PCB Assembly Pick and Place Example

Small Fragile Electronic Component Assembly - Challenges and Solutions

The smallest SMD components are shrinking in size, currently 0.2 x 0.4 mm (type 01005). The solder area’s on these components is 0.07 x 0.2 mm. The interspacing between components needs to be as small as possible in order to maximize the component density. A distance between components of 0.05 mm has been realized. This interspacing requires a placement accuracy of 10 micron (sigma value). This includes the deviations due to rotary shaft run out. In order to reduce the contribution of the run out, the repeatable deviations can be calibrated reducing the contribution of the rotary run out to 2 micron. The remaining 8 micron error budget is available for the X-Y-Z robotic system.

The optimal force for placing an 01005 is about 0.3 N. This force takes care the solder paste between the component and the solder pad is not squeezed out creating bridges with neighboring components.

The challenge is to develop an actuator with the capability of moving the part quickly to the surface with an impact that will meet the 0.3N requirement.

Since the formula 2MV/T is relevant and since V must be maintained or increased in order to meet output requirements M is obviously important. Programmable precise force control is also very important now. The ability to precisely rotate parts is also critical. Actuator width must narrow due to the smaller pitch of feeders associated with the smaller electronic components. And the trend towards multiple actuator manifolds to increase output means overall mass of the actuator is important.

A new actuator specifically designed to meet these challenges has been released by SMAC Corporation. This is the LCR 13.

The LCR 13 has independently controllable linear and rotary motors. The linear is based on SMAC's patented Moving Coil design. The design has a number of advantages over ball screw and moving magnet technology:

• Direct drive so no backlash
• Low moving mass (<90 grams) means higher accelerations and lower impact than moving magnet designs
• Energy efficient.  Very low operating currents - normally in the 1.5 amp peak range.
• Soft-Land™ function. The light moving mass and SMAC's patented surface detection capability means that the actuator tip can softly land onto a surface and report back its position.
• Long cycle life proven beyond 300M cycle.s
• Micron repeatability in linear shaft movement.
• Thin profile - width of 13mm.

The rotary servo motor is built into the linear motor piston. Its shaft acts as the shaft of the entire system thereby eliminating the need for a coupling. Key features are:

• Direct drive eliminates backlash, increases acceleration, and results in fast and precise moves even of partial degrees.
• 96 mnm torque
• SMAC's very compact rotary encoder mounted directly on the rear shaft of the motor - eliminating the tether.
• Shaft runout of less than 5 micron.
• Rotary encoder resolution of 24K and higher.
• Built in through hole in the shaft allows dust picked up by vacuum to pass completely outside the unit thereby increasing seal life.

For placing BGA components (Ball Grid Array, square shape up to 40 mm, pitch 1 mm) the rotary accuracy is crucial. The SMAC actuator has a rotary resolution of 0.0088 degree. This allows accurate placement of all balls of the BGA. The optimal placement force varies per type (and number of balls) with several Newtons. A too high force leads to bridging (solder paste touching between pads). Too low forces lead to poor/weak connections.

An interesting and very useful version of the Moving Coil Actuator is the Linear Rotary Actuator.

This product has a compact direct drive servo motor integrated into the linear motor piston. 

When equipped with SMAC's very compact rotary encoders (resolutions up to 50,000 counts) a very useful pick and place device is the result. 

SMAC's LAR 31 was first developed as the Pick and Place actuator for one of the world's leading Circuit Board Assembly Machine manufacturers. Today many customers around the world use variations of this product. With over 40,000 in the field and a mean time between failure of over 250,000,000 million cycles the LAR 31 has been the most technologically advanced pick and place actuator in the industry. 

The LAR 31 has a number of unique technical features that are advantageous in Electronic Assembly. 

SMAC pioneered the Moving Coil Linear Servo Motor based actuator. These design has significant advantages over older technology ball screw actuators and Moving Magnet Linear Motors. 

• No backlash like that found in ball screw actuators
• Direct drive results in high speeds and precise positioning
• Much lower moving mass than moving magnet linear motors since only a light coil is moving - not a heavy magnet. 
• Much high accelerations - F= MA - M is lower so A is higher
• Energy efficient. Much lower operating current - SMAC Moving Coil Actuators operate typically with 1.5 amp peaks vs 5 amps + used in moving magnets. 
• Soft-Land™ function. The light moving mass and SMAC's patented surface detection capability means that the actuator tip can softly land onto a surface and report back its position. 
• Long cycle life - proven over  1 Billion Cycles 
• Micron repeatable linear shaft repeatability

Integrated into the Moving Coil Piston is a small proprietary brushless dc motor expressly developed for the actuator. This is the independent "rotary" axis found in the LAR actuator.  It features:

• The built in brushless motor shaft also serves as the shaft of the compete device. This eliminates the coupling. The run-out of this shaft is less than 5 microns, making it extremely accurate.
• The shaft has a through hole that serves as the vacuum passage. This runs completely through the motor and via tubing out of the actuator. This ensures that any dust picked up from the work surface by vacuum exits the device to prevent build up. 
• The SMAC manufactured very compact rotary encoder is mounted directly onto the motor. Thus there are no bearings to wear out and no tether. The rotary encoder disc is bonded directly to the motor shaft and has less than a 5 micron "wobble".
• The moving cables have been life tested to well over 1 billion cycles.
• The rotary "direct drive" solutions is very fast and precise. Position moves in small fractions of a degree are easily realized. 

SMAC has recently released a new Linear Rotary actuator: the LAR 13. This device has the same linear and rotary performance found in the LAR 31 with significant additional advantages:

• Moving mass less than 50% of the LAR 31 - important both for higher acceleration and lower impact force.
• More compact - 13 mm width vs 35mm - allows stacking to meet 15 mm feeder pitch. 
• Much lighter - 95 grams vs 320 grams.      

The LCR 13 has the same low shaft run-out of 5 microns or less found in the LAR 31. It also incorporates a vacuum thru the motor shaft capability like the LAR 31.

 Electronic chip design trends continue to emphasize smaller and more fragile package designs. The LCR 13 (and its linear only version the LCA 13) gives the electronic test and assembly machine manufacturers a new and very useful tool to help them meet this challenge.  

Simple impact contact force can be calculated as 2MV/T. Since manufacturers wish to keep machine assembly speeds up while dropping impact forces down - (Current target is 1.0N and future requirements call out for 0.3N impact) - the reduction of mass is quite important. 

The LCR 13 incorporates a small (13mm) and powerful 0.096mNm direct drive DE brushless motor. The motor is equipped with SMAC's newest and most compact rotary encoder with a diameter of 13mm and a resolution of 24k/rev.  This mass plus that of the small coils and linear guide carriage result in a mass well below 100 grams. The result is an impact that meets the 1.0N target while moving at a relatively fast contact velocity.

Future Development

The continual push for smaller components means Electronic Assembly is a moving target. Actuators must continually shrink and size and improve in performance. Further development requires:

• Continual reduction is moving mass - with the goal of under 50 grams a target.
• Thinner devices with 8mm pitch a goal.
• Technology that reduces the time required to soft land.
• Continual effort to reduce cost and hence price to the customer.