Step 6 Component Placement
December 31, 1969 |Estimated reading time: 15 minutes
By Scott Wischoffer
The concept of pick-and-place itself is very simple it is the process of taking the components used to build a product from the reel, stick or tray and placing them in the correct place on the board in the correct direction. This article introduces some details of that process, as well as provides some practical guidelines about approaching the assessment of pick-and-place equipment.
By the time a circuit board is ready for pick-and-place, it has gone through either a screen printing or adhesive dispensing process that has put material on the printed circuit board (PCB) to hold the parts in place.
Levels of PlacementManual Placement. Pick-and-place can be accomplished in many ways, the simplest of which would be using a pair of tweezers and a table. In this method, a person would simply take the components and put them on the appropriate spot by hand. Manual placement still is used in many areas today mainly in rework but sometimes with the addition of a late part or with very small prototype builds.
There are many reasons why this is not the best component placement method. Four of them are key:
- Accuracy: The likelihood of errors increases significantly when someone is hand placing a part. When getting into fine-pitch components, it is very difficult to achieve the accuracy necessary for functionality when placing a part by hand. Additionally, the likelihood of placing the wrong components or placing the part in the wrong place is high.
- Speed: Placing parts by hand is simply too time consuming.
- Orientation: Even if a part is hand-placed in the right spot, it is easy to place it in the wrong orientation. Many components are polarized and placing them in the wrong direction will cause the board to fail.
- Technology: As parts get smaller and smaller, they become more difficult to handle. Typically, they have few identification marks, often requiring underside inspection to be placed manually. The newest generation of mini components is the 0201. They are barely bigger than grains of sand and have no markings to identify their type or value.
Semiautomatic Placement. A tabletop unit designed to assist the operator in placing components typically does this next level of pick-and-place.
Semiautomatic placement machines allow the component to be absorbed onto some type of vacuum pipette and sometimes they incorporate camera or magnifying systems to target the component over the placement point before allowing mechanical placement of the part. Such machines allow the operator to adjust for X-Y and rotational compensation upon placement, but this is done with a mechanically assisted hand operation.
Whereas manual placement may attain a couple hundred parts per hour, semiautomatic placement may yield placement rates of 1,000 parts per hour.
Fully Automated Placement. The third level of placement, fully automated machines, comes in a wide range of sizes and capacities. They start with what generally is termed an "entry-level" machine a relatively inexpensive, small but versatile piece of equipment that can pick-and-place a large variety of parts with a fair amount of speed and accuracy. Entry-level machines typically place parts in the range of 3,000 to 5,000 parts per hour.
Slightly more advanced and more expensive than entry-level machines, mid-range machines essentially have the same capabilities as their entry-level counterparts. However, they often will hold more component feeders, and they place components faster. A mid-range machine usually will place between 8,000 to 14,000 components per hour. High-end machines take placement speed considerably further to the 20,000 to 30,000 per hour range.
At this point, placement machines typically are split into two categories: high speed and flexible placement. The reason for this is the inherent conflict between speed and accuracy.
Usually, about 80 percent of the components on a board are resistors and capacitors (Rs & Cs), which need to be placed quickly. With these components, accuracy is not as critical as with other components on the board. Larger, more complex components (e.g., flat package integrated circuits [IC], ball grid arrays [BGA], direct attach chips) demand higher placement accuracy, so speed may be sacrificed in these instances.
Finally, the ultra-high-speed chip placers are machines that typically place 40,000 to 70,000 components per hour. They are amazingly fast, quite easy to changeover and use many parts reels.
At the high- and ultra-high-end equipment are the chip-shooter-style machines, which are available in three distinct types. The most popular is the turret-style machine, the second is the gantry-style machine and the third is the walking-beam-style machine. Each style has its own unique characteristics.
Figure 1. Turret machines use a number of placement heads, each with a selection of vacuum nozzles that allow the machine to pick up the component of the appropriate size.
Machine TypesTurret-style machines. This style machine uses a rotary turret also known as an indexing head (Figure 1). The turret will contain numerous placement heads. Each head is equipped with a selection of vacuum nozzles that will allow the machine to pick up the components with the most appropriate size, ensuring that the component will be held firmly through the placement process. The parts feeders are positioned on a table that can shift from left to right to correctly position the feeders at the pick point. The component then is absorbed onto the nozzles and carried to the opposite side of the turret. While traveling from the pick point to the placement point, the component is inspected by a vision system to ensure it is the correct size and shape, has the correct number of leads, and the leads are the correct length, width and pitch. Once this information is verified, the part can be rotated to the exact placement angle. Any compensation necessary for X and Y is made by the worktable, which is holding the circuit board.
The main advantage of turret-type machines is a simple design concept they are inherently very fast and have relatively few moving parts. Optimizers and other programming tools allow the feeders to be located in the most optimum positions, reducing excess motion and increasing speed. Additionally, the components on the PCB are grouped by the computer to create the shortest distance between placements, also increasing the number of boards that can be assembled in a day. Most of the speed comes from the fact that the gear-driven turret can move very rapidly from one position to the next while generating only minimal inertia. The variety of nozzle sizes in a random access design ensures the correct nozzle is always available and every placement head is used every time. Finally, PCB load time can be reduced to almost nothing, reducing the total board assembly time to the minimum.
Most of the speed comes from the fact that the gear-driven turret can move very rapidly from one position to the next while generating only minimal inertia. The variety of nozzle sizes in a random access design ensures the correct nozzle is always available and every placement head is used every time. Finally, PCB load time can be reduced to almost nothing, reducing the total board assembly time to the minimum.
Figure 2. In gantry-style machines, placement heads are mounted on a beam, allowing access to all feeder positions as well as the circuit board.
Gantry-type machines. Gantry-style machines use a placement head mounted on a beam, allowing the placement head access to all the feeder positions as well as the circuit board (Figure 2). The component feeders and the PCB all remain stationary in this design, and the pick-up head mounted on the gantry goes to the feeders, gathers the components, image processes the parts and then delivers them to the correct placement point on the circuit board.
To increase speed, gantry-style machines often have many nozzles on a single head and may use more than one gantry per machine, in sort of a "tag team" approach. As one head is collecting parts, the other is placing parts, and vice versa.
The negative side to the gantry design is the time lost to motion. Large distances often must be traveled to reach all the feeders to gather the components, and then traveled over again to return to the placement point on the board. Additionally, multiple gantry machines often must wait for each other in order to maintain minimal distances from one another for collision avoidance. Because of these considerations, gantry-style machines generally experience higher derations the difference between rated speed and actual throughput than similarly rated turret designs.
One way to overcome the inherent downtime is to put multiple machines in a row, but there are disadvantages to this, as well. The cost will be higher, the floor space greater, the number of placement heads to maintain will increase, along with the number of operators. Also, the greater the number of machines in a single line, the greater the deration of each unit. The reason for this is the influence one machine has on the others. If any machine in the line stops for more than a minute or two, the machine in front chokes and the machine behind starves because the line flow has been interrupted.
So balancing is critical to gantry-type performance. If there are multiple robots side-by-side, one should not be waiting for another because the line only can move as fast as the slowest robot.
Walking-beam machines. The walking-beam-style machine is designed for extremely high-volume production environments. PCBs usually are mounted on a pallet just prior to assembly and then the pallet is indexed step-by-step through a number of placement stations. This style machine is extremely popular with a very small, but highly respected market segment. Although the design is very efficient, it is not very friendly when changing from one product to another. To change to a different product, the pallets must be adjusted, the feeders repositioned and the heads retooled, limiting popularity to products requiring nonstop production of a limited number of products.
Flexible placement machines. Flex-placers always are gantry-style design. The main reason is that speed is not as important as flexibility and accuracy. High-speed machines only accept parts from a tape-and-reel supply source and generally are limited to a maximum tape width of 32 or 44 mm. Flexible placement machines must accept input from a full range of tape-and-reel, stick and tray component supply mediums. The ability to place large and fine-pitch components at a reasonably fast speed is the requirement in this market. Many flex-placers also have the ability to handle mechanical chucks and use both front and rear lighting methods for image processing. Other features include coplanarity checking and precise placement pressure control systems.
Production and Component ConsiderationsThere are many different production environments that ultimately determine the type of pick-and-place machine needed.
Production volume is the first consideration how many boards need to be built every week.
For companies that build boards infrequently, small or prototype machines can adequately meet their needs, the low cost and flexibility of these units make them extremely popular, but for companies building several hundred thousand boards a week, a machine's ability to meet production demands depends on numerous factors:
- Which components need to be placed on the board? This determines the acceptable limitations of the machine. Maximum component size, weight, supply medium and quantity can influence the purchase decision.
- How many different components must the machine place? And what size tapes, tubes or trays do they come in?
- What is the production environment? Whether it is high-, medium- or low-mix (mix referring to the number of different boards, i.e., products, being run on the line) determines how many times the machine has to "changeover" from one product to another.
Changeover time is critical in a medium- to high-mix environment, as downtime occurs when changeover is taking place. Most new pick-and-place equipment allows the manufacturer to change products rapidly. If there are additional feeders and reels on the premises, setups can be changed within minutes sometimes within seconds. However, the number of changes expected will determine the number of additional feeders to purchase and the number of additional feeder pallets that will be required.
The component types being placed typically dictate the equipment used. High-speed machines normally are used for resistors, capacitors, small tantalum chips, MELFs and small SO packages. Flexible or fine-pitch placement machines usually handle larger, high-end components.
Other less mainstream packaging types also can be used in pick-and-place, but these represent a very small percentage of total components placed.
The Basic ProcessFeeders present components to the machine be it tape-and-reel, stick or tray delivering the parts individually so the machine can pick them up and inspect them. Most machines are fitted with vacuum nozzles. The vacuum nozzle is a tool that absorbs the component onto the machine and carries it through the processes of inspection, alignment and placement.
The board is brought into the machine either manually (with low-end equipment) or by conveyor on fully automated machines. There are three types of automated loading systems conveyor load, servo load and walking-beam.
A conveyor loading system brings the boards in on a belt from the load position to the assembly position. There, the board is built, then exited out the other side by the conveyor. Cycle time for a conveyor-driven system is between 3.5 and 5 seconds per board.
Servo loading systems are the fastest loading systems. A high-speed servo system will move the X-Y table to the in-conveyor position and locate the board. After assembly is completed, the board is delivered by the same high-speed servo system to the exit conveyor where it will be removed from the machine using a standard conveyor. This type of loading system often can load two boards at one time, reducing the PCB loading time to between 0.6 and 1.4 seconds, and increasing the amount of product built at the end of the day.
The third type of loading system is called a walking-beam. The walking-beam generally is used for extremely high-volume applications. The board is loaded onto a carrier or pallet, which then is indexed through the machine station by station. After assembly is completed, the board is removed from the pallet and replaced on the conveyor to continue down the production line.
Once loaded, most PCBs use some sort of optical correction to determine if the board is exactly where it should be and make corrections if it is not. Boards can be located by using tooling pins; however, the more accepted method is by using fiducial marks, and most machines can accept a wide range of fiducial sizes and shapes. Fiducial marks are used to locate the stencil to the board, the adhesive to the board and the components to the board. The advantage of fiducials is they are placed on the board at the same time as the land patterns for the components and are aligned perfectly with these patterns. Using fiducials, the machine can compensate for less-than-perfect boards by still placing the correct components in the correct places.
Machine Selection CriteriaIt is very rare that a manufacturer will be able to match the product they build exactly with a specific placement machine. Potential purchasers need to ask a lot of questions from the start, such as:
- What is the maximum circuit board size I am going to build?
- How many different part numbers are required to build my boards?
- What type of components will I need to build the board?
- How many changeovers do I expect to do in a day? A week? A month?
- What is the average number of placements per panel?
- How many boards per hour do I need to produce?
- Is the equipment footprint important?
- How many placements per hour can the machine achieve?
- What is the cost per placement of the machine on my product mix?
- What is my return on investment?
A single manufacturer may offer several different sizes of machines, designed to handle small, medium or large boards. Keep in mind that a small board always can be put in a large machine, but a large board cannot be put in a small machine.
Cost and QualityCost is always a factor. Consider the machine's resale value; generally higher quality equipment will be worth more of its original cost when it is sold later. When resale value is high, leasing costs are lower. There can be considerable variance from one machine to another, and parameters like throughput, quality, reliability and uptime are associated directly with cost.
Quality will this machine last and be reliable as long as I need it? Listen to it. Does it sound like it is beating itself to death? Noise means metal is hitting metal. This causes vibration, harmonic resonance and general wear and tear. A loud or "clickity-clack" machine may not be reliable over a long period of time.
Once you put together a matrix based on the metrics described above, generally, you can bring your selection options down to three or four different manufactures. Then you go and look at them.
Choosing the SystemOnce the possible machine solutions for the board building requirements have been determined, go to each of the manufacturers of these machines. Look at the machines and run your product on them.
It is a mistake to accept demonstration boards as "proof of operation" in the final evaluation stage. Demo boards often are designed to make the machine look the best it can; but in actual production, there are not as many opportunities for gang picks, gang nozzle changes and gang placements as there are in a controlled demonstration. Run your boards and time them on the same machine model that you are considering before making your final selection.
Go beyond the machine and check out the intangibles. What kind of training, spare parts, service and applications engineering does the machine vendor provide after the sale? Can the vendor help develop your process? The right vendor will make an effort to deliver everything you need to do a complete job when building your circuit boards.
Consider the urgency of your needs. What kind of lead time is the vendor promising? How long can you wait for the machine? How much time will installation take? How and when will training take place? When can you expect to be in production with the machine?
Understand the controller. With most placement machines, some type of controller is needed to operate the machine. Is the machine PC-based? Is the controller part of the equipment or an additional purchase?
The point is to be exhaustive in the upfront exploration. Lay out your questions. Put together a needs or functionality matrix. Eliminate machines based on what they cannot do.
Your decision will be an important one in supporting your company's ongoing success; make sure it is an educated decision based on all the facts.
SCOTT WISCHOFFER, national applications manager, may be contacted at Fuji America Corp., 171 Corporate Woods Pkwy., Vernon Hills, IL 60061; (847) 913-0162; Fax: (847) 913-0186; E-mail: scottw@fujiamerica.com; Web site: www.fujiamerica.com.