This article aims to help buyers analyse and select the features that affect decisions on the purchase of an appropriate automatic pick-and-place machine.
As with any complex machine, there will be tradeoffs between cost and capabilities, some of which specifically relate to production accuracy and yield. In particular, we will address:
1. Component positioning methods
2. Machine construction
3. Solder paste fluid dispensing
4. Component feeders
When starting the evaluation process, there are two defining factors to keep in mind which determine what category fits your machine needs. The first principal factor is CPH (components per hour) and the secondary factor is machine capability. In this article, we will address three aspects of machine capability that have a direct impact on final board quality and production yield.
Accuracy and repeatability
For production machines, we typically recommend looking for a machine with accuracy of ±0,001” (25,4 microns) and fine-pitch capability down to 12 mil (304,8 microns) on a repeated basis. Less expensive machines rarely meet this spec, so that’s something to be aware of. There are several types of positioning systems employed, differences in construction methods, and a variety of component feeders, all of which have an impact on quality and yield.
Component positioning systems
There are four methods for pickup and placement:
• No centring mechanism
• Mechanical (jaws)
• Laser centring
• Vision centring
Method 1: No centring mechanism other than relying on the component’s pick-up point for placement. In other words, the part is not physically centred after being picked up by the tool head, and if it’s picked off-centre on the tool, it will be off-centre when placed on the board. This is obviously not a very accurate placement method because there is no definable tolerance.
You can expect to find this method used by hobbyists or instructors, but certainly not in any type of precision production environment. There are not many options available either, and long-term reliability is questionable.
Pros: Low cost.
Cons: Low accuracy, repeatability and long-term reliability; no options or spare parts.
Size range: No definable tolerances.
Method 2: Mechanical centring jaws or fingers. In this method, the component is picked up and moved into its centre position in the X and Y axes on the pick-up head. Typically, this method is easy to set up and repeatable within ±25,4 microns accuracy. This centring method is generally found in low- to mid-range machines.
Pros: Easy to learn and set up; repeatable; one of the fastest methods currently available; a true ‘on-the-fly’ system; low cost.
Cons: Physically touches the component which may not be appropriate for certain types of parts, especially those with delicate leads.
Size range: 0201 packages up to 35 mm square.
Method 3: Laser centring. In this method, the component is picked up in-line with a laser beam which detects the component’s centre position on the tool head and recalculates the zero point of the part according to its position in the X and Y axes and rotational position relative to the head for an accurate placement on the board.
Pros: Touchless; on-the-fly (similar to mechanical method).
Cons: It is less reliable. There are limitations on the types of parts it can handle, such as very thin components (If 50 mil thin, they may need to be reset because of part variations, even from the same vendor); requires longer setup time, since the Z axis (part thickness) must be defined; more costly than mechanical centring but about the same as vision.
Size range: cannot centre parts below 0402 packages or larger than 35 mm square.
Method 4: Vision centring. Here, there are two types: look-down and look-up. Look-down vision will view the top of the component prior to picking it up for its pick-up location. It then calculate its centre, compares it to its image file from the stored database, then picks up the component and transports it to its position on the board.
Pros: True touchless centring; can handle odd-shaped and delicate components; accurate to ±101,6 microns capability.
Cons: Typically longer setup times due to the need to teach the vision system how to identify part images which are stored in the machine’s database; a slower method of centring due to time slice required for processing; vision is more costly than the mechanical method; for look-down vision, the part may move from its pickup point to its placement on the board.
Size range: 0402 – 15 mm.
The look-up vision method is the most accurate centring method available. The component is first picked up from the pickup area, moved to a camera station that looks at the bottom of the component, and calculates its centre position.
Pros: True touchless centring; handles delicate components; accurate down to ±25,4 microns positioning capability.
Cons: Typically a longer setup time due to the need to teach the vision system how to identify the image, stored in the machine’s database; a slower method of centring due to processing time; vision is more costly than the mechanical method.
Size range: 01005 – 50 mm (can see smaller and more detail).
The pick-up and centring method you choose will have a great deal of influence on the quality and speed of your production needs, along with how to relate this accuracy back to the machine.
1. Component positioning methods
After each component is picked up and centred in the tool by one of several methods, it must then be positioned accurately on the board, in an X-Y position. There are three methods commonly used for positioning:
• Positioning with no feedback system (open loop system).
• Positioning with rotary encoders (closed loop system).
• Positioning with linear encoders (closed loop system).
Method 1: No positioning feedback loop. In this system, the motor drives the part to a location on the board defined in the program by the number of steps in each X-Y axis, but there’s no way to tell if it actually ends up in the right place. These systems use stepper motors for positioning.
Pros: Low cost.
Cons: Unreliable accuracy; not recommended for high-quality production.
Method 2: Positioning with rotary encoder. In this method, an encoder is mounted directly on the motor shaft and delivers position feedback to the control system; however, it only reports the motor position, and not the actual position of the X-Y axis. This is dependent upon the remainder of the mechanical components that make up the machine. These machines can use stepper or servo motors (which also has cost implications).
Pros: Low cost; this system is widely used on entry-level machines.
Cons: Typical positioning accuracy of ±127 microns.
Method 3: Positioning with linear encoder. In this method, linear scales are mounted to the machine’s X-Y axes table and an encoder is mounted on the travelling beam that will be carrying the components. This method will report its actual position back to the control system and make corrections to the position programmed, if needed, to within a few microns of actual X and Y location for the component placement (which is typically 12 800 increments – or steps – for each inch of travel). The best machines in this category use servo motors.
Pros: Very high accuracy, to within 12,7 microns; very repeatable.
Cons: More costly, but necessary for high-value production.
NOTE: The quality of the encoder (the position feedback sensor) is an important element in the whole system and does affect accuracy.
2. Machine construction
When selecting a pick-and-place machine, you should be aware that its construction will dictate its effective CPH range and footprint, including considerations for the number of component feeders it can accommodate.
All-welded steel: The most accurate machine will have a frame that is constructed of solid welded structural steel tube. This provides significant stability necessary for accurate positioning and high-speed movement of X and Y axes. This construction method is recommended for any production environments, and it will remain stable without requiring ongoing calibration.
Bolt-together frame: Extruded aluminium or formed sheet metal frame will come with a lower initial accuracy than a welded frame and will need to run more slowly because it can’t handle the rapid inertia shifts of X –Y axis movement. Further, it will likely go out of calibration frequently, which will adversely impact labour time, downtime and yield. Lower cost usually reflects a weaker construction.
3. Solder paste/fluid dispensing
Any pick-and-place machine should be capable of offering fluid dispensing systems. Most common liquids include solder pastes, adhesives, lubricants, epoxies, fluxes, glue, sealants and more. This is a valuable option when building prototypes or one-off PCB assemblies that do not warrant the cost of a dedicated printer stencil or foil.
4. Component feeders
If the machine’s production will be dedicated to a small number of components and type of job, it’s very easy to identify the number and type of feeders. However, that is not usually the case with contract assembly shops, since they don’t know what type of board and how many different components the next job will require. Some OEMs also need flexibility for a wide range of board configurations, especially if they intend to use the same machine for prototypes and several different production boards.
It is useful in those cases to consider a machine with the greatest number of feeder position and options, that can accommodate the footprint your space can handle.
Types of feeders include:
1. Cut strip holders are usually associated with the low-volume world.
2. Matrix tray holders are used for components that are not available on tape.
3. Tube feeders dispense components supplied in tubes.
4. Electric tape (and reel) feeders.
Electric tape and reel feeders are usually more costly initially, but offer the best long-term investment. They are available as single units in a variety of sizes, and cover the range of 0201 components up to 56 mm large components. Many manufacturers now offer a multiple feeder (known as bank feeder). These are usually available for 8 mm tape, and can come with up to 12 - 8 mm feeder lanes per unit.
Since components are packaged in many forms, e.g., discrete components on tape, quad packs, matrix trays, tubes, cut strips, etc., your choice of feeders would depend on your production but also on any size restrictions you may have. A good starting point is to purchase the most feeders you can get in the footprint you have available.
Vendor support
When evaluating any type of SMT machine, consider factory support as one of the most important assets of your purchase. The best way to learn how a company treats its customers is by word of mouth. Talk to several customers to find out how happy they are with the machine, the seller, and the support they provide.
Where is the manufacturing plant? Can they help troubleshoot alignment issues over the phone? Do they offer field service? Do they have spare parts in stock for immediate shipment? While there isn’t much of a used market for manual, machine-assisted or enhanced manual pick-and-place machines, it’s still a good idea to ask your supplier about their older machines in the field, and if down the road spare parts are available, and about their capability to customise a spare part if the machine becomes obsolescent.
Ask what the expected life-cycle of the product is; the industry standard is seven years.
For more information contact Test & Rework Solutions, +27 (0)11 704 6677,
[email protected], www.testandrework.co.za
| Tel: | +27 11 704 6677 |
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| www: | www.testandrework.co.za |
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