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Optimal Design and Solder Paste Printing Compatible with QFN Component Mounting

The constant improvement of electronic product assembly density leads both electronics components and devices to miniaturization, fine pitch and even no leads. This article will discuss excellent solder paste printing technologies compatible with QFN (quad-flat no-leads) components and introduce QFN components and LCCC (leadless ceramic chip carrier) components whose features will be elaborated. QFN structure and pad design will also be introduced based on QFN package appearance design, QFN pad design, and QFN stencil opening design. Finally, excellent solder paste printing technologies of QFN components will be analyzed from the perspectives of solder paste ingredient, stainless stencil properties and parameters, printing environment, solder paste printing technology design and printing equipment with leading defects of QFN component solder paste printing discussed and practical experience introduced of excellent solder paste printing implementation compatible with QFN components.

QFN and LCCC are the commonest two types of lead-less components that are unusual. Compared with lead components, both PCB (Printed Circuit Board) pad and metal stencil opening feature different pads from pads for fine and long leads, especially in terms of solder paste printing technology.

Essential Advantages of QFN

The leading material of LCCC packages is ceramic while that of QFN is plastics with such low prices that are more accepted by consumer electronics products. As a result, QFNs are widely applied in small-scale home appliances. QFN components perform as squares or rectangles, which is similar with that of CSP (chip size package). The only difference between them is that QFN components hold no solder balls below so that electrical and mechanical connection between PCB board and QFN totally relies on solder paste that is melted during reflow soldering and will become solder connections after cooling. Because contact distance is the shortest between QFN and PCB pads, leading to better electrical performance and thermal performance than majority of lead components, which is especially more suitable for electronic products calling for higher requirement on thermal dissipation and electrical performance. Compared with traditional PLCC (plastic leaded chip carrier) components, QFN components dramatically decrease in terms of package area, thickness and weight with parasitic inductance reduced by 50% so that they work better especially for cell phones and computers.

PCB Pad Design for QFN Components

• Shape Design of QFN Packages

As a newer IC (integrated circuit) package form, QFN components contain a soldering end that is parallel to pads on circuit board. Naked copper is usually designed in the middle of components, providing better thermal conductivity and electrical performance. Accordingly, I/O soldering ends for electrical connection can be distributed at the surrounding of central cooling fins, which makes it more flexible to carry out PCB tracing. I/O soldering ends come in two types: one is to get component bottom exposed with other parts packaged in component while the other type is partial soldering end is exposed at the side of component.

With punching or zigzag type applied, copper leads are then used to make internal wafer and central soldering end copper chip and surrounding soldering ends connected to generate a frame structure. Resin is then leveraged to fix it through mold fixation and encapsulation, leading central soldering ends and peripheral soldering ends to be exposed outside package.

• Pad Design for QFN

Since large copper sheets for thermal dissipation are available at the bottom of QFN components, excellent PCB pad design and metal stencil design should be implemented to generate reliable soldering connections on QFN components. Pad design for QFN contains three aspects:

a. Peripheral I/O pin pad design

Pad for I/O on PCB board should be designed to be a little bigger than I/O soldering ends of QFN. Internal side of pad should be designed to be a circle to be compatible with the shape of pad. If PCB features sufficient design space, perimetric length of I/O pad on circuit board should be at least 0.15mm while internal lasting length should be at least 0.05mm to guarantee sufficient space between pads that are around QFN and those in the central part, prohibiting bridging taking place.

b. PCB Solder Mask Design

PCB solder mask design mainly comes in two categories: SMD (solder mask defined) and NSMD (non-solder mask defined). The former category of solder mask features openings that are smaller than metal pads while the latter category of solder mask features openings that are bigger than metal pads. Since NSMD technology is easier to be controlled in copper corrosion technology, solder paste can be placed around metal pad with soldering connections' reliability greatly improved. SMD technology should be picked up in central thermal-dissipation pad solder mask design with a relatively large area.

Solder mask openings should be 120 to 150μm larger than pads, that is, a spacing of 60 to 75μm should be kept between solder mask and metal pad. Cambered pad design should have a corresponding cambered solder mask opening that is compatible with it. Especially sufficient solder mask should be maintained at a corner to stop bridging from occurring. Solder mask should be covered at each I/O pad.

Solder mask should cover through holes on the pad for thermal dissipation to stop solder paste from flowing off from thermal through holes since it will possibly cause void soldering between QFN central naked soldering end and PCB central thermal-dissipation pad. Through-hole solder mask primarily comes in three types: top solder mask, bottom solder mask and through hole. Diameter of through-hole solder mask should be 100μm larger than that of through hole. It's suggested that solder mask oil is coated to block through holes on the back side of PCB, which can generate many cavities on the front side of thermal-dissipation pad, which is beneficial to gas release during reflow soldering process.

c. Central Thermal Pad and Through-Hole Design

Because pad is designed for thermal dissipation at the central bottom of QFN, it features excellent thermal performance. To efficiently conduct heat from internal part of IC to PCB board, a corresponding thermal pad and thermal-dissipation through hole have to be designed at the bottom of PCB. Thermal pad provides reliable soldering area and thermal-dissipation through hole provides thermal dissipation function.

Air holes will be generated during soldering by large pads at the bottom of components. To reduce number of air holes to the minimum, thermal through holes should be opened at thermal pad, quickly conducting heat and beneficial to thermal dissipation. Number and size design of thermal through holes depends on application field of packages, extent of IC power and electrical performance requirement.

• QFN Stencil Opening Design

a. Peripheral I/O Pad Leak Hole Design

Metal stencil opening design generally conforms to the principle of area ratio and width-thickness ratio since certain type of components possibly takes advantage of the principle of local thickening or local thinning.

b. Central Thermal-Dissipation Large Pad Opening Design

Since central thermal-dissipation pad belongs to a large scale and gas tends to be escaped with bubbles generated. If a large amount of solder paste is applied, more gas holes will be caused with numerous defects generated as well such as spatter and solder balls etc. To reduce the number of gas holes to the minimum and obtain optimal amount of solder paste during thermal-dissipation large pad design, a net leak hole array is selected to replace a large leak hole and each small leak hole can be designed to be a circle or square whose size is unlimited as long as solder paste coating amount is within the range from 50% to 80%.

c. Stencil Type and Thickness

Metal stencil thermal-dissipation pad opening design is directly associated with solder paste coating thickness, determining connection height of assembled components.

Excellent Solder Paste Printing Technology

Elements determining QFN solder paste printing quality mainly include solder paste, PCB pad, metal stencil, solder paste printer and manual operations.

Solder paste features a much more complicated ingredient than pure tinlead alloy, containing solder alloy particles, flux, rheological regulator, viscosity control agent and solvent. Because QFN components are leadless device containing a large thermal-dissipation pad in the central part, relatively high requirement has been set to viscosity and viscosity control technology. Viscosity of solder paste shouldn't be too high since too high viscosity will make it difficult to go through openings on the stencil. Furthermore, printing traces are incomplete with low viscosity.

The smaller solder paste particles is, the more viscous solder paste will be. The higher amount of particles included, the more viscous solder paste will be. Solder paste features the highest viscosity with circular particles and vice versa. When it comes to ultra-fine spacing printing, solder paste with thinner particles has to be used to acquire better solder paste resolution.

Solder paste printing is such a complicated process that contains so many technical parameters each of which will bring forward much damage if they are unsuitably adjusted. All those parameters mainly include scraper pressure, printing thickness, printing speed, printing method, scraper parameter, demolding speed and stencil cleaning frequency. When scraper features low pressure, solder paste will fail to effectively arrive at the bottom of stencil opening and to fall on pad. When scraper features too large pressure, solder paste will be too thin or even damage stencil. Agreeable thickening of solder paste printing is good to improve QFN components' assembly reliability.

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