder plating of the electronic component and HASL substrate. Chemical reaction between the tin oxide and the flux generates water steam.美丽中国下载
< SnO + 2R-COOH → (R-COO)2Sn+H2O
The volume of this water steam may become larger in proportion to the degree of oxidation.
If this is the ca, it is desirable if additional oxidation is minimized during the heating process in the reflow process primarily in the pre-heating which should be reduced either by shortening the exposure time and temperature or by using inert atmosphere.
Considering the above points, we would recommend to design the reflow profile as below.
1) Complete solvent evaporation
Make sure enough heat energy (temperature and time) has been cured.
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(2) Prevention of excess re-oxidation
Shorten pre-heat time and/or reduce temperature (low ramp rate)
(3) Enhancement of evacuation of flux gas
Extend the time above the melting point.
Again, as the voiding may also be dependant on the volatile nature of the flux constituents, proper lection of solder paste itlf may help solve the problem.留学点评
2) Component
Quality of the BGA component itlf may have an influence on the occurrence of voids in the solder joint.
sheep的音标Quality issues related to voiding are surface tension, oxidation and the co-planarity of bumps.
1. Surface tension
Due maybe to alloy composition and/or bump oxidation of solder, flux gas generated during soldering tends to be entrapped in the joint.
In the normal situation, when solder bump melts and merges with solder from the solder paste, the BGA tends to sink towards the substrate or the height of bump becomes smaller due to lowering the surface tension and wetting effect of the solder to the substrate.
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What this means is that gas entrapped in the solder would tend to be trapped in the joint as the BGA joints do not sink and do not push trapped gas out.
Plea e to the attached pdf. “BGA voiding” for your reference.
computergraphics2. Oxidation - High surface tension
Oxide layer formed on the surface is hard to break due to its surface tension and prevents voids
512护士节文章from evacuating from the solder joints.
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3. Co-planarity of solder bumps
If the co-planarity is poor and there are some height differences between the bumps, as some of the bumps cannot sink toward the substrate, it could result in entrapping a certain volume of flux gas.
remain Voids evacuate
Normal BGA Problematic BGA remain Problematic BGA evacuate
This phenomenon is of similar effect to “1. Surface tension”.
Placing this component on the alumina plate, we measured the space between the surface of the alumina plate and solder bumps by microscope.
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As shown above, it is clearly obrved that the component is not completely flat.
While the very center of the component was in contact with the alumina plate, some open space was obrved at the bumps on the edges of the component with quite large spaces at the bumps on the corner edges.
It would indicate that the substrate has upward warp, or co-planarity of the bumps is poor.
It shall be necessary to check if the co-planarity level of each BGA is within specification and acceptable.
hostage4. Pad design
It may be a very rare ca, but it has also been noticed that the shape of the BGA pads might also have some influenced in entrapping the gas.
The concave pads prevent the gas from evacuating from the joints, and would make it easier to retain more voids in the joint.
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