Placing Tiny Parts Precisely
You?ve heard the phrase "bigger is not necessarily better." Well, that certainly holds true for manufacturers of PCBs.
There used to be a time when having enough real estate on a PCB was no problem. However, those days are gone. Today, the goal is to squeeze as many components as possible onto a board that is no larger than it absolutely has to be.
Shrinking PCBs and ComponentsThe desire for smaller products, especially in the mobile device market, is the main driver behind the shrinking size of PCBs and surface mount components. "Ten years ago, a cell phone was just a cell phone. You dialed, called and that was it. Now, you can store messages. It?s more of a time-management feature. They also have games. And with the reduction and miniaturization of the size of components, this allows more features to be added into something that is easy to carry around," says Cal Driscoll, applications manager at Manncorp Inc. (Huntingdon Valley, PA).
According to Prismark Partners LLC (Cold Spring Harbor, NY), since the 1970s, the size of passive components has decreased in area by more than 28 fold. At that time, the typical passive component was a 1206 (3.2 by 1.6 millimeters). Ten years later, 0805s (2 by 1.25 millimeters) had become the most common size, and by the end of the 1990s, 0603s (1.6 by 0.8 millimeters) were the most commonly used components.
But with the consumer desire for smaller handheld devices, components smaller than 0603s were needed. So along came 0402s (1 by 0.5 millimeter). Prismark Partners states that implementing 0402s alongside 0603s did not require processes that were much different. The 0402s have been around for several years. And most manufacturers are now comfortable with 0402s, at least in the mobile device market, even though they represent a small segment of that market.
But even now, the desire for smaller and smarter portable devices has not abated. Therefore, a new component has come onto the scene to challenge the dominance of 0402s and to challenge the automated assembly processes of PCB manufacturers.
The 0201The 0201 (0.6 by 0.3 millimeter) is currently the smallest component being produced and used. Compared with 0402s, 0201s are 75 percent smaller in both volume and weight.
They are already used in portable consumer electronic products. "Currently, 0201s are produced at a rate of 6.8 billion per year. However, they represent only 0.6 percent of total passive production," says Prismark Partners.
There are a couple of reasons for the low percentage of 0201s being used. "Some have shied away from 0201s because of the much higher design characteristics that are involved for the pads and the reflowing process. Manufacturers are using 0402s in their place," says Frank Ruiz, service area manager at Juki Automation Systems Inc. (Freemont, CA). Also, as components decrease in size, they have lower power-handling capabilities. "So you might actually need more of them or you might not be able to go that small if you need to use higher amounts of power. So you?re just going to see the smaller components in smaller handheld devices—things that don?t require excessive amounts of power," says Ruiz.
0201 Design ChallengesAccording to Jim Adriance, process research engineer at Universal Instruments? surface mount technology laboratory (Binghamton, NY), if a manufacturer is comfortable with assembling 0402s, then 0201s should not be a problem. "No special machinery is required. Parameters need to be correct, but it does not require a new system or new technology," he says.
However, there are assembly issues that need to be addressed before 0201s can be successfully implemented into a project.
One of the most important issues is knowing whether the pick-and-place machine is adequate. "There are a lot of things that have been added onto the machines to ensure the accuracy of the picks," says Ruiz.
Typically, the pick-and-place machine should have a high degree of accuracy, because the components are so small. "If you pick up right in the center of the component, ideally you shouldn?t have any problems. If you pick up off to the left, off to the right or on the edge, when it starts assembling it, there could be placement issues," says Driscoll. A pick-and-place machine with an autocorrect feature can be helpful if the machine picks up the component off center or if the paper carrier tape?s position is varying. The autocorrect feature will allow the machine to recognize that it picked up the part a certain amount off. If this amount of offset is consistent, it will automatically move the pick position to that location.
Inspection is an integral part of the PCB assembly process. Typically, computer-aided design (CAD) data sets the positioning where the component should be placed. Some pick-and-place machines have an onboard camera, so when the component is placed, the camera can see if it has been placed correctly in reference to the CAD data. Or afterwards, you could have auto-mated optical inspection equipment outside of the machine to inspect placement and position.
When the component is in the feeder, the operator should know how low to go in the feeder to pick up the component. Therefore, most pick-and-place machines should have a height-measurement system to gauge the exact Z position of the tape feeder. By determining the exact distance needed to pick up the component, the machine will run faster.
Pick-and-place machines use vacuum nozzles to pick 0201 components from a paper carrier tape. Manufacturers should be aware that the nozzle should be a special 0201 nozzle. Nozzles are typically made of plastic, steel or ceramic. Driscoll suggests using a ceramic nozzle. "With plastic nozzles, when you get down that small, you start to sacrifice a degree of robustness. With steel, you could have problems with scoring the component, or if the steel gets dented or damaged, it could sacrifice the suction and cause the component to get dropped," he says.
Ruiz adds that with 0201s, the part should be picked up and placed with no more than 3 newtons of force. If the nozzle goes down too hard, the component might tombstone, or it might fly out of the nozzle pocket. Therefore, the nozzle needs to have a delicate spring.
Manufacturers should also be able to automatically detect whether the tip of the vacuum nozzle is clogged. This might prevent a component from being placed incorrectly. Pick-and-place machines with a component-release check have a laser that checks the end of the nozzle to ensure that the component has been delivered to the board and is not sticking to the side of the nozzle.
Another important issue when incorporating 0201s into a PCB project is choosing the correct type of stencil and the screen printing process. "If you?re dealing with smaller components and you?re using a larger solder paste or the solder ball size is larger, it?s not going to fit through the holes. The balls are going to be larger than what the holes are for the 0201 components," says Driscoll.
Going hand in hand with choosing the stencil is solder paste selection. Solder paste selection should be based on the particular application, and generally most applications use no-clean solder paste. No-clean flux chemistry combines chemicals to accomplish a number of tasks in the soldering process, such as cleaning the parts to be soldered and keeping the solder powder from oxidizing during storage and reflow heating. These fluxes produce a solder paste that can be applied as required, will stay in place, will hold components in place prior to reflow and be inert after the soldering process.
Additionally, because solder paste has a definite ball or particle size once the flux is activated, there is a solder paste best suited for 0201s. "Most suited for a 0201 would be a size 3," says Driscoll.
Poor board planning up front can lead to defects later on during the reflow process. Pad size, pad dimension and distance will affect the reflow profile. According to findings from Universal Instruments, a pad design of 12 by 15 by 9 mils is optimal for 0201s. Component pad spacing is also critical. For example, the optimal pad spacing for 0402s and 0603s is 0.3 millimeter from one component to the other. For 0201s, spacing has decreased to 0.15 or even 0.1 millimeter.
The reflow profile also depends on the density of the board. If the board is thick, it will require more heat or a longer time in the oven. If it is a thin board, it will need less heat or less time in the oven.
Common DefectsThe two most common defects found with 0201s are tombstoning and solder bridging.
Tombstoning occurs when a component is misaligned so that it stands on one of its sides. This typically happens when one side is pulled harder than the other. One side of the component reaches a liquid state before the other, creating a surface tension where the molten solder pulls the component vertically. Other causes for tombstoning include heterogenous flux wetting speed, using attachment pads or component terminations that have poor solderability, and excessive heating speed or inadequate flux activation time.
Solder bridging is the formation of an electrical short when sections of solder from two adjacent components run together, forming a conductive path between the two components.
"Many parameters will influence defects. It goes back to pad design and stencil design. The solder paste print process is crucial. Placement accuracy is very important, along with the type of solder paste that you?re using and the reflow profile," says Adriance.
How Small Can Components Get?There are components coming online that are smaller than 0201s. The next size smaller than a 0201 is the 0105. Adriance believes that these will be predominantly used in handheld products. They could also go into other niche applications, such as a hearing aid.
Driscoll has another idea about the use for components smaller than 0201s. "A lot of Arial, the smallest components get embedded underneath components like a quad flat pack, or embedded into circuit traces. Even though our demand for smaller and smaller components is definitely out there, I can?t see, just for shear mechanical limitations of the equipment that we?re using, it going much further. At that point, it will just go to embedded circuitry. They?ll just take that component and put it right into the land pattern of the PCB, and that will be done at the PCB house. The current mechanical limitations of the machines just can?t process it," he says.
Adriance agrees. "I believe that they are somewhat approaching [the smallest] at the moment with the 0105s. I?m not saying that they won?t go smaller, but I don?t see the need or the drive to go considerably smaller than that in the near future."