Prototyping tools TRANSFORM design dreams into reality

WHO HAS THE PARTS?

When you have that pc board in hand, you’re well on your way to having a prototype

to debug and demonstrate. But there’s one serious obstacle you still have to overcome: You need to put the electronic components on the board.You can do this step in one of several ways. You can send it to an assembly house, but then you have to provide the necessary documentation, plus all your components on reels or in carefully bagged and

labeled packages. This preparation is time-consuming and again puts you at the mercy of an outside vendor (see sidebar “Can I go out now?”). Furthermore, checking your documentation and resolving placement issues become more difficult.

Alternatively, you can use a pick-andplace machine that targets low volumes, such as the Expert 5000 from Manncorp (www.manncorp-smt.com). This $8000 semiautomatic unit guides the operator via mapping software and a magnified screen image; the device shows where each part should go with placement accuracy of 0.6 mm (25 mils) as standard and 0.4 mm (16 mils) with an optional fine-pitch package. A motorized partssupply tray holds your components, and you can also use tape or stick feeders for components that you are using in quantity.

Don’t rule out the old-fashioned hand-load method, either.Using vacuum tweezers, a foot-operated solder-paste dispenser, and a wide-view 3 or 43magnifier with sufficient lighting, a careful operator can load a pc board one electronic assembly component at a time and can then solder the

board in a regular production setup.Yes, it’s laborious, and you don’t want to do it for more than a few boards, but it takes virtually no setup time, and your costs are just a few hours of labor and some basic equipment that you probably need anyway.

HAVE IT YOUR WAY

These board-producing machines offer options in addition to higher speed spindles for smaller dimensional work and automatic tool changing. These options include a vacuum unit that sucks up the debris that the cutter tool generates, a sound-reduction box that cuts the

typical 80-dB sound level at 3 ft by about 10 dB, and a high-power magnifier— typically 503—for visually checking your fine-pitch design.Consider this last item more of a necessity than a luxury, because you probably need to inspect the board and its design in a few critical areas.

Don’t think that cladding removal is the only way to go, either. In some applications, such as those in which you need special claddings other than copper or those that need a circuit built on a substrate that is unavailable with copper cladding, you may want to consider a

precision writing system (see sidebar “Don’t forget to write”).

MAKE A CASE FOR YOUR PRODUCT

When I first saw a stereolithographyapparatus (SLA) system in action at a trade show, my initial thought was how it functionally paralleled the handy “replicator” of the Star Trek series (in which the crew used the device to create— on the spot—replacement parts, food, or whatever it needed). The SLA I watched magically produced a detailed, 3-D part that matched solid-modeling engineering figures on a nearby screen. To add to the science-fiction aura, this solid rendition of the image I saw on the screen rose from a pool of liquid polymer and emerged ready to use (see sidebar “How’d they do that?”). As the finished

part rose from the pool, I had a distinct Terminator 2 flashback: The sight reminded me of the ultra-advanced,metallic, morphing cyborg who comes after Arnold Schwarzenegger in the movie.

But SLA is not science fiction. SLA and the broader area of rapid prototyping and solid freeform fabrication (SFF) are significantly changing the way that manufacturers are making mechanical parts for both prototyping and short production runs. A design team can plan its enclosure on a PC or workstation using solid-modeling software tools supplemented by special SFF application software; can see whether and electronic assembly how the board, connectors, power source, antenna, and other pieces fit; and can then have an exact prototype of the enclosure or housing in hand—complete with mounting ears, tabs, and openings for a display and keyboard.

Follow Heuristic Guidelines To Make Surface-Mount PC-Board Footprints

Guideline 5: Use a 1:1 ratio for solderstencil openings for all pins/terminal pads, except any exposed heatsink pads or other large apertures. For large apertures (3 by 3 mm and greater), shrink the periphery of the aperture by 0.25 mm to allow sufficient clearance between the large pad and surrounding pin/terminal pads, and break the large pad's aperture into multiple "window panes."

For the terminal pads, the task is automatically accomplished if the PCB-layout software is reset from the "cream" option to "on" for the terminal pads that were created. This creates a 1:1 aperture for each terminal pad's stencil layer—a value that works very well for both fine-pitch and standard-pitch leads when using a 6-mil thick stencil.

For the exposed pad polygon, however, you will need to create the pattern manually. Start by turning on both the top-copper layer (so you can see the polygon) and the stencil layer, where you will draw the corresponding aperture. Next, choose the rectangle function in your PCB-layout software, and create a set of window-pane apertures within the area of the polygon. (Make sure that you're placing these rectangles on electronic assembly the stencil layer.) Figure 5 shows the finished stencil openings for the package overlaid on the top-copper pattern.

Note how the exposed pad stencil openings are pulled away (inward) from the perimeter of the polygon, and especially from the corners. This reduces the likelihood of excess solder and resulting solder shorts.

Top Silkscreen: The top silkscreen carries the human-interface information you wish to appear on top of the PCB. This silkscreen doesn't serve any circuit function, but it is important to prototyping. It will help orient and place components during board assembly and assist in testing and debugging the prototype.

Conventionally, the silkscreen consists of the component-name layer, place layer, and value layer. The component-name layer supplies the IDs for the part on the board—for example, U1, R1, C5, and Q2. The value layer would include any associated values for the component— for example, 1k or 0.01 F. And, the place layer contains an outline and orientation for the component. We'll consider the top-place layer as the minimum requirement for the device.

Guideline 6: Use minimum 7-mil (0.007-in.) line widths for silkscreen features. Change the font type to "vector" and adjust the ratio and size as required to create readable letters and numbers.

Don't be tempted to place numbers by every pin, but do label pin 1 on SOICs. On QFN packages, it's helpful to have the silkscreen note the corner "pin" numbers. Make the top-stop layer visible while drawing on the topplace layer. Be careful not to place the silkscreen numbers/letters/lines too close to the solder-mask-stop apertures. Otherwise, the ink will bleed over into the associated depressions in the solder mask on the finished PCB.

Adjust the font's ratio and size to achieve the most readable text while maintaining the minimum line width. It's also helpful to place an outline around the part perimeter. Figure 6 shows the minimum silkscreen content for the example device, with the soldermask-stop layer shown for reference.

By closely following these steps, you can create any PCB layout package footprint you need. The result is a PCB footprint that, while not optimized for production, is robust for prototype PCB applications. In particular, the achievable goal is to substitute schematic-capture software for "napkin sketches" and quick-turn PCBs with electronic assembly solder stencils in place of hand-wired breadboards, while maintaining a rapid concept-to-labwork cycle that supports the engineer's creative process.

Learn the trade and the tricks will follow

I’ll never forget the 11th grade English class when Mr. Ervin departed from his usual lessons in literature, and lectured us on this topic for the full hour. “Learn the trade and the tricks will follow.” As he repeated this simple advice, he Needle Roller Bearing bemoaned the fact that everyone was looking for the shortcuts, looking for the quick and easy, trying to bypass learning any more than was necessary.

Where did he learn this advice? As a carpenter’s apprentice, he had noticed that the master carpenter always had some little trick or technique to apply in difficult situations to make them easier. Carpentry was hard work, and he though if he could learn all of these techniques, it would be much easier to learn. So he asked the master Pressed Bearing carpenter to teach him the tricks of the trade. “Learn the trade and the tricks will follow,” was the reply.

Did this advice stop me from trying to find the shortcuts? Of course not. But, like Mr. Ervin described, I found that those shortcuts didn’t seem to work so well for me. But when I learned the fundamentals of the business (whatever business Special Bearing I tried), then the tricks of the trade became more useful. The framework provided by learning the basics gave me insight into how, and when, to apply more advanced or less obvious techniques. “Learn the trade and the tricks will follow.”

The decades that have passed have proven Mr. Ervin’s advice to be correct. And so I want to pass it on to you. I want to pass it on to those who look for tools or code to copy that will make software development something you can do without thinking. Learn the trade, and the thinking becomes easy. I want to pass it on to those who look for the framework to allow inexpensive non-programmers to develop business applications. Hire Thrust Bearing people who know the trade, and developing the applications will follow. Learn the trade of hiring and retaining high-quality developers, and the ability to develop the systems you want, when you need them, will follow.

“Learn the trade and the tricks will follow.”

Advantages and functioning of ceramic ball bearings

Centuries ago, the concept of ball bearings was conceived and that concept has evolved over many centuries. Today, smooth ceramic ball bearings have become quite popular and the introduction of using ceramics has greatly benefited the manufacturing process.

Ancient man used round rollers to relocate the heaviest of items. Present day ball bearings most likely evolved through the use of logs. These rollers were placed beneath anything heavy and the item would be dragged along as the logs rolled forward underneath it. There are even images of ancient Egyptians using this type of system to drag the huge stone blocks used in the creation of the great pyramids. Eventually this concept evolved to the point that the rollers were attached to the item being moved and the first vehicle with wheels was born.

As it turned out, the world would have to wait awhile until the concept was further developed. It was not until very late in the eighteenth century when the first bearings were actually designed. Philip Vaughan was an iron-master in Wales and in 1794 he submitted a patent for a carriage axle. That original design was improved upon through Thrust Bearing the next century and into the early 1900s. The development process was ultimately accelerated by the emergence of bicycles and automobiles.

The most recent technological advances in this field involve the use of ceramics in the ball bearing manufacturing process. Ceramics has revolutionized the industry with its durability and provides additional advantages when used as the base material.

Ceramic ball bearings are particularly useful when used in high speed systems. Ceramics is the preferred material because the high rigidity and low density are able to dampen friction, heat and vibration when it use. Low friction means the whole system runs smoother.

The same properties that reduce friction also give ceramics an electrical insulation component that is quite useful when used in electric motors, various hand tools and fluid machinery. The ability to perform despite less than ideal operating conditions also makes ceramics a popular material to work with.

One of the biggest benefits of ceramic ball bearings is the durability. Ceramics far outlasts other materials and can be effective up to ten times longer than a bearing made out of steel. Durability is the key to keeping machinery running at optimum capacity. Anything that can be done to reduce maintenance costs and down-time for equipment will allow you to realize a huge cost savings in the future.Centuries ago, the concept Special Bearing of ball bearings was conceived and that concept has evolved over many centuries. Today, smooth ceramic ball bearings have become quite popular and the introduction of using ceramics has greatly benefited the manufacturing process.

Ancient man used round rollers to relocate the heaviest of items. Present day ball bearings most likely evolved through the use of logs. These rollers were placed beneath anything heavy and the item would be dragged along as the logs rolled forward underneath it. There are even images of ancient Egyptians using this type of system to drag the huge stone blocks used in the creation of the great pyramids. Eventually this concept evolved to the point that the rollers were attached to the item being moved and the first vehicle with wheels was born.

As it turned out, the world would have to wait awhile until the concept was further developed. It was not until very late in the eighteenth century when the first bearings were actually designed. Philip Vaughan was an iron-master in Wales and in 1794 he submitted a patent for a carriage axle. That original design was improved upon through the next century and into the early 1900s. The development process was ultimately accelerated by the emergence of bicycles and automobiles.

The most recent technological advances in this field involve the use of ceramics in the ball bearing manufacturing process. Ceramics has Pressed Bearing revolutionized the industry with its durability and provides additional advantages when used as the base material.

Ceramic ball bearings are particularly useful when used in high speed systems. Ceramics is the preferred material because the high rigidity and low density are able to dampen friction, heat and vibration when it use. Low friction means the whole system runs smoother.

The same properties that reduce friction also give ceramics an electrical insulation component that is quite useful when used in electric motors, various hand tools and fluid machinery. The ability to perform despite less than ideal operating conditions also makes ceramics a popular material to work with.

One of the biggest benefits of ceramic ball bearings is the durability. Ceramics far outlasts other materials and can be effective up to ten times longer than a bearing made out of steel. Durability is the key to keeping machinery Needle Roller Bearing running at optimum capacity. Anything that can be done to reduce maintenance costs and down-time for equipment will allow you to realize a huge cost savings in the future.

Sensor Mounting Techniques

Introduction

While upgrading one of my Mini-Sumo robots, I found that sensor placement and orientation can have a big impact on your final results. This article will discuss the problems I ran into, trouble-shooting, and solutions.

Time to Upgrade

Mini-Spat has competed in 6 PAReX events, as well as several demos at swap meets, high schools, and colleges. In other words, there's a lot of mileage on the little guy. During the Nov. 2003 event it died twice in competition, and in the Nov. 2004 event, there were a few times when it lost it's mind and wandered the ring aimlessly. The original circuit board was a low-profile wire-wrap board. With the extremely short wraps I was concerned about it's reliability and decided to upgrade to a PCB.

Upgrade Woes

The PCB was finished and mounted to the robot. There were no software changes made, except swapping the left and right servos, due to electronic assembly routing issues. Fired up the robot and watched it act just like it did with the old board, in other words, a robot possessed. Since the PCB seemed to be correct, and nothing else had changed, I started looking at what I had done in software, trying to figure out what went wrong.

Basic Trouble-Shooting

The basic problem was the robot was seeing things that weren't there. When Mini-Spat drops, it's suppose to move forward, then start spinning in a circle looking for the opponent. If there's nothing in range, it should just keep spinning. Most of the time it wouldn't even make one full turn before wandering off in search of a ghost. Other times it would work fine for several runs.

By inserting some breakpoints in the code, I would stop the robot when it thought it saw something, and flash the LED to tell me which sensor was acting up. Turns out the right range sensor, a Sharp GPD012, was at fault. Further investigation showed that two of the three connector pins had broken solder joints! Ahh problem solved!! A quick fix with a soldering iron and it was back on the ring, failing exactly the same way. Insert your favorite expletive here!

Digging Deeper

Well at least I found the intermittent problem. Most Mini-Sumo's don't have a display on them so it can be hard trying to figure out what's going on sometimes. I have a diag routine which can blink an LED to show me an 8-bit sensor value, but you can't use it while the robot is running. I guess the ideal interface would be a wireless link to a PC, then you could receive all kinds of data to see what's going on. Well those links aren't cheap so I fell back on the RS-232 link that I had brought over from the original board. A long 2-wire cable connects the robot to the laptop, but it's still not practical, even on the 30" ring. Since the failure was a range sensor, I did all kinds of tests without the motors running, and the sensor performed perfectly.

Ok, since this is an analog sensor, maybe the motors are generating some electrical noise and hosing things up. Putting the robot on blocks and running the motors during the tests, made no difference, the sensors worked as expected. The only thing left was to monitor the sensors while the robot was running normally. To do this without the cable attached, I would save the sensor readings to RAM, and then connect the cable and dump the data to the laptop.

I had about 250 ram locations to use and as fast as the sensors are updated, it would fill in about 38ms. So I once again set the robot to stop when it saw something and send me the last 250 readings. What I saw was quite interesting, the left sensor read a 0 or 1 the entire log. The right sensor varied from 0x20 to 0x28, before hitting my 0x2C threshold and stopping. The 0x2C is about the length of the ring, so I would "see" things in the ring, but didn't care about what laid beyond. If I did the same test with the motors  electronic assembly off I got readings just like the left sensor, nothing greater then 2. If I ran the motors on blocks, again the readings were good. I changed out the sensor and again had no changes. So the actual motion of the robot had something to do with it.

Requirement of cleaning

 

Cleaning of printed boards is required to remove flux residues and other contaminants which are left behind after soldering . Cleaning or washing of the boards prevent potential electrical failures due to electromigration. Cleaning operations does the removal of the following contaminants:

I.) Ionic contaminants

ii) Non-ionic contaminants

iii) Particulate contaminants

Fluxes that are water soluble generally produce ionic(also called polar) contaminants that require aqueous cleaning. Non-ionic(also called non-polar) electronic assembly contaminants produced by rosin fluxes require non-ionic solvents such as trichloroethane.

2. Cleaning options

Note: Rosin and Rosin Mildly Activated (RMA) fluxes need not be cleaned. However, for high reliability applications and for aesthetic reasons, it may be necessary to clean these boards.

Water soluble fluxes need to be cleaned thoroughly due to corrosive elements that are present in the flux residues, aqueous cleaning is ideal. Because rosin is not soluble in water, when aqueous cleaning is used for rosin fluxes, alkaline chemicals called saponifiers are added to the water. The efficiency of cleaning treatment is significantly increased if the cleaning is assisted by ultrasonic vibration. However, the ultrasonic vibrations are not confined to the cleaning fluid and the surfaces to be cleaned, but are also transferred into the electronic components, which may be damaged . Bonding wires, inside active components between the die and the bonding pads may break if the bonding wires are free and have not been tightly encapsulated in plastic or any other packaging material. High-power, high frequency vibration induce fracture in external leads also. The following set of parameters is generally accepted:

§ maximum frequency of 40 kHz

§ maximum time of ultrasonic load 1 to 5 minutes

§ maximum power of 10 W/ litre

§ boards in racks, so that they cannot touch each other.

In all cases, the time interval between  electronic assembly soldering and cleaning be reduced to a minimum (less than an hour) to obtain good cleaning results.

Prototyping techniques help verify analog-circuit performance

The result is a board similar to the final manufactured double-sided pc board, except that the final board lacks plated-through-hole capability, and you must wire and solder on both sides any vias between the two layers of the board. Minimum trace widths of 25 mils (1 mil=0.001 in.) and 12-mil spacing between traces are standard, although you can achieve smaller trace widths. The size of the milling bit, which is typically 10 to 12 mils, dictates the minimum spacing between lines.

An example of such a prototype board is a daughterboard that interfaces an AD9562

dual PWM board in a 44-pin PLCC package to a test-set motherboard (Figure 4). The

leads are on 50-mil centers, and the traces are approximately 25 mils wide. This board

illustrates the resolution of the milling machine, but you can use the technique to

produce more complex boards.

IC sockets can degrade the performance of high-speed or high-precision analog ICs.

Although low-profile sockets ease prototyping, even these sockets often introduce

enough parasitic capacitance and inductance to degrade circuit performance. If you must use sockets in high-speed circuits, an IC socket comprising individual pin sockets, sometimes called "cage jacks," mounted in the ground plane board may be acceptable (Figure 5). Clear the copper on both sides of the board by about 0.5 mm around each ungrounded pin socket and then solder the grounded pin sockets to ground on both sides of the board. Both capped and uncapped versions of these pin sockets are available from electronic assembly Amp (Harrisburg, PA) (part no. 5-330808-3 and 5-330808-6, respectively). The pin sockets protrude through the board far enough to allow point-to-point wiring interconnections between them.

The spring-loaded, gold-plated contacts within the pin socket make good electrical and mechanical connection to the IC pins. Multiple insertions, however, may degrade the performance of the pin socket. The uncapped versions allow the IC pins to extend from the bottom of the socket. After the prototype is functional and requires no further changes, you can solder the IC pins directly to the bottom of the socket, thereby making a permanent and rugged connection.

Special prototyping considerations

These prototyping techniques apply only to single- or double-sided pc boards. Multilayer pc boards do not easily lend themselves to standard prototyping techniques. If a design requires multilayer-board prototyping, you can use one side of a double-sided board for ground and the other side for power and signals. You can use point-to-point wiring for additional runs, which would normally exist on the additional layers a multilayer board provides. Unfortunately, it's difficult to control the impedance of the

point-to-point wiring runs. The high-frequency performance of a circuit prototyped in this manner may differ significantly from that of the final multilayer board. Other difficulties in prototyping may occur with op amps and other linear devices that have bandwidths greater than a few hundred megahertz. Variations of greater than 1 pF in parasitic capacitance between the prototype and the final board can cause subtle differences in bandwidth and settling time.

If you use DIP packages for the prototype but SOICs for the production packages, you can see differences between the performance of the prototype and the final pc board. For instance, the AD8001 current-feedback op amp (approximately 800-MHz bandwidth for G=1) is available as both an eight-pin DIP and an eight-pin SOIC.Table 1, which contains data collected with the use of an evaluation board, reflects the difference in performance between the two packages after you optimize the feedback (RG) and feed-forward (RF) resistors (Figure 6a). The resistor values in Table 1 produce the highest 0.1-dB flatness bandwidths. The SOIC package's bandwidth is higher because of lower package parasitics. All resistors and capacitors on the board are surface-mount types for low parasitics.

Evaluation boards can be extremely useful in evaluating new analog ICs; these boards let you verify the IC's performance with minimum effort and without constructing your own prototype. Evaluation boards can range from relatively simple ones, with just op amps, for example, to rather complex ones for mixed-signal ICs, such as ADCs. ADC evaluation boards often have onboard memory and DSP μPs for 6 von 8 9/25/00 4:09 PM EDN -- 02.15.96 Prototyping techniques help verify analog-circuit performance http://www.ednmag.com/reg/1996/021596/04df3.htm analyzing the ADC's performance. IC manufacturers often provide software so that these more complex boards can interface with a PC to perform complex signal analysis, such as histogram and FFT testing.

Most manufacturers of analog ICs provide evaluation boards, usually at a nominal cost. Regardless of the product, the manufacturer takes proper precautions regarding grounding, layout, and decoupling to ensure optimum device performance. The layout of the components on the evaluation board can guide both the prototype electronic assembly and the final pc-board layout. The artwork or CAD file is usually free, should you wish to copy the layout directly or change it to suit the application.