How to Work with Small Parts Like 0201?
Almost all users prefer smaller, thinner, and more manageable gadgets. To pack more functionality into their electronic products, designers use multi-layered printed circuit boards (PCBs) and smaller electronic components. Unfortunately, manufacturers cannot make very small leaded components, and instead, they have resorted to making surface mount devices (SMDs) that do not require extended leads—leading to very high component density of PCBs.
Fortunately, the electronics industry has standardized the SMD packaging, which has made life easier for both designers and manufacturers. However, the demand for smaller size of products has been driving down the SMD sizes, and while 0402 was the smallest size even a few days ago, there is an even smaller SMD size in the market—0201. The new entrant measures barely 0.6 x 0.3 mm (0.024 x 0.012 in.), which makes them 66% smaller and five times lighter than their predecessor, the 0402.
Apart from the requirements of component density on the PCB, manufacturers are making components smaller to improve the reliability of electronic equipment as they too grow smaller and offer constraints of space to designers. Almost all electronics is now portable, including digital cameras, robotics, tablets, smartphones, and so on, thanks to the availability of miniature components. The main advantage with smaller components is their better high-frequency capabilities, ESL, and the smaller space they occupy on the PCB. However, the small size of the SMDs demand delicate handling during the assembly process.
Designing with Small SMDs
Miniature dimensions of small SMDs the size of 0201 make it imperative that the design and assembly follow some strict guidelines for successful completion of the projects. Rush PCB offers easy to follow guidelines for working with such small parts. The guidelines involve the design of PCB mounting pads and their solderable metallization, and suitable width of traces associated with these pads. For assemblers, the guidelines offer selection of suitable solder paste, stencil screening, package placements and overages, reflow profiles, and final inspection.
Solder Pad Guidelines for Small Parts
Commonly, designers use two methods of defining the copper pad dimensions for SMDs. One is the solder mask defined (SMD) pad, and the other non-solder mask defined (NSMD) pad.
For SMD pads, designers use a large pad, and restrict the solder flow by placing a solder mask with a smaller opening over the pad. On the other hand, the NSMD pads are smaller, while the solder mask opening is larger. During reflow, the solder flows all over the pad, and an absence of solder mask in a thin ring around the pad prevents solder from flowing beyond the pad.
PCB designers favor NSMD pads over the SMD pads, as the absence of solder mask around the pad in the former reveals the quality of etching, while the larger opening of the solder mask improves the reliability of the solder joint. Of course, the etching process for NSMD pads demands a higher tolerance for the copper pad than the solder masking process requires.
Also Read: WHAT IS A SOLDER JOINT?
Metallization of Pads
Fabricators typically use three types of solderable coatings to protect the exposed pads for SMDs. These are Hot Air Solder Level (HSAL), Electroless Nickel/Immersion Gold (ENiAu), and Organic Solderability Protectant (OSP).
Although HSAL is more common, Rush PCB does not recommend this process for high-density boards, especially with tiny SMDs of 0201 size. The HSAL process makes it difficult to control the amount of solder that goes on to each pad, and this results in a highly uneven PCB surface for mounting small parts.
Depending on the temperature excursions the board undergoes in its lifetime, it will face internal material stress. However, these are likely to occur during reflow, as high temperatures are necessary to make the solder melt and form joints. The temperature excursions also determine the thickness of the ENiAu coating. However, Rush PCB does not recommend allowing the gold metallization surpass 5% of the overall solder volume, as this makes the joint brittle.
Rush PCB recommends a coat of OSP covering the bare copper pad as it adequately protects the pad for soldering. The thickness of coating typically ranging from 0.25 to 0.35 microns is adequate. An OSP coating on the board allows the assembler to pass it multiple times through the reflow oven, if necessary.
A deposit of common solder pastes forms a bump on the tiny pad for components of the size 0201, causing the component to tilt and slide off the pad during reflow. Using a PCB trace with a width equal to that of the mounting pad can prevent the component from tilting, to some extent.
Solder paste has a high impact on processing and assembly yields of PCBs with 0201 and smaller sized components. Characteristics of solder paste that affect this yield include its viscosity, particle size, alloy composition, and the composition of the solder flux in it.
For components of the size 0201 and smaller, Rush PCB suggests using Type 4 solder paste. This is really helpful particularly for fine-feature printing, as Type 4 solder paste produces more consistent volume deposits and has a better release.
Using a stencil is a common and effective method for accurately administering solder paste on to the pads on the PCB. For components of size 0201, Rush PCB recommends a stencil thickness of 0.1 mm as the most suitable. Furthermore, providing the sidewalls of the stencils with a taper of approximately five degrees, and electro-polishing them helps in discharging the paste cleanly when removing the stencil.
However, Rush PCB prefers the use of fully automated jet printers for dispensing paste for components as small as 0201. Not only do these printers eliminate the need for a stencil, but the operator is able to monitor in real time the amount of solder paste that the printer deposits on each pad. Jet printing is ideally suitable for depositing solder paste, specifically for small components, as it is possible to program the printer to adjust the area coverage, volume, height, and layers of solder paste for each pad, depending upon the component type and package.
The above capability of jet paste printing makes these printers especially important for parts with fine pitch, such as those with a pitch of 0.33 mm. They are suitable for the most difficult applications such as with microBGA and even 01005 packages.
Also Read: EFFECTIVE PCB MANUFACTURING PROCESS FOR 2019
Placement of Small Packages
Placing small packages of 0201 size or smaller accurately with proper orientation on the PCB is a difficult job. Rush PCB recommends using pick and place machines equipped with dual-image systems. This helps in proper alignment of the bottom side of the package with the pad on the PCB.
Component suppliers usually offer larger packages up to 0402 in reels with pitch spacing of 4 mm. However, this does not provide adequate accuracy when placing packages of 0201 sizes and smaller. Rush PCB recommends using cartridges with component pitch of 2 mm and a dedicated feeder when using such small size components.
Pick-and-place machines typically use round pipe nozzles for 0402 components. However, for mounting smaller components of 0201 and 01005 sizes, this nozzle design is highly inadequate, Rush PCB recommends nozzles with a rectangular shape and with two vacuum ports for providing a better grip on the tiny components and for an improved performance.
PCBs with multiple packages on board need special care during setting up the reflow profile. Usually, the recommendation from the solder manufacturer is a guideline for the ideal profile. Minor adjustments are usually necessary to account for the distribution of packages on the PCB, and the flux type under use.
Rush PCB recommends raising the temperature to a maximum rate of not more than 2 degree Celsius at the pre-heating stages in the reflow oven. Extending the time that the PCB spends in the soak zone typically reduces the risk of voiding. Designing the PCB such that the SMD component has both ends in the same temperature zone simultaneously helps prevent tombstoning during reflow.
Reflow soldering using a nitrogen atmosphere in place of air potentially causes solder bridging as nitrogen produces less oxide, thus allowing for faster wetting. Rush PCB recommends air atmosphere that wets solder joints more slowly, maintaining a balanced force between both end terminations.
For delivering projects on-time, it is necessary that the assembler has all components for the project at any time. Rush PCB advises adding component overages to account for any losses that occur, meaning purchasing some extra parts than those the project requires.
SMDs are far smaller than most through-hole components, and 0201 and 01005 are practically invisible. Although very accurate, SMD parts tend to drop off during mounting and dismounting from the feeder. Rush PCB recommends using pick-and-place equipment fitted with magnification, dual-image optical systems, and with a standard tolerance of ±0.5 mm as crucial for working with small parts of 0201 and smaller sizes.
At present, 0201 and 01005 SMDs have taken up a large portion of the modern component market, and this share is likely to grow with time. Although there are issues with manufacturability, cost of use, and inspection methods that tend to limit wider adoption of these components, trend for smaller products in many industries are driving an increase in the adoption resulting in improving practices in assembly. According to Rush PCB, as the market matures, applications, where reliability is paramount, will also begin adoption of these small parts.