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Q:What caused solder paste printing defects ?2017-12-30
In SMT PCB production, solder paste printing is a critical step. Since the solder paste is used to directly form the soldering joint, the quality of solder paste printing affects the performance and reliability of the surface mount assembly. Quality solder paste printing guarantees a quality solder joint and final product. Statistics demonstrate that 60% to 90% of soldering defects are related to solder paste printing defects. So it is very important to understand what causes defects in solder paste printing.
The following table lists an analysis of solder paste printing defects:
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Item
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Factors
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Analysis
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1
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Solder Paste
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Powder formation
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The irregular shape of solder powder will easily clog stencil apertures. This will cause a big slump after printing. It can also cause solder ball and short bridge defects after reflow.
A spherical shape is best, especially for fine-pitch QFP printing.
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Particle Size
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If the particle size is too small, the results will be poor paste adhesion. It will have a high oxygen content and cause a solder ball after reflow.
The particle size should be controlled to about 25 ~ 45 μm in order to meet the requirements for fine-pitch QFP soldering, If the partical size desired is 25 to 30 μm, it should applied with less than 20 μm solder paste for an ultra fine-pitch IC.
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Flux
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Flux contains a thixotropic agent, which allows the solder paste to have pseudoplastic flow characteristics. Since the viscosity decreases when the paste passes through the stencil apertures, the paste can be applied to the PCB pads rapidly. When the external force stops, the viscosity will recover to ensure no deformation occurs.
The flux in the solder paste should be controlled to between 8 and 15 percent. A lower flux content will result in excess amount of solder paste applied. Conversely, a high flux content will result in an insufficient amount of solder applied.
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2
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Stencil
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Thickness
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A stencil that is too thick will cause a solder bridge short.
A stencil that is too thin will cause an insufficient solder to be applied.
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Aperture size
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When the stencil aperature size is too big, a solder bridge short can occur.
When the stencil aperature size is too small, and insufficient solder paste will be applied.
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Aperture shape
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It is best to use a circular-shaped stencil aperture design. Its size should be slightly smaller than the PCB pad size, preventing a bridging defect during reflow.
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3
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Printing parameters
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Blade Angle Speed & Pressure
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The blade angle affects the vertical force applied on the solder paste. If the angle is too small, the solder paste will not be squeezed into the stencil apertures. The best blade angle should be set around 45 to 60 degrees.
A higher the printing speed means that less time will be spent in applying the solder paste through the stencil aperture surface. A higher printing speed will cause insufficient solder to be applied.
The speed should be controlled to around 20 ~ 40 mm/s.
When the blade pressure is too small, it will prevent the solder paste from being cleanly applied to the stencil.
When the blade pressure is too high, it will result in more paste leakage. The blade pressure is typically set at about 5N ~ 15N / 25mm.
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4
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Printing process control
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PCB moisture
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If the PCB moisture is too high, the water under the solder paste will quickly evaporate, causing the solder to splash and creating solder balls.
Dry the PCB if it was fabricated over 6 months ago. The recommended drying temperature is 125 degrees for 4 hours.
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Paste storage
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If the solder paste is applied without a temperature recovery period, the water vapour in the surrounding environment will condense and penetrate the solder paste; this will cause the solder to splash.
Solder paste should be stored in a refrigerator at 0 to 5 degrees.
Two to fours hours before use, place the paste in a normal temperature environment.
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Q:What affects solder paste viscosity ?2017-12-30
Paste viscosity can be dependent on many factors including:
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Formulated (initial viscosity) plus the metal content, of course. The more powder per unit volume, the thicker the paste will be. Fine powders have more surface area and will present as a more viscous paste.
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Shear-thinning through squeegee action. Many pastes are thixotropic (e.g., subject to thinning through mechanical perturbation.)
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Environmental conditions (temperature and humidity.)
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If an automatic bottom-side wipe is employed, a wet-wipe may slowly contribute solvents to the paste and alter paste viscosity with time. Be careful of this. Dry wipe is generally a better option.
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Q:Why do we tend to buy long-life lead-free tin?2017-12-30
Lead-free solder composition: 99.3Sn0.7Cu. EU standard less than 1000PPM, lead-free solder Melting point: 223 °. So
far there is no internationally accepted definition, reference
standards: lead welding solder and Zhang Hua key lead content of less
than 0.2wt% (United States), 0.10.2wt% (Europe). Brazing temperature below 300 ℃, the surface of the liquid solder as
mirror bright; the amount of slag is only about 1/7 of the normal
solder;
Lead-free
environmentally friendly tin has a wetting characteristics of a short
period of time, the expansion rate is superior to the general solder;
due to the addition of antioxidants so bright, beautiful, and reliable
solder joints. Suitable for wave soldering and hot dip soldering of electronic products.
According to the melting temperature to classify:
Can be divided into low-temperature lead-free solder, room temperature solder, high temperature solder.
By metal alloy materials to classify:
Can be divided into tin and lead alloy solder, pure tin solder, tin
copper alloy solder, tin silver copper alloy solder, tin-bismuth alloy
solder, tin-nickel alloy solder and special tin alloy solder.
According to the chemical composition of solder paste additives to classify:
Can be divided into rosin core solder, no-clean solder, solid solder,
fat-type solder, single-core solder, three-core solder, water-soluble
solder, aluminum solder, stainless steel solder.
For
lead-free environmentally friendly tin, a single lead-free solder tin
prices low, but we still can not be careless in the purchase. Because although a single lead-free tin bar cheaper, but when the purchase of thousands of the required costs will be more. Since the purchase of lead-free tin need to spend a lot of money, then
the time of purchase, must not be careless, to take into account all
aspects, in particular, to take into account the life of lead-free green
tin.
Perhaps
the same type of tin single root price is lower than the lead-free tin,
but if its service life is far less than lead-free tin, then large
quantities of tin need to spend more money. In addition, the purchase of tin life is very short, then use a period of time and then have to face the large cost of tin. This calculation, the purchase of long lead-free tin is still more cost-effective.
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Q:Lead-free environmentally friendly tin slag cause?2017-12-30
In the welding process will also produce solder slag, which is normal oxidation phenomenon. Lead-free
environmentally friendly tin from pure tin and trace of copper, in
which the addition of antioxidants, compared with the traditional tin
wettability, good liquidity, more easily on the tin, while the bright
spot full, Weld situation does not appear, Become the main choice of wave soldering now. So what is the reason for the production of solder slag?
First, the content of tin:
Tin metal in tin bars is the main ingredient. If the content and
purity of tin is not enough, it means the impurities and other elements
will increase, which also increases the chance of tin dross overflowing.
Second, the furnace temperature control:
Whether
it is a lead-free solder paste that has a fixed melting point and
operating temperature, improper control of the temperature of the solder
can accelerate the oxidation process and spill more tin residue. When the furnace temperature is high or low will continue to emerge black tin oxide in the solder bath surface.
Third, tin metal features:
Whether
lead-free environmentally friendly tin is made of tin alloy, tin in
contact with the air in the process of chemical reaction occurs tin
oxide. In the welding operation of tin through the high temperature from
liquid to solid state, which will greatly exacerbate the oxidation rate
of tin, tin slag will be more and more.
Fourth, anti-oxidation elements:
In
the production of lead-free environmentally friendly tin, we must add
anti-oxidation elemental metal, the purpose is to suppress the oxidation
rate of air in the blast furnace temperature and reduce tin slag. If you join the antioxidant element is not enough, that tin slag will overflow more.
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Q:Tin wire smoke is why? How to solve?2017-12-30
Tin wire in the welding of smoke is a big dilemma, the big smoke wire on the welder body have a certain impact. In operation, it is inevitable that choke nose, smoked eyes and so on. As a welder is best to wear protective masks for welding. However, this method is only a temporary solution.
If we want to solve the problem better, we should understand where the source of the problem lies. We
all know that solder is the main component of the metal, the metal is
not organic, it is difficult to appear such a big smoke in the case of
not reaching the boiling point. So what is the culprit behind the smoke? We come up with a volume tin wire analysis is easy to find, in fact, a small hole in the tin wire, not solid. The culprit is the tin core stuff - rosin.
Speaking
of rosin this thing a bit embarrassed, and some people think the
culprit since it is rosin, then buy without rosin tin line is not
enough. In actual fact, the production of tin wire manufacturers containing rosin, must have its effect. Rosin role in the tin line is the role of flux, as the name suggests is to help the role of welding. Tin wire in the welding time without flux can not be completed
welding, so the tin wire is either built-in rosin, or need additional
purchase of rosin for welding.
Talking
about some friends here will be puzzled, since the rosin is necessary,
rosin is the culprit smoke, to say it does not also can not solve the
problem! Here we need to popularize a concept that is the concept of tin rosin content. What is the rosin tin content, that is, the proportion of rosin tin line. In the general market, the proportion of lead-free tin is 2.5%, and the content of lead-tin rosin is 2.0%. If the tin wire on the market when welding smoke, you can contact the tin wire manufacturers custom solder content less silk.
Rosin content is generally more than 1.4% or can be completed welding, but not recommended to take over the limit value. Of
course, even if the use of small rosin solder wire, welders who think
for their own sake, wearing a mask welding is still necessary.
