Surface Mount Technology

By Ray Miles, KC0BR

13752 Deodar, Tustin, CA

Surface mount technology (SMT) is a relatively new process used in the construction of printed circuit boards. SMT uses a technique of adhering components to the PC board by special pastes and other bonding techniques, rather than through-hole soldering. It is important to the electronics industry and Amateur Radio operators because as its popularity increases, every electronic product will be affected. Japan already uses SMT on 70 to 75 percent of all board applications.1 Amateur Radio equipment is included in the plan. (Carefully open your Kenwood TH-21, for example, and locate the dual-tone, mul-tifrequency circuit for a visual display of the finished SMT product.)

In this article, SMT processes, their components and substrates, component placement methods, soldering techniques, testing, and problems facing the users of this technology are examined. How SMT will affect the Amateur Radio experimenter can only be argued at this time, since much available equipment continues to be fabricated with leaded components.

Why Surface Mount?

The rationale underlying surface mount devices is best determined by examining the benefits of this technology. Improved electrical performance—as a result of lower lead inductance and faster circuit operating speeds—can be acquired. Automated assembly featuring flexible "pick-and-place" equipment is available. Virtually all surface mount devices can be automatically placed on these machines; they are smaller and lighter than through-hole components and feature a higher resistance to shock and vibration. Most SMT processes can be fully automated. Additionally, SMT involves overall smaller parts (less stock room area required) and smaller equipment (less assembly floor space required). Each of these improvements contribute to lower manufacturing cost.

Components

Most components are becoming increasingly available in surface mount form. Available components include capacitors, crystals, inductors, filters, resistors, trimmer capacitors, thermistors,

'Notes appear on page 6.

4 QEX

Photo 1—An assortment of microcompo-nents are compared to a standard Vi-watt resistor. Clockwise from top left, a 0.001/iF chip capacitor, package style 0805; a resistor (101 = 10 x 10 = 100); a 0.47 nF capacitor, package style 1210; and a 150-kilohm resistor (154 = 15 x 104).

trimmer potentiometers, small outline transistors (SOT), small outline integrated circuits (SOIC) and leaded- and leadless-chip (IC) carriers. (Small outline describes the size of the transistor or IC. The package can withstand very high temperatures and features "footprints." Footprints on the package are tiny pads that adhere to footprints on the circuit board during soldering processes.)

Components are generally packaged using one of four methods:

• Bulk—these components are loaded into vibratory feeders connected to the automated machines.

• Tube (similar to DIP IC packaging)— they are mounted so as to allow the components to slide down to a pickup point. Tube slides are often vibration assisted.

Photo 2—A resistor lies in its 8-mm wide package waiting to be plucked by an automated pick-and-place machine.

• Tape and reel—the tape is fed into a feeder that opens the cover to expose the components, and ratchets the tape into the pickup position. The standard tape size is 8 mm wide, although other tape sizes are becoming standard for larger components.

• Matrix Tray—also called a Waffle Pack because of its appearance. The Tray is made of plastic and the parts fit into each of the tiny pockets.

Substrates

The word substrate is another expression for supportive material or base. It can also be considered a foundation from which a layered circuit board can be fabricated. Two predominant substrate materials commonly used are:

Alumina (ceramic)

The phrase "hybrid circuit" generally refers to SMT on ceramic. Ceramic substrates usually are not larger than 4 inches x 4 inches because larger units may warp during the curing process.

The circuit conductor paths and resistors are deposited onto the substrate with a screen printer that spreads a paste of glass-like material impregnated with conductive or resistive ingredients. The substrate is then placed in a dryer to evaporate the volatile paste vehicles. Repeating the previous two steps, additional layers may be screen printed onto the substrate and dried (I have seen as many as forty layers printed onto a well-designed substrate.) After all the layers have been printed, the substrate is placed in a furnace to be fired.

During firing, the dried paste melts on the substrate and actually becomes part of its surface. After this process, the resistors that are required to be low-tolerance devices are laser-trimmed to value.

Glass Epoxy

Glass epoxy boards (standard circuit boards) are treated essentially the same for SMT as they are for through-hole circuits, except for trace and hole dimensions and solder mask requirements. Current state-of-the-art techniques produce 0.005- to 0.007-inch traces and 0.010- to 0.015-inch holes. The technological goal is to soon manufacture 0.002-to 0.003-inch traces, and 0.006- to 0.007-inch plated-through holes. The purpose of these small holes (called VIAs) is not to allow passage of part leads, but rather to provide circuit paths between the top, bottom and intermediate layers of the board.

The Solder Mask

There are two predominant types of solder mask—the wet mask and dry film. The wet mask method is the most common and the least expensive solder mask to produce. A liquid is screened onto the PC board and then dried in much the same way that designs are silk screened onto a T shirt. This method is not accurate for most surface mount boards because the liquid does not hold its consistancy or position tolerance well.

Dry film uses a thin plastic-like material that is supplied in sheet form. It is bonded to the PC board and photographically exposed and developed. The result is a precise and uniform solder mask.

The copper on a PC board may be tinned with solder before or after the solder mask is applied. If the solder mask is applied over tinned copper, when the board is wave or reflow soldered, the thin layer of solder under the mask becomes molten. This allows the mask to move and additional solder may flow under the mask. (You may have seen the bottom of a PC board that has wrinkles over large areas of masked tinned copper.) In wave solder, having the soider get under the mask and into the solder joint is not a problem. In surface mounting, it is a different story.

During the surface mounting process, a precise quantity of solder paste is applied to the solder joint. If some of that solder flows under the mask during reflow, there will not be enough solder left at the joint. If the PC board is solder tinned after the solder mask is applied (solder mask over bare copper or SMOBC) these problems are eliminated.

Component Placement

Component placement can be done by simply using tweezers to pick up and position components. This method may be adequate for boards with few parts, but as part quantities increase, the chance for error also increases rapidly. This leads to the next method: Semiautomatic placement.

With semi-automatic placement, an operator sits at a placement machine with a vacuum stylus in hand. The machine turns on a light at the correct component bin, and also shines a light on the substrate, thereby identifying component location and polarity. The operator merely has to"follow the lights" to construct the board.

Another system uses a fully-automatic pick-and-place machine. One or more placement heads on a machine using an X-Y positioning system contain different stylus sizes. The styli can be changed automatically, each capable of handling a limited range of component size. Multiple head/stylus sizes are preferable. Head utilization is not limited to parts placement: One of the heads may be a dedicated epoxy dispenser.

Next is a more "component-size-dedicated" system. Here, the tradeoff is flexibility for speed. The placement head is fixed and the circuit board is moved. The component input magazines move to a fixed pickup point, or a shuttle brings the component to the pickup point. The part is put onto the placement head, the board is moved into position and the head puts the part onto the board.

Fastest and most costly, is the mass-placement method. The magazine is loaded with components that are pre-oriented exactly as they will be placed onto the board. The multi-stylus head picks up and places all parts in only one operation.

Clearly, the mass-placement method is the most efficient system. However, considering the dedicated tooling required, this system is only a viable candidate for high-quantity consumer product lines.

Soldering

Two major soldering methods are used in SMT:

Solder wave

The components on the bottom of the board can be soldered with a dual-wave flow solder system. Components soldered in this manner must be cured (glued to the board with epoxy) before proceeding to wave solder.

Dual-wave soldering is a system whereby the board travels through a first wave that is turbulent (mostly vertical speed component) and then travels through a second wave that is the usual, smooth type (horizontal speed component). The first wave gets the solder into areas that the normal wave may miss; the second wave smooths out the solder, removing most of the "icicles" and bridges.

Presently, there exists in the industry some controversy as to whether a "hotair knife" is also required. The hot-air knife is a device that follows the dual wave. It creates a narrow, high velocity air jet at a temperature above that of molten solder. The air jet biows away icicles, bridges and excess soider.

Reflow

This process applies intense heat to an area previously loaded with soider paste and components. The heat causes the solder to reflow and creates solder joints. The solder is usually deposited with a screen printer.

Infrared and vapor phase techniques are also used in reflow soldering. In the infrared method, the board is transported on a conveyor through multiple heat zones. The temperature in the first few zones is set low to evaporate existing paste fluids. The final zones heat the board to a temperature at which the solder reflows.

The vapor phase method also requires that solder paste fluids be evaporated from the board before reflow. Then, the assembled board is immersed into a hot, saturated vapor generated by a pool of boiling Fluorinert.2 As the hot vapor condenses onto the board, it releases the latent heat of vaporization. The colder the part, the more the vapor is condensed on it; the hotter the part, the less condensation. Therefore, heating transfer is rapid, uniform, and relatively independent of component geometry. One drawback of this process is that components become more prone to misalignment because of liquid condensation on the work piece surface.

Testing

If an in-circuit component test is to be performed, this should be considered during the initial board layout design. Solder pads with surface-mount components often do not lend themselves to "bed-of-nails" test fixtures. Test fixtures are available, however, such that the board fits in vertically. Contact plates on both sides of the board possess bed-of-nails test fixtures, but even with this arrangement, there may be test nodes inaccessible to the test nails.

Wherever possible, the board should be manufactured with VIAs to provide contact points to test points otherwise located in an inaccessible area of the board. The best situation is to have all test nodes on the top of the board, brought down to the bottom of the board by the use of VIAs.

Actual inspection, testing and repair of SMT boards can be performed visually, by X-ray or by laser. A heated collet can be used to solder or unsolder only one component. Let's say that a replacement chip must be inserted. The new component is aligned with its position on the board, and solder paste is applied to the bottom of the chip. Then, the chip is placed on the board and a heating element inside the collet walis applies intense heat until the solder reflows. While the collet is lifted from the work area, a center pin holds the replacement part in place until the solder is cool.

Another device used for manual removal or insertion of a component is a heated probe. With a handle shaped like scissors, this tool is heated like a conventional soldering iron. The component-handling end is sized to fit the four sides of a chip carrier, and several sizes are available.

Another system uses a hot-air repair terminal to apply a stream of air to the component connections, while a tweezer-like tool removes the component. Nitrogen, nitrogen and hydrogen, argon or an infrared light that concentrates heat on a certain area is also used in a similar process.3 These inert gases are used to reflow the solder so a faulty component can be removed. This process keeps oxidation to a minimum.

Problems

Because most chips consist largely of ceramic substrates, the components made of this material must always be watched for signs of thermal stress. Ceramic has a relatively low coefficient of thermal expansion, compared to that of glass-epoxy circuit boards. During heating and cooling cycles, the expansion differences tend to place a strain on the solder joints. Uneven solder fillets (concave junction formed by the solder between the component pad and the circuit board pad) multiply this problem.

For example, we may have a component that has a large fillet on one side, and a small fillet on the other side. With thermal expansion, the large fillet may hold while the small fillet gives a little. Instead of both fillets sharing the strain evenly, the small fillet receives nearly the entire strain.

This can be a significant problem with large parts. One solution to this problem is to use leaded chip carriers, SOTs and SOICs. With leaded parts, the leads absorb most of the thermal stresses.

Tombstoning and Solder Balls

Fluids may remain in the solder paste during the solder reflow process. When this occurs, the remaining fluid boils out of the paste, creating minute explosions that can blow one end of a component upward from the board. This causes the component to stand on one end, an effect known as "tombstoning." Tombstoning may also occur if one side of a component is in the solder paste and the other side is not, or if the paste on one side melts before the paste on the other side does. Solder surface tension pulls the part to a vertical position. Surface tension is not always a bad thing! During reflow, solder surface tension may serve to pull the parts to the center of their solder pads (if the pads are well designed) aiding alignment of the component.

Let us return to the first reason for tombstoning (fluid remaining in the solder paste during reflow). Its mechanism (minute explosions) also creates solder balls. The explosions throw small bits of solder paste over other parts of the board. When the paste melts during reflow, balls of solder form as a result of surface tension.

Photo 3—Three integrated circuits. A standard 14-pin IC is shown in the upper left hand corner. The remaining chips are small outline integrated circuits (SOICs). Their pins are shaped in a "gull-wing" fashion (flared) for placement on the circuit board pads to which the ICs will be adhered. SOICs have the same pinouts and electrical specifications as DIPs of the same generic number.

Photo 3—Three integrated circuits. A standard 14-pin IC is shown in the upper left hand corner. The remaining chips are small outline integrated circuits (SOICs). Their pins are shaped in a "gull-wing" fashion (flared) for placement on the circuit board pads to which the ICs will be adhered. SOICs have the same pinouts and electrical specifications as DIPs of the same generic number.

Conclusion

In this article, we have discussed the specifics of what SMT is and the manufacturing processes involved. You might ask, "If the technology proves itself, and has overall advantages compared to the conventional way of fabricating printed circuit boards, why then, is SMT use not more widespread?" There are several concerns manufacturers must address before converting to SMT. New equipment to perform SMT functions would have to be installed. This costs money. Employees who have been educated to manufacture design boards and use through-hole soldering and mounting techniques would have to be reeducated. Again, this costs money.

What impact will this new technology have on Amateur Radio? Should the experimenter be concerned? Yes. Pat Hawker, G3VA, calls SMT "throwaway

Photo 4—A 20-pin PLCC (plastic leaded chip carrier). The chip leads curve back under itself for ease of placement on the footprint pads. The PLCC is also referred to as J leaded because the curved lead configuration resembles the letter J.

electronics." If a faulty piece of equipment needs modification or repair, how does the experimenter go about fixing it? An SMT printed circuit board would have to be sent back to the factory, or even worse, disposed of entirely. Faulty components would be difficult to trace on densely-packed boards, and some surface mount devices are too small to carry identification marks or values. This is definitely not technology for experimental breadboard units, G3VA opines.4

SMT could be beneficial to the Amateur Radio experimenter as well. Companies using SMT can afford to pass on their production savings to the buyer. Result? Low-cost equipment. If a single device is faulty and traceable, hand tools might be made available to designers and as easy to obtain as a soldering iron is today. Educational facilities might include courses of study on SMT for engineering students. These are only some of the many possibilities that can make SMT work for Amateur Radio. The results of one survey predicts that US manufacturers would not become greatly involved in SMT until the 1990s—but that's not a long way off.5

Notes

1Jerry Lyman, "What's Holding Back Surface Mounting," Electronics, Feb 10, 1986, Vol 58, No. 6, pp 25-29. 2See note 1.

3Jerry Mullen, How to Use Surface Mount Technology, (Dallas: Texas Instruments Information Publishing Center), 1984. "Pat Hawker, "Technical Topics, "Radio Communications, Mar 1985, pp 188-187. 5See note 1.

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