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Prototyping: think small and work smart

by: Dec 25,2013 753 Views 0 Comments Posted in Engineering Technical

PCB assembly printed circuit board prototype a PCB

label: prototype a PCB,printed circuit board,PCB assembly

No matter whether you are soldering, testing, or just trying to get a part out of a reel, you need to be able to see the tiny part. First, make sure that you have an abundance of light in your work area. Electronic-ballast lighting provides good general illumination at a reasonable electricity cost. You might also want a halogen spot lamp or compact fluorescent bulbs in a reflector fixture for illuminating your work area. Articulated lamps that mount on your bench are equally useful, and several flashlights of different sizes also come in handy. You often must look into an enclosure as you troubleshoot, and a small flashlight lets you clearly see the interior. Flashlights that mount to eyeglass frames are also available, as are the more traditional doctor's light that you wear on your head. These hands-free lamps can be especially useful if you need to crawl under some equipment to troubleshoot your prototype.

Once you have enough light on your subject, you need to magnify it. Although magnifiers are great for checking details and reading part numbers, you need a microscope to solder parts onto a board. Olympus, Nikon, and Leica offer good working models. Microscopes from Wild are of the highest quality but are available only used and from old stock. For those on a budget, Motic offers microscopes that have the look and feel of a Wild microscope at a fraction of the price. For even tighter budgets, you can find any of these stereomicroscopes, including older models from Bausch & Lomb and American Optical, on eBay. You should ensure that the microscope has optics that provide a long visual reach, so the objective lens can be 5 in. or more from the work. This feature allows you to work with soldering irons, scope probes, and X-Acto knives while looking at the board.

Eyepieces should offer a maximum of 10× magnification, and a zoom of 4× is more than adequate. A zoom of 20× offers too much magnification and will require you to constantly move the board to see what you are soldering. You can fit the Motic microscopes with an optional 0.5× objective to reduce the zoom level by 50%. Lower-powered inspection, or assembly, microscopes allow a good working distance between the objective and the work piece. You can illuminate the area under the microscope with a gooseneck fiber illuminator or a ring lamp around the objective lens. If you find it difficult to work with your face touching an eyepiece, you might consider Vision Engineering's patented Dynascope or Mantis microscope. By rotating a lenticular mirror inside the viewing head, the Dynascope provides a stereo image through a screen 10 in. away from your face. This technology does not come cheap, however; even on eBay, newer models sell for approximately $2500. Vision Engineering sells the smaller Mantis Compact model for $1625 on its Web site.

Once you have enough light and a microscope to see your prototypes, you still need tools to handle the small boards and parts. A thorough examination of the prototyping section of Digi-Key, Newark, Mouser, Jameco, or Allied catalogs should give you an idea of the selection of tweezers, tiny screwdrivers, and other tools you will need. Spend as much as you can reasonably afford: a $25 pair of Swiss tweezers is preferable to a $5 pair that will bend, warp, and not close properly. You can use air-suction tools to handle prototypes, but a vise to hold the board or assembly is more important. Panavise, a leader in this area, offers a variety of vises, including models that can hold a PCB (printed-circuit board) by the edges. One problem with conventional Panavise vises is that applying a soldering iron to the board can melt the vise's polypropylene jaws. To address this problem, Panavise offers Teflon jaws for its vises. Some engineers are blasé about antistatic procedures, but this attitude may be a result of where they live and work. Those in humid climates may work for years without an ESD (electrostatic-discharge)-induced failure, whereas someone in Michigan on a dry winter day could destroy every circuit he touches.

You also have to store your prototype parts in an organized and orderly fashion. Think of your bench as a small manufacturing operation for the board. Use the BOM (bill-of-materials) list that comes with your CAD (computer-aided-design) package to print a series of labels. You can then apply these labels to the bins of a parts cabinet. This organized inventory eases board assembly, and you can give the cabinet to a contract manufacturer for a handmade production run of boards. It is also useful for scenarios in which you accidentally damage a part: You know exactly where to get the spare. If you are away from your office, you can explain to a technician or another co-worker exactly were the parts are for your board.

Soldering your prototype is your next problem. You might want to consider soldering irons from Metcal. The company's irons include a base unit that supplies RF energy to the tip, which absorbs the RF and turns it into heat. When the compound covering the tip reaches its curie point—the temperature above which it loses its characteristic ferromagnetic ability—it stops absorbing RF energy, controlling the soldering iron tip's temperature. The infinite number of temperature-control loops provide better thermal-energy transfer down to the joint. This thermal energy travels to the pad and lead through the tip's shank and length.

"Look for soldering systems with shanks and tips that are short and fat," says Joe Curcio, field-application engineer at National Semiconductor, who used to work in technical marketing at Metcal. "This [type] provides the best thermal pathway to deliver heat to the joint." This principle is always valid, whether in tools for which RF supplies the heat, as in a Metcal soldering tool, or in conventional resistance-heated tips. In 2005, Curcio found that the Weller Silver series gave the best performance. Wayne Yamaguchi, owner of Yamaguchi Consulting, prefers a tool from JBC. Techni-Tool and Howard Electronic Instruments also distribute JBC irons. This type of tool is expensive, though: A complete kit, including a control unit, a 50W handpiece, microminiature tweezers for desoldering, and stands and cartridges, costs $1200 on Howard Electronic's Web site, for example.

Another valuable weapon in your arsenal of soldering technology is a hot-air iron, such as those from Hakko. This type of iron is similar to a heat gun, but it allows you to carefully control the temperature and flow; scores of replaceable metal nozzles ensure that the heat goes where you want it. For solder-bump parts, such as SMD (surface-mount-device) packages and other CSPs (chip-scale packages), a hot-air iron may be the only way, short of a full reflow oven, to solder the part.

Small circuits requiring tight clearances in modern prototypes also require small heat-shrink tubing. For this requirement, the 250W Weller 6966C heat gun is more useful than the large guns, which are better suited to stripping the paint off battleships.

Yamaguchi points out you don't need a $20,000 tool to do an oven-reflow process. He uses a $20 commercial toaster oven to reflow his prototypes. The trick is in characterizing the process for times, temperatures, and preheats, just as a board house does.

Another challenge in soldering parts arises when you encounter die-attached paddles—the large pads in the center where heat exits the part. You typically solder this pad to a part that thermally connects to a heat sink, so it is difficult to solder. You could use the brute-force method: Put some liquid flux on the part, use a big soldering iron, and just pour heat into the plastic area of the part until the die-attached paddle reflows. The lower thermal conductivity of the plastic means that you will sometimes need a second iron on the bottom of the board to heat the vias that carry the heat away from the paddle. A preferred method is to use a hot-air iron or even a heat gun on the back as well as hot air on the part itself. Ultimately, you may have to use a hot-air-rework station, in which a small vacuum pump applies suction though a hole in the center of the soldering-iron tip.

Unsoldering prototypes is even more challenging than soldering. Yamaguchi reports that he has used his toaster oven to heat an entire board to the point at which all the parts fell off. For a more selective approach, engineers have long used hand-operated "solder suckers" to draw solder from through holes. When you perform this job correctly, you'll draw virtually all the solder from the hole. You then use the soldering iron to pry the lead from the side of the hole; it will pop off with a distinctive click. Once you've loosened all the leads in this way, you can pull the part off the board. Use a Teflon nozzle on your solder sucker so that the soldering iron does not melt the nozzle into a shapeless blob. You might want to invest in a rework station, in which a small vacuum pump applies suction through a hole in the center of the soldering-iron tip, for situations requiring a lot of unsoldering. As with a manual sucker, once the rework station has drawn out the solder, you use the side of the tip to pop the lead off the side of the hole.

Another valuable technique is solder wick, a copper mesh embedded with flux. You place the wick on the solder joint and heat it with any soldering iron. The wick then draws up the solder with capillary action. As with a solder sucker, you must pop the lead off the inside barrel of the hole with a momentary re-application of heat. The use of flux is also critical in rework situations. The flux not only cleans the joint but also provides the primary method of transferring heat to the solder. If you fail to remove all the solder, you may need to resolder the joint or fill it with liquid flux. This approach allows the joint to fully melt and likely remove all the solder on your second try. You can obtain both liquid flux and flux encased in felt-tipped pens from distributors' catalogs. Although the Internet offers a wealth of information, diligent engineers may want to flip though the paper catalog just to see what parts and tools others are using.

You can unsolder surface-mounts part by either applying heat sufficient to elevate the entire part and its leads to the melting point or using two irons—one on each pad of resistors, capacitors, and diodes. Metcal's Talon soldering tweezers provide a more convenient approach, however. The device features two heated tips, so you can use it to simultaneously heat both solder pads. Some engineers use the tweezers not only to grip but also to place components on the board. Using the Talon for placement requires a deft touch, however; you must be careful not to draw the part off the pads or "tombstone" the component—that is, make it stand upright on one pad.

Reworking a BGA (ball-grid-array)-type package is more challenging. Getting the BGA off the board is straightforward enough; you can use a hot-air iron or even a heat gun to remove the BGA from the board. Resoldering a new part onto the board is difficult, however. Spreading a uniform layer of solder paste requires using a stainless-steel stencil with holes that match the locations of the BGA pads. You use a squeegee to drag the paste over the stencil, applying a precise and repeatable amount of solder paste. The next problem is alignment. When you lay out the board, make "witness marks" in the copper to show you where the edges of the BGA package are. This approach might enable you to align the part as you drop it onto the board. There is also the problem of resoldering. You need a uniform method to reheat the board with a hot-air gun, a toaster oven, or some other tool.

All these problems have given rise to hot-air-rework stations. These stations apply hot air from both the top and the bottom so that ground planes and heat-sink copper do not draw off the heat and prevent a good solder joint. The topside heat discharge is configurable with specially made nozzles that fit the BGA. This approach prevents the solder joints of components other than the BGA from melting.

The shield needs some clearance around the BGA, and assembly houses expect you to accommodate for that clearance when you design your board. They, too, need to be able to get that nozzle around the BGA when they rework your board. Although the heat control and profiling of these hot-air-rework stations are important, the most critical feature is a dual-image-camera system. This camera slides out between the BGA part and the board before you place the BGA on the board. The camera supplies one image of the bottom of the BGA and another of the top of your board. You can then manipulate the board in the X and Y directions and rotate the part on the air-suction nozzle to perfectly align the BGA to the board. You then push the camera out of the way and start a computer-controlled motion that brings the BGA to a point just above the board. At that point, you turn off the vacuum, and the BGA drops a few mils (thousandths of an inch) onto the solder paste on your board. Because you control the heat and position of the entire process, you can achieve solder-joint success on par with a pick-and-place machine and reflow oven.

PCB assembly: home-brew or send out?

No matter whether you carved a board out of copper clad, milled it with an LPKF machine, or sent it to a fab house for manufacture, you still have to assemble it. Whether you wrote your parts list on a cocktail napkin or you have a computer-generated BOM (bill of materials) from your schematic tool, you can now get parts in a day from all the major distributors. Be aware that if you shop for parts at a salvage yard, you may get gray-market parts that someone stole from a dumpster and that have failed final testing. You may also need to make many more prototypes or enter production, and you don't want to risk being unable to find a sufficient number of salvage parts. It is better to stick with sources that sell new parts and report real-time stock.

You should always attempt to assemble the boards you have designed if only to better understand the problem areas and difficulties the design may have created for the contract manufacturers. Contract manufacturers report that designers' choices during the design phase determine as much as 80% of the cost of a product, and no amount of second-sourcing or negotiating with vendors can appreciably lower the cost of your product. Even if your job is small, don't hesitate to call Flextronics, Aeroflex, or one of the other giant contract manufacturers (Reference A). They can steer you to a local contract-assembly house for your prototypes. Sierra Proto Express will take over the entire design of your project. The company can enter the schematic and lay out, fabricate, and assemble the board—all in a couple of weeks.

You can use the Internet to find a small contract manufacturer in your area. A local source is always convenient for emergencies when you need someone to rework or assemble a board overnight. Assembly houses with good track records include Rapid SMT Assembly and Naprotek. Some small contract manufacturers use their rework staff to hand-assemble your boards. Others insist on at least a yard of every tape-and-reel part so that they can run your board through their pick-and-place machine—not a bad idea; it verifies the validity of your insert file that has the X-Y location of all the components in it. Just as you can do remote PCB fabrication and get the boards in a day, so, too, can you get board assembly done quickly. One company is Advanced Assembly (Reference B). Another company that will handle all phases of your build is VSE. Assembly companies are realizing what circuit-board companies figured out a decade ago. They can make good money helping engineers deliver a form, fit, and function prototype in a few days. It also gets them involved on the ground floor so they have a shot at high-volume manufacturing, as well. This approach is good for everyone.

Many engineers do not have the budget for a contract manufacturer. Study the first sidebar to this article and become familiar with handling and soldering tiny components. Often there will be a technician or production assembler who you can put to work on assembling your prototype. But realize he might not be so willing to work through the night to get your prototype assembled. Just as it is good to understand what a board house does with your files, you should watch or assemble at least one of your boards yourself. Doing so will give you an idea of problems in the manufacturability of your design, which often involves difficulty in rework or access to components. Understanding these issues will allow you to lay out the next board with better results.

If you do have to hand-assemble a prototype board, you may need a solder stencil. This stainless-steel sheet has holes the same shape as the solder pads on your board. You smear solder paste on one side of the stencil, lay it on top of your board, and use a squeegee to distribute a uniform paste of solder on every pad. You can then stick your components into the solder and either reflow-solder the board in an oven or hand-solder the parts, or use a hot-air iron for BGAs and other parts that have no exposed leads. Advanced Circuits will provide a stencil for your board at a moderate additional cost.

If your board has BGA or LLP or other package types with no exposed lead, assembly rework and inspection will be far more difficult. Some medical devices require visual inspection. They cannot even use these types of packages. If you do have a BGA on your board, sometimes the only way to inspect it is with an X-ray machine. Ally yourself with a local assembly house as soon as you can, and make sure it has an X-ray machine and all the rework tools needed to replace every component on your board.

As circuit cards become more complex so, too, do the tools and techniques needed to prototype them. You can get a lot done in a garage environment, but to produce state-of-the-art boards, you will need partnerships with fab houses, prototype assemblers, and contract manufacturers. The sooner you try them out, the sooner you can get a leg up on your competition.

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