SMCs (Surface Mount Components) have become increasingly popular in the industry of electronics manufacturing due to their merits of small size, low cost and high reliability. Up to now, SMCs depend on reflow soldering as the main manner to get them fixed onto PCBs (Printed Circuit Boards) and the performance of reflow soldering is directly close to that of end products. As a common defect usually met in SMT (Surface Mount Technology) assembly process, solder balls, however, are caused due to lots of reasons and are so difficult to control that they have become a main concern in SMT assembly process.
Generally speaking, solder balls feature a diameter in the range from 0.2mm to 0.4mm and are usually found mainly at the side of chip components. Sometimes, solder balls can be found around the pins of ICs and connectors. On one hand, solder balls affect the appearance of electronics products. On the other hand, solder balls will possibly fall off, leading SMDs (Surface Mount Devices) to be shorts, substantially reducing the reliability of electronic products, which is especially troublesome for assembled PCBs with high density and fine pins.
Up to now, PCBCart has become a professional provider of PCB solutions, including PCB fabrication, components sourcing and SMT assembly after more than 10 years’ efforts and focus. The process engineers from our warehouse have been constantly studying essential measures to defeat solder balls in SMT assembly process and some handy measures have been summarized based on their deep analysis on the cause of solder balls.
Although solder balls are finally exposed after reflow soldering, each link of the whole assembly process “contributes” a little to their final forming. First, some solder paste may be left outside pad possibly due to collapse or squeezing. Then, the residual solder paste usually comes together around pad, specifically at both sides of chip components. Finally, the residual solder paste will be melted in reflow soldering oven and become solder balls as temperature cools down. If too much solder paste is squeezed out, more solder balls will be generated.
It’s obvious that solder balls are generated due to a lot of causes during SMT assembly process. The causes can be generally classified into two types: material causes and technology causes.
• Material Causes
→ Solder Paste
a. Low coefficient of thixotropy;
b. Cold collapse or slightly thermal collapse;
c. Too much flux or low activity temperature;
d. Tin powder oxidation or uneven metal particles;
e. Humidity absorbing;
→ PCB
a. Small spacing between PCB pads;
b. Pads or components featuring low solderability;
→ Stencil
a. Opening wall featuring burrs;
→ Scraping Blade
a. Too low in weight;
b. Distorted scraping blade.
• Technology Causes
a. Too large amount;
b. Residual solder paste is available between stencil and PCB;
c. Imbalanced energy or inappropriate setting of soldering temperature;
d. Too high mounting pressure;
e. Too much space between PCB and stencil;
f. Scraping blade featuring a small angle;
g. Stencil featuring small openings;
h. Solder paste is improperly applied;
i. Other causes including staff, equipment and environment.
Measure#1: Pick up the solder paste compatible with SMT requirement.
The selection of solder paste directly affects soldering quality. Solder balls tend to be made when solder paste features inappropriate metal content, oxidation, IMC particles and thickness on pad. Prior to solder paste determination, it is necessary to try it out to confirm whether it can be applied in volume SMT assembly. Solder paste compatible with SMT requirement features the following attributes:
a. High content of metal
Normally solder paste features metal content from 88% to 92%. As metal content rises in solder paste, the viscosity of solder paste goes up as well, which is capable of effectively defeating the stress generated through vaporization. Additionally, the higher content of metal leads metal powder to be compact so that metal powder is easy to be combined instead of being parted. Plus, the higher content of metal is capable of stopping solder paste from being collapsed with solder balls difficultly being formed.
b. Controlled oxidation of solder paste
As far as solder paste is concerned, higher metal oxide content always leads to a higher combination resistance among metal powder. Thereafter, insufficient wettability is caused between solder paste, pads and SMDs, reducing their solderability. It’s been summarized that the occurrence of solder balls is directly proportional to metal oxide. Therefore, the oxide should be rigorously controlled below 0.05% in solder paste so as to stop solder balls from being generated.
c. Larger metal particle size
The smaller metal particle size is, the larger the overall surface area of solder paste will be, leading to a higher oxidation that adds the opportunity of solder ball generation.
d. Reduced solder paste thickness on pad
The normal thickness of solder paste on pad is between 0.1mm and 0.2mm. When the solder paste on pad is too thick, it usually results from collapse, making solder balls.
e. Controlled flux content and activity
Too high flux content tends to lead solder paste to be partially collapsed, making solder balls. If flux features lower activity, it will perform badly in deoxidation, generating solder balls.
f. Proper storage and application
Generally speaking, solder paste should be stored in the temperature range from 0 to 10℃. Before its application, solder paste should receive warming up treatment and it can never be applied unless its temperature totally rises up to room temperature. It should be stirred in accordance with the regulated instructions. After sufficient solder paste is taken out from the bottle, the cover should be recovered at once. Boards through printing have to go over reflow soldering in two hours.
Measure#2: Stencil opening should be properly designed.
Stencil thickness should be appropriately designed and opening ratio has to be rigorously controlled. Stencil thickness is determined by the SMDs with the finest pitches on PCB. Relatively thin stencils should be picked up and thick stencils should be avoided.
Some defects may be aroused when stencil openings feature inappropriate ratio and opening shape, leading to the generation of solder balls. When openings feature inappropriate ratio, solder paste tends to be printed onto solder mask so that solder balls will be formed during reflow soldering process.
Measure#3: Stencil cleaning quality should be improved.
Stencil cleaning quality improvement is beneficial for printing quality improvement. In the process of solder paste printing, stencil surface should be carefully cleaned and residual solder paste should be in time eliminated so as to stop solder balls from being formed during reflow soldering process. If, however, stencil is inappropriately cleaned, the solder paste left at the bottom of stencil opening will be accumulated around openings so that solder balls will tend to caused.
Measure#4: Mounting stress should be reduced.
Actually, mounting stress is also a leading cause for solder balls but it arouses little attention from people. Mounting stress is determined by some factors such as PCB thickness, component height and chip mounter nozzle pressure setting. If the mounting stress is too high, solder paste will be squeezed outside pad and the squeezed solder paste will become solder balls after reflow soldering. To solve this issue, mounting stress can be reduced to an extent that components can be placed onto solder paste printed on pad and can be appropriately pressed down. Different components need different levels of mounting stress and it should be rationally set.
Measure#5: Solderability of components and pads should be increased.
The soldeability of components and pads features a direct effect on the generation of solder balls. If both components and pads suffer from severe oxidation, some flux can be consumed due to too much oxide so that solder balls can be also generated due to incomplete soldering and wettability. Thus, the incoming quality of components and PCBs must be guaranteed.
Measure#6: Soldering temperature curve should be optimized.
Solder balls are authentically manufactured in the process of reflow soldering whose process contains four phases: preheating, temperature rise, reflow soldering and cooling down. The aim of preheating and temperature rise is to reduce the thermal attack towards PCB and components to guarantee that the melting solder paste can be partially volatilized to prevent temperature from rising too quickly to arouse collapse or splash that is the main cause for solder balls.
To acquire optimized temperature curve in reflow soldering oven, the solution is to control the temperature of reflow soldering and stop the temperature from rising too rapidly in preheating phase. The speed of temperature rise should be controlled at 2℃/s or lower and the temperature of solder paste, components and pads should climb to the range from 120℃ to 150℃. As a result, thermal attack can be reduced from components in reflow soldering phase.
Measure#7: Other elements should be well controlled.
Normally, the optimal temperature of solder paste printing is in the range from 18 to 28℃, RH (Relative Humidity) from 40% to 70%. If the temperature is too high, solder paste will feature low viscosity; if RH is too high, solder paste will absorb more water. The result of both the situations lies in solder ball generation. Therefore, the temperature and RH of workshop should be well controlled.
Solder ball defect generation is such a complicated process that it results from quite a lot of reasons. Therefore, comprehensive elements have to be taken into consideration to stop solder balls from being aroused. In summary, stencil should be accurately designed with opening parameters being compatible with SMT requirement; solder paste has to be stored and applied in accordance with rigorous regulations; mounting pressure should be controlled to proper extent; reflow soldering temperature curve should be optimized.
According to PCBCart’s more than 10 years’ SMT assembly experience, it’s been found that 60% to 80% of solder balls are caused due to inappropriate mounting pressure. Thus, the greatest attention has to be paid to mounting pressure setting on chip mounter so that solder paste won’t be squeezed outside pad with chances of solder paste generation improving.
