How to do PCB Pad Design for QFN Components
2019-11-20 17:22Writer: qyadminReading:
• Form Design of QFN Packages
As a more recent IC (built-in circuit) bundle form, QFN components include a soldering end that is parallel to pads on circuit table. Naked copper is usually designed in the center of components, providing better thermal conductivity and electric performance. Appropriately, I/O soldering ends for electric connection can be distributed at the encompassing of central cooling fins, rendering it more versatile to handle PCB tracing. I/O soldering ends come in two types: the first is to get element bottom uncovered with other areas packaged in element as the other type is partial soldering end is uncovered beside element.
With punching or zigzag type applied, copper leads are then used to make inner wafer and central soldering end copper chip and encircling soldering ends linked to create a frame framework. Resin is then leveraged to repair it through mildew fixation and encapsulation, leading central soldering ends and peripheral soldering ends to be uncovered outside package.
• Pad Design for QFN
Since large copper linens for thermal dissipation can be found in the bottom of QFN components, excellent PCB pad design and metallic stencil design should be applied to create reliable soldering contacts on QFN components. Pad design for QFN contains three aspects:
a. Peripheral I/O pin pad design
Pad for I/O on PCB panel should be made to be considered a little bigger than I/O soldering ends of QFN. Internal part of pad should be made to be a group to be appropriate for the form of pad. If PCB features sufficient design space, perimetric amount of I/O pad on circuit plank should be at least 0.15mm while inner enduring length should be at least 0.05mm to ensure sufficient space between pads that remain QFN and the ones in the central part, prohibiting bridging occurring.
b. PCB Solder Mask Design
PCB solder mask design mainly will come in two categories: SMD (solder mask described) and NSMD (non-solder mask described). The previous group of solder mask features openings that are smaller than steel pads as the latter group of solder mask features openings that are bigger than metallic pads. Since NSMD technology is simpler to be managed in copper corrosion technology, solder paste can be positioned around steel pad with soldering cable connections' reliability greatly improved. SMD technology should be found in central thermal-dissipation pad solder mask design with a comparatively large area.
Solder mask openings should be 120 to 150μm bigger than pads, that is, a spacing of 60 to 75μm should be held between solder mask and metallic pad. Cambered pad design must have a related cambered solder mask starting that works with with it. Especially sufficient solder mask should be managed at a part to avoid bridging from occurring. Solder mask should be protected at each I/O pad.
Solder mask should cover through openings on the pad for thermal dissipation to avoid solder paste from flowing faraway from thermal through openings since it'll possibly cause void soldering between QFN central naked soldering end and PCB central thermal-dissipation pad. Through-hole solder mask mainly will come in three types: top solder mask, bottom level solder mask and through opening. Diameter of through-hole solder mask should be 100μm bigger than that of through gap. It's recommended that solder mask essential oil is covered to prevent through openings on the trunk aspect of PCB, which can create many cavities on leading part of thermal-dissipation pad, which is effective to gas release during reflow soldering process.
c. Central Thermal Pad and Through-Hole Design
Because pad is created for thermal dissipation at the central bottom level of QFN, it features excellent thermal performance. To effectively conduct warmth from inner part of IC to PCB table, a related thermal pad and thermal-dissipation through opening need to be designed in the bottom of PCB. Thermal pad provides reliable soldering area and thermal-dissipation through gap provides thermal dissipation function.
Air openings will be produced during soldering by large pads in the bottom of components. To lessen quantity of air openings to the minimal, thermal through openings should be opened up at thermal pad, quickly conducting temperature and good for thermal dissipation. Quantity and size design of thermal through openings depends on software field of deals, degree of IC power and electric performance requirement.
• QFN Stencil Starting Design
a. Peripheral I/O Pad Leak Hole Design
Metallic stencil starting design generally conforms to the theory of area ratio and width-thickness ratio since certain kind of components possibly requires benefit of the basic principle of local thickening or local thinning.
b. Central Thermal-Dissipation Large Pad Starting Design
Since central thermal-dissipation pad belongs to a big level and gas is commonly escaped with bubbles generated. If a huge amount of solder paste is applied, more gas openings will be triggered with numerous defects produced as well such as spatter and solder balls etc. To lessen the amount of gas openings to the minimal and obtain ideal amount of solder paste during thermal-dissipation large pad design, a net leak opening array is chosen to replace a sizable leak gap and each small leak opening can be made to be a group or sq . whose size is unlimited so long as solder paste coating amount is at the number from 50% to 80%.
c. Stencil Type and Thickness
Steel stencil thermal-dissipation pad starting design is straight associated with solder paste coating thickness, identifying connection elevation of assembled components.
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