Extensive development has been made in the package technology of electronic components and products. The size of electronic component is shrinking day by day with these advancements. Moreover, the quality and performance of the components is also enhanced. Therefore, development trend in the electronic components is so as to decrease the spacing among its output and input terminals as well as increasing functioning density of the components. This is achieved through Surface Mount Technology (SMT). For the implementation of component’s surface mount, first step is the creation of corresponding pads on the Printed Circuit Board (PCB), for obtaining structure of PCB. After pad’s creation, stencil printing is used for covering the solder paste present over the pads of PCB surface.
The last step is conduction of heating for the transformation of soldering paste to liquid form among PCB pad and the pins of components. The melted solder paste confined with the area of solder pad for its prohibition among solder joints for the automated assemblage of chips on to the PCB. There are different types of packages each having different pads, however rectangular and circular padding are extensively used which is known as the Quad Flat no-lead (QFN) and ball grid array (BGA) packages respectively. This article will cover detailed information on QFN packages.
The Quad Flat no-Lead (QFN) Packages
When compared with various types of packages used in the fabrication process of PCBs, QFN packages are fashioned so as these may directly be soldered on the substrate of PCB. The QFN package is having numerous advantages, such as it has capacity to offer enhanced heat dissipation on its exposure to the metal pads in the bottom. Moreover, the QFN package is also featuring an advance capacity of superb electrical performance because its pins are very short when compared to the components having extended packages. Therefore, it is better to have QFN design in PCBs for maintaining and ensuring the enhanced performance and reliability of PCBs.
The Wetting Angle of QFN
As discussed earlier, the spacing among the pins of QFN is very small along with size of pins itself, there is a chance of occurrence of solder joint bridge because of accurate quantity of coating of tin paste. The success rate of the soldering is dependent over the size of pad of PCB which is based on stencil’s thickness (h0). Considering the wetting angle of solder tin over the soldering pad (θa) to be 30° whereas the wetting angle of the soldering tin over the solder mask (θr) to be 160°.
In the case, if the roughness of surface of pads is neglected, then the wetting angle can be considered as the advanced angle of three phased line of contact. When compared to the practical soldering craft of the QFN components, there is a rational control over temperature curves of reflow soldering considering the ideal situation with fully melted soldering tin and the surface of pad to be wetted is having capability of insurance of efficiency of soldering assisting the components reaching automated soldering balance.
If the design of pad is rational, then the ideal condition of the solder joints is not meeting the requirement of the electrical performance only but also fulfilling the requirements of mechanical connections and is avoiding the solder joints to fail, for instance pseudo soldering and bridging etc. In such conditions, the solder joints must be fulfilling the following criterion as per mentioned in formula below.
- In case of fully distributed solder joints in the QFN package on PCB pad.
- In case when tin outside QFN tends to grow inside pad.
The Design of QFN Pad
In the above formulas, θs3 is considered equal to that of θa and both are actually the wetting angle of the soldering tin on the pad. The equation of static equilibrium mentioned below is bridging the liquid in vertical direction.
At the bottom of the solder joints, the intensity of pressure is given as following
In the equations above is referring to the liquid density of the soldering tin, T is referring to the surface tension of the liquid solder joint, x4(0) and x3(0) are referring to the slippage of the both ends of solder liquid joint at the bottom position. θ2(0) and θ1(0) are referring to the angles of contacts which are formed on the both sides of the liquid joints along with bottom of surface of pad through liquid-gas interface. θ4(0) and θ3(0) are referring to the angles of contact on each sides, appearing because of liquid-gas interface. V0 is referring to the volume of the solder joint and Wz is referring to the force applied on the pad at end of solder joint and chip in that of vertical direction.
From the equations 1 and 2 the frame curves of the solder joints are making boundary conditions which are equivalent to the upper ends of the joints and initial conditions grounded on the actual technique of solutions to that of initial values. Because of the incapability of the initial values to meet the requirements of z being equal to 0, it is then transforming to an equivalent issue which is given through following formula.
Therefore the application of the minimized objective function is possible for the determination of the ideal design of the size of QFN pad. Furthermore, there are certain geometric features of the tin wrap angle of the PCB which must be taken in to consideration. Therefore, the expansion size of the pad is given through equation. Here, Dh is referring to the thickness of pad which is exposed outside the chip.
RayPCB Offering QFN Technology PCBs
RayPCB is offering ites extended services to made customized PCBs based on QFN technology. RayPCB is ensuring the suitable pad design for QFN in order for achieving higher electrical performance from both PCBs and components used in its fabrication. With professional PCB manufacturing, fabrication, and assembling capacity, RayPCBs are turning ideas in to realities.