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Stencil Design Requirement on QFN Components for Optimal Performance of PCBA
Recent years have witnessed wide applications of QFN (Quad Flat No-lead) package components owing to their comprehensive advantages including excellent electrical and thermal performance, light weight and small size. As no-lead package, QFN components have received numerous focuses from the industry since they feature low inductance between leads. QFN package components feature square or rectangle whose package forms are similar with those of BGA (Ball Grid Array) package components. Different from BGA, QFN has no solder balls at the bottom side and its electrical and mechanical connection with other components is achieved through solder joints that are generated through reflow soldering before which solder paste has to be printed on pads that are on the surface of printed circuit board (PCB).
Solder paste printing is such a significant phase during PCBA (Printed Circuit Board Assembly) process that it will further determine the final quality and performance of assembly. Solder paste printing can never be smoothly or accurately done unless appropriate stencils are designed and used, which is why this article is generated.
Solder paste printing plays such an essential role in surface mount assembly (SMA) and electronic assembly technology that quality of solder paste printing through stencil is directly associated with First Time Yield (FTY) of surface mount electronic/electrical component soldering. It is concluded that 60% to 70% of soldering defects derive from low quality of solder paste printing carried out through stencil. Therefore, it's necessary to implement comprehensive study on all the aspects concerning solder paste printing through stencil technology.
When it comes to stencil design, excellent opening is the first element guaranteeing optimal and reliable soldering joints.
• Stencil Frame Design
Stencil frame is usually made of aluminum alloy with its size being compatible with parameters of printer. Automatic manufacturing requires that stencil has to go onto production line and the size of stencil has to be accepted by printer. Neither too large size nor too small size can push smooth manufacturing.
• Stencil Stretching Design
Stretching refers to the process during which stainless stencil plate is stuck to frame. Glue and aluminum paste tape are usually used in stretching. Glue agent is first coated at the connection of aluminum alloy frame and stainless stencil plate and then a protective coating is evenly scraped on. During stretching process, an internal distance of 25mm to 50mm needs to be left between stainless stencil plate and frame to guarantee excellent flatness and stretch during solder paste printing. A newly manufactured stencil should maintain the tension in the range from 40 to 50N per centimeter.
• Stencil Fiducial Mark Design
Automatic positioning should be carried out by printer during automatic manufacturing line so that stencil needs to contain fiducial marks. Fiducial mark design is based on dimensions of marks in Gerber file of PCB and then opening is set at a ratio of 1:1 with etching carried out at the back of stencil. Generally speaking, at least two fiducial marks are required on a stencil at two opposite angles.
• Stencil Design for I/O Pads around QFN Components
Stencil opening size should be equivalent to that of peripheral I/O pads so that such opening size is able to ensure solder joints can be formed with a solder paste height of 50 to 75μm after reflow soldering around pad. When it comes to fine line QFN components, especially those whose I/O pitch is less than 0.4mm, stencil opening width should be reduced a little than PCB pads to avoid bridging between ambient I/O pads. Stencil opening ratio between width and thickness (W/T) should be more than 1.5.
• Stencil Design for Central Thermal-Dissipation Pads of QFN Components
Inappropriate central pad for thermal dissipation opening design will cause all kinds of defects. When QFN components are going through reflow soldering, solder paste on big pads will be melted with melting flux generating some air running over, leading to issues such as air holes, pinholes, solder splash and solder balls. Although it's almost impossible to eliminate those issues, their bad effect can be reduced through some measures. For example, multiple small mesh opening array is picked up instead of a large opening. The shape of each small opening can be either circle or square as long as 50% to 80% of pads for central heat dissipation are covered by solder paste, which will ensure a solder paste height of 50 to 75μm.
• Stencil Category and Stencil Thickness
Stencil material is suggested to be stainless while etching method is suggested to be laser cutting. Electrolytic polishing is carried out on hole wall so that hole wall will be smooth with friction reduced, beneficial for solder paste demoulding and molding.
Stencil thickness plays a decisive role in solder paste amount printed on PCB. Either too much or too little solder paste will lead to defect generation during reflow soldering. Too much solder paste tends to cause bridging while too little solder paste tends to cause open soldering. It's suggested that fine-line QFN components (pitch is 0.4mm below) depend on stencils with thickness in the range from 0.12mm to 0.13mm and high-spacing QFN components (pitch is 0.4mm above) depend on stencils with thickness in the range from 0.15mm to 0.2mm.
• Stencil Inspection
Stencil should be carefully inspected prior to authentic SMT assembly with the following inspection items:
a. Visual inspection should be picked up to inspect the flatness of stretching and make sure opening is located at the center of stencil.
b. Visual inspection should be picked up to ensure that stencil opening positions are compatible with PCB pads.
c. Stencil opening size (length, width) should be inspected.
d. Microscope is used to inspect the smoothness of opening hole wall and stencil surface.
e. Tensiometer is leveraged to measure stencil tension.
f. Stencil thickness should be validated through solder paste printing result.
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