Is Your Solder Mask Process Ready for the Fine Pitches?

Component Technology and Miniaturization

Component technology and assembly have historically been driving new material developments to meet requirements for solder mask materials and application processes. The requirements for smaller component pitches with advanced functionalities have rapidly increased in the past few years. There is a continuous request for miniaturization of electronic designs, especially in consumer handheld electronics, where the component pitches are reduced to the minimum. The solder mask material has to be able to form smaller features on the surface combined with high registration and imaging accuracy on large production panels.

Fine pitch designs from 0.4 mm are in volume production while 0.3 mm has been tested and will be introduced in the next few years. The PCB producer needs to be able to process large production panels with fine pitch designs in both outer layer and solder mask HDI production processes.
So far this has been possible with standard contact printing equipment using optical registration technology. For the next-generation components this will be extremely challenging and it will be difficult to achieve acceptable production yields. Depending on pad size designs, solder mask clearances are down to 20 µm, with small features of 50 µm between pads (see Figure 1).

LDI Imaging Technology
LDI imaging technology was introduced in PCB manufacturing some years ago. PCB producers started to introduce the LDI imaging process to improve the registration accuracy in the production of inner and outer layers. There are a number of compatible resists available for LDI primary imaging where dry films are the most common materials in use for inner and outer layer imaging.
Digitalization, through elimination of artwork, will help producers to complete the whole PCB process, with high imaging accuracy and yield. The solder mask process has to follow at the end of the PCB production cycle without creation of a single artwork.
LDI equipment manufacturers are using single wavelength light sources of 355 or 405 nm. Recent developments increased the effective output energies, shortening the exposure time, but mechanical and registration cycle time has also been reduced for a faster total cycle time and higher productivity. Other market drivers are fast prototyping to eliminate artwork and reduce total lead times through production (see Figure 2).

Can Standard Solder Mask Material Be Used for LDI?
Not in an optimized way. The main reason is the absorption during imaging is different from LDI. The polymer/photo system of the solder mask material has to be able to absorb at a specific single wavelength of 355 or 405 nm, while a traditional standard exposure is emitting energy at a wider UV spectra (see Figure 3, Fe-doped lamp [blue] and Ga-doped lamp [orange]). The photo package can then absorb at various wavelengths, which is at an optimum for each photo initiator component.
The standard solder mask material will not absorb enough UV energy at LDI wave lengths to perform acceptably with fast and complete photo reaction. For high performance, a high cross linking density is needed to complete the layer thickness. If the cross linking density is too low, then undercut and low resolution will result, which in turn will not form small solder mask features such as 50 µm solder dams, on the surface. With decreased undercut, the risk of penetration or entrapment of chemicals in post-solder mask processes is reduced (see Figure 4).

LDI Imaging at 355 nm
An LDI solder mask applied in various thicknesses results in a solder dam thickness of 30 to 50 µm. To achieve a 50 µm (2 mil) dam in 30 µm film thickness, a thickness commonly applied for HDI outer layers, the exposure energy needs to be 50 to 60 mJ but requires 80 to 90 mJ at approximately a 40 µm thickness.

The exposure values for this process will be strongly influenced by the developing process, which has to be optimized for the aspect ratio of the panels, minimum hole diameters, and buried vias to be cleaned out from solder mask material. This also means that the wet solder mask application process will influence the exposure values and how much solder mask material is present in the plated holes or vias, which need to be cleaned out. The optimal application process is electrostatic spray or spray application in general as less volume of material is present in the plated holes (see Figure 5).

LDI Solder Mask
The excellent imaging properties of an LDI solder mask are demonstrated in Figure 6 to 8. The 50 µm (2 mil) dam in 30 µm thickness is processed on a UV 355 LDI (50 mJ exposure energy). The dam feature shows excellent shape and sidewall with a very low undercut. The demonstrated LDI products are formulated from IP-protected high-performance polymer binders, and they are modern state-of-the-art solder mask formulations that are halogen-free, RoHS-conforming, and UL-listed. LDI solder mask ranges are used for all conventional solder mask application technologies like screen print, spray, or curtain coating.

Split Imaging Process
PCB producers, in some cases, do not need to use LDI imaging for all panels they produce. The required registration accuracy could be handled in a standard contact exposure process while some panels do require LDI imaging due to tight tolerances. An LDI solder mask could be applied and processed on the same wet application line. After coating and drying, the processing of panels could be either imaged in LDI or in common used standard exposure using good vacuum.

PCB producers can benefit from high production flexibility where the difficult designs could be done with LDI imaging and conventional designs with standard exposure, using only one product applied in one wet solder mask application line.

Printed Circuit Boards

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MAJOR DOMESTIC MARKETS AND TRENDS

The seven basic markets for printed circuit boards are described below.

• Automotive: engine and drive performance, convenience and safety, entertainment (radios), and other applications for diagnostics display and security.

• Communication: mobile radio, touch tone phones, portable communication, pagers, data transmissions, microwave relay, telecommunications and telephone switching equipment, and navigation instruments

• Consumer Electronics: watches, clocks, portable calculators, musical instruments, electronic games, large appliances, microwave ovens, pinball/arcade games, television, home entertainment, video recorders, and smoke and intrusion detection systems.

• Computer/Business Equipment: mainframe computers, mini computers, broad level processors, add-on memory, input/output devices, terminals, printers, copy machines, facsimile machines, word processors, cash registers, teaching machines, gas pumps, and taxi meters.

• Government/Military/Aerospace: radar, guidance and control systems, communication and navigation, electronic warfare, ground support, sonar ordinance, missiles, and satellite and related systems.

• Industrial Electronics: machine and process control, production test and measurement, material handling, machining equipment, pollution, energy, and safety equipment, numerical controls, power controls, sensors, and weighing equipment.

• Instrumentation: test and measurement equipment, medical instruments and medical testers, analytical, nuclear, lasers, scientific instruments, and implant devices.

Computers are the major U.S. market for PCBs, with communications being the second largest application market. The Institute for Interconnecting and Packaging Electronic Circuits (IPC) indicates that nearly 39 percent of printed circuit boards produced in 1993 were used by the computer market, while 22 percent were used by the communication industry.

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Outer Layer Circuit Formation

This section describes the processes used to create the printed circuit design on the two outer layers of the panel.

1 RESIST APPLICATION OPERATIONS
The types of resist and application methods used by the PCB industry for Outer Layer Circuit Formation are the same as the Inner Layer Circuit Formation.

2 EXPOSURE UNITS
The resist that has been applied to the outer layers of a circuit board panel is selectively exposed to UV light using photo design tools and electric Exposure Units similar to those for inner layer circuit formation. However, the outer layer circuits are normally produced using a negative photoresist process. In a negative photoresist process, the resist covering the desired circuit traces remains soft because it is not exposed to the UV light, while the resist covering the undesired copper areas is exposed to the UV light and polymerized.

3 DEVELOPER
The unexposed resist is removed by a Developer to reveal the copper foil circuit pattern. this developer normally contains potassium carbonate to dissolve the soft unpolymerized resist.

4 ELECTROLYTIC COPPER PLATING LINES
Additional copper is added to the copper foil circuit pattern by electrolytic copper plating. The electrolytic copper plating line includes several cleaning or etching baths, rinses, and a copper sulfate plating bath.

5 TIN AND TIN/LEAD PLATING LINES
The new copper circuit design is protected from subsequent processing steps by adding a second layer of metal. Most PCB facilities use tin or tin/lead alloy for this step. The tin or tin/lead alloy is added to the plating line which may include cleaning baths, etching baths, rinses, and electrolytic plating baths.

6 RESIST STRIPPERS
Resist Strippers are used to dissolve and remove the polymerized resist to reveal the unwanted copper foil.

7 ETCHERS
unwanted copper foil is removed by ammoniacal etchers.

8 TIN AND TIN/LEAD STRIPPERS
The protective layer of tin (or tin/lead) is removed from the copper circuits using either a chemical stripping process or an electrolytic stripping process. Both processes normally use inorganic solutions. Occasionally this step is skipped per customer requirements, and the tin/lead plate is left on the panel for reflow.

Printed Circuit Board

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Printed Circuit Board Manufacturing

Printed circuit boards are electronic circuits created by mounting electronic components on a nonconductive board, and creating conductive connections between them. The creation of circuit patterns is accomplished using both additive and subtractive methods. The conductive circuit is generally copper, although aluminum, nickel, chrome, and other metals are sometimes used. There are three basic varieties of printed circuit boards: single-sided, double-sided, and multi-layered. The spatial and density requirement, and the circuitry complexity determine the type of board produced. Printed circuit boards are employed in the manufacturing of business machines and computers, as well as communication, control, and home entertainment equipment.

Production of printed circuit boards involves the plating and selective etching of flat circuits of copper supported on a nonconductive sheet of plastic. Production begins with a sheet of plastic laminated with a thin layer of copper foil. Holes are drilled through the board using an automated drilling machine. The holes are used to mount electronic components on the board and to provide a conductive circuit from one layer of the board to another.

Following drilling, the board is scrubbed to remove fine copper particles left by the drill. The rinsewater from a scrubber unit can be a significant source of copper waste. In the scrubber, the copper is in a particulate form and can be removed by filtration or centrifuge. Equipment is available to remove this copper particulate, allowing recycle of the rinsewater to the scrubber. However, once mixed with other waste streams, the copper can dissolve and contribute to the dissolved copper load on the treatment plant.

After being scrubbed, the board is cleaned and etched to promote good adhesion and then is plated with an additional layer of copper. Since the holes are not conductive, electroless copper plating is employed to provide a thin continuous conductive layer over the surface of the board and through the holes. Electroless copper plating involves using chelating agents to keep the copper in solution at an alkaline pH. Plating depletes the metal and alkalinity of the electroless bath. Copper sulfate and caustic are added (usually automatically) as solutions, resulting in a growth in volume of the plating solution. This growth is a significant source of copper-bearing wastewater in the circuit board industry.

Treatment of this stream (and the rinsewater from electroless plating) is complicated by the presence of chelating agents, making simple hydroxide precipitation ineffective. Iron salts can be added to break the chelate, but only at the cost of producing a significant volume of sludge. Ion exchange is used to strip the copper from the chelating agent, typically by using a chelating ion exchange resin. Regeneration of the ion exchange resin with sulfuric acid produces a concentrated copper sulfate solution without the chelate. This regenerant can then be either treated by hydroxide precipitation, producing a hazardous waste sludge, or else concentrated to produce a useful product.

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Printed Circuit Board Manufacture

The manufacture of a multi-layer circuit board typically starts with preparing the board laminates. Each laminate has a core of fiberglass-reinforced polymer resin, with both sides of the core laminated over with copper foil surfaces. The resulting laminate is then baked, to cure and stabilize the polymer core. Subsequent processing etches the copper surfaces, so that the copper circuit patterns are formed on each surface. After additional processing, the laminates are layered onto each other using isolation sheets, and fabricated into the multi-layer printed circuit board. Holes are then drilled through the board, according to the intended circuit design. After etching and deburring the drillholes, the board is electroless-plated with copper to deposit a conductive copper coating onto the wall of each drillhole. This conductive surface is then electroplated with copper, to strengthen and build up the copper surfaces on the drillhole walls. The purpose of the copper-coated drillhole walls is to connect the circuit pattern on each board laminate into a continuous circuit design among the laminated layers of the completed printed circuit board.

The copper-coated, multi-layered board then undergoes additional processing to complete the circuitry. Initially in this process, a dry film mask is applied onto the exposed copper surfaces on both sides of the board. The mask is applied as a pattern so that a circuit design is traced onto the copper surface. Subsequent processing exposes the copper tracings by removing the dry film mask according to the circuit design. The exposed copper tracings are plated with solder to protect the copper circuit. The mask is then stripped from the unplatted copper areas, which are then etched to remove any unsoldered copper strips. The ammoniacal etchant removes the unmasked copper, but does not remove the tin/lead solder plating that protects the copper circuit tracings. Then the panel is rinsed, and processed in 10% sulfuric acid as an oxidation inhibitor.

Wastestreams
For Circuit Board Manufacturers, the most prevalent hazardous wastestreams are spent aqueous process solutions, spent rinsewater, other metal-laden wastewaters, and the metalladen sludge that is precipitated by the treatment of the spent solutions, rinsewater, and other wastewaters. Many of the innovative source reduction measures listed here would reduce or eliminate sludge generation. Some of the measures would reduce the volume of aqueous wastestreams.

Source Reduction
Hazardous waste source reduction reduces or eliminates the quantity of hazardous waste generated at the source generating the waste. Source reduction can simplify and economize hazardous waste management.
Source reduction of process solutions includes several process substitution measures that reduce or eliminate the use of some process baths that generate hazardous waste. Some of the substitutions still do generate spent process solutions, but often at reduced volume, or as spent solutions that can be easily regenerated for reuse.

Recycling
Onsite and offsite recycling are a complement to source reduction. While source reduction avoids generation of waste at the source, recycling manages the waste to derive further benefit. Like source reduction, recycling is a preferable alternative to treatment or disposal of the waste. Recycling can recover spent process chemicals or rinsewaters. Recycling can also recover contaminants like copper and other etched metals. Like source reduction, recycling can reduce the risk and impact to human health and the environment. Recycling can reduce or delay the need for storage, handling, transport, and disposal of spent process chemicals.

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Intel Makes a Digital Coin Tosser for Future Processors

An all-digital random-number generator will help keep encryption strong and data safe for chips with features 45 nanometers and smaller 29 June 2010—Random-number generators make cryptography possible, thereby making safe digital communication possible, but because the generators rely on analog components, they are notoriously difficult to reduce in size. Engineers at Intel’s Circuit Research Lab, in Hillsboro, Ore., bet they could build one without the analog parts using the complementary metal-oxide-semiconductor (CMOS) processes that will soon be turning out chips with feature sizes as small as 32 nanometers (and eventually 22 nm).

At the VLSI Symposium earlier this month in Honolulu, the Intel researchers revealed they were close to winning that bet. They reported that they had made an all-digital version of a random-number generator using the 45-nm CMOS process that has been used to build Intel processors since 2007. ”Historically, RNGs have been analog,” says Greg Taylor, director of the Circuit Research Lab. ”But porting to smaller technology nodes [with analog devices] requires a lot of fine-tuning that is unnecessary with digital versions.”

Analog circuits require extra design work to manage things such as the signal-to-noise ratio, Taylor explains. “Supply voltages scale down as we move to more advanced technologies. This supply voltage reduction reduces signal power without changing device noise, consequently reducing the signal to noise ratio.”

The benefits of going digital were immediately obvious. ”The device generates billions of random bits per second and can run at very low voltage,” says Taylor. What’s more, making the generator all digital made it more random. The circuit takes advantage of a phenomenon that is a bugaboo for designers of logic that uses more than one clock. The Intel team engineered the number generator so that in each string of numbers turned out by the machine, every bit is the result of ”metastability.”

Usually, you sample a digital device’s output when it has settled on a definite value, either a one or a zero. Metastability is what occurs when the voltage is sampled during a transition and the bit is caught between a zero and a one. Eventually, the bit drops down to one state or the other, but there is no way to tell on which side it will land. The Intel researchers deliberately sample during transitions; they enhanced the randomness even further by tuning the metastability so that the bit falls to one or zero with fairly equal probability, ”making it essentially a coin flip,” says Taylor.

Taylor says his research group is trying to help make the Internet a secure place, and the all-digital random-number generator is just one piece of the puzzle.

Resource: Printed Circuit Board

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Build a Custom-Printed Circuit Board

Breadboarding a new circuit is a key skill and an important step in many projects—especially early on, when you need to move wires around and substitute components. But that very flexibility also makes it easy to knock wires out. Eventually, if your project is a keeper, you’re going to want something with a bit more permanence.

Printed circuit boards (PCBs) solve all those shortcomings. But most people don’t even consider translating a one-off project into a PCB design. For one thing, PCB fabrication has traditionally been expensive, viable only in commercial quantities. (One alternative is to do it yourself with etches and silk screens, a messy and time-consuming process.) Also, there are technical constraints involved with PCB designs that are daunting to the casual hobbyist. But it turns out that nowadays you can produce a professional PCB very inexpensively.
I only recently delved into the mysteries of PCB fabrication, for an upcoming IEEE Spectrum article on building a robotic digital microscope. Part of the project is a variable LED illumination system, and to dim LEDs you need to design a pulse-wide-modulation circuit. With a little research, I found a design based around a 555 timer chip, a power transistor, a couple of diodes, two resistors, and three capacitors. Not a huge number of components, but enough that I was dreading getting them all hooked up on a breadboard. Since I already had the design in hand, I was pretty confident it would work out of the box. That made it a perfect candidate for doing it as a PCB.

I had looked into custom PCBs a while back. I had even downloaded Eagle, a PCB design program from CadSoft Computer. I liked Eagle (and eventually returned to it for this project), but back then the cost of a single board—US $75 and up—stopped me. This time I discovered BatchPCB.

BatchPCB consolidates a bunch of individual projects onto a single large board and then cuts them apart when they come back from the factory. Small boards can be fabricated for under $20 this way. There’s a three-week turnaround, but I wasn’t in a hurry. I dove into designing my LED dimmer board.A basic review of PCBs is in order here. A PCB consists of a thick, rigid insulating layer with conductive traces on the top and bottom. Because the middle insulates, the traces on the top and bottom can run over each other. To get a signal from one side to the other, you drill a hole called a via, which is like a trace running through the board vertically. You also drill holes for ICs, resistors, diodes, and other discrete pieces. The traces lead to the holes, and the components have solder pads, allowing them to be soldered in place.

Some components may be connectors to bring signals to the board, and you can also leave larger holes with solder pad rings around them to solder external wires directly. You can even add layers with traces in between the top and the bottom, although this can get pricey. At BatchPCB, a two-layer board costs $2.50 per square inch (about $0.40 per square centimeter), while a four-layer board costs $8 for the same area (about $1.24/cm2).

The first step in creating a custom PCB is laying out the schematic view. Place your components (which usually come from a component library included with your design software) onto a canvas, and then connect the pins with lines representing electrical connections. You may find yourself faced with multiple choices for the same part number. That’s because many components come in different packages, such as a DIP (dual in-line package) chip or a surface-mount chip. For hobby PCBs, you almost always want to go with the big, clunky DIPs and SIPs (system-in-packages), because they’re easily found at hobby venues and are easier to solder than surface-mounted devices, which are meant for commercial applications. Although the various packaging options may look the same in the schematic view, they will appear very different when you go into the layout view to actually design the board.

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