Tag Archives: PCBs

What is a PCB Via and How is it Made?

Vias are actually holes drilled into PCB layers and electroplated with a thin layer of copper to provide the necessary electrical connectivity. Three most common types of plated through via are in use—plated through holes, blind holes, and buried holes—with plated through holes running through all the layers of the PCB. These are the simplest type of holes to make and the cheapest. However, they take up a huge amount of PCB space, reducing the space available for routing.

Blind vias connect the outermost circuit on the PCB with other circuits on one or more adjacent inner layers. As they do not traverse the entire thickness of the PCB, they increase the space utilization by leaving more space for routing.

Buried vias connect two or more circuit layers in a multi-layered PCB, but do not show up on any of the outer layers. These are the most expensive type of vias and take more time to implement, as the fabricator has to drill the hole in the individual circuit layer when bonding it. However, designers can stack several buried vias in-line or in a staggered manner to make a blind via. Therefore, buried vias offer the maximum space utilization when routing a PCB. Fabricators of high-density interconnect (HDI) boards usually make use of buried vias, most often using lasers for drilling them.

Drilling a Via

At positions for the vias, the fabricator drills holes through the PCB using a metal drill of small diameter. He or she then cleans the hole, de-smears it, and de-burrs it to prepare it for plating. Rather than removing copper as is normally the case with the etching process, the fabricator then adds a thin layer of copper to the newly formed hole through a process of electroplating, thereby connecting the two layers. For a two-layer board, the fabricator then etches circuit patterns on both sides. Via usually have capture pads on both layers.

The process of drilling a via hole using a laser is somewhat different. In general, fabricators use two types of lasers—CO2 and UV—with the latter able to make very small diameter via holes. UV laser-drilled via holes are about 20-35 µm in diameter. As the laser beam is able to ablate through the thin copper layer, capture pads with a central opening are not necessary. Most fabricators program a two-step process for drilling a hole with a laser beam.

In the first step, a wider-focused laser heats the top copper layer, driving the metal rapidly through the melt phase into the vapor phase prior to gas-dynamic effects expelling it from the surface. The laser repeats this for all via positions on the PCB layer.

In the next step, the program focuses the beam tightly and controls the depth the laser can burn. For blind vias, It allows the laser to burn through the intervening dielectric and stop when it has reached the bottom copper layer, before moving on to the neighboring via position.

The same process of electroplating as above deposits a thin layer of copper along the walls of the holes left behind by the laser beam, thereby connecting the two layers. The rest of the process for etching the circuit pattern on the two sides remains the same.

How useful are PCB Vias?

Designers use a plated through via as a conduit for transferring signals and power from one layer to another in a multi-layer printed circuit board (PCB). For the PCB fabricator, the plated through via are a cost-effective process for producing PCBs. Therefore, vias are one of the key drivers of the PCB manufacturing industry.

Use of Vias

Apart from simply connecting two or more copper layers, vias are useful for creating very dense boards for special IC packages, especially the fine-pitch components such as BGAs. BGAs with pitch lower than 0.5 mm usually do not leave much space for routing traces between neighboring pads. Designers resort to via-in-pads for breaking out such closely spaced BGA pins.

To prevent solder wicking into the via hole while soldering and leaving the joint bereft of solder, the fabricator has to fill or plug the via. Filling a via is usually with a mixture of epoxy and a conductive material, mostly copper, but the fabricator may also use other metals such as silver, gold, aluminum, tin, or a combination of them. Filling has an additional advantage of increasing the thermal conductivity of the via, useful when multiple filled vias have to remove heat from one layer to another. However, the process of filling a via is expensive.

Plugging a via is a less expensive way, especially when an increase in thermal conductivity does not serve additional value. The fabricator fills the via with solder mask of low-viscosity or a resin type material similar to the laminate. As this plugging protects the copper in the via, no other surface finish is necessary. For both, filled and plugged vias, it is important to use material with CTE matching the board material.

Depending on the application, fabricators may simply tent a via, covering it with solder mask, without filling it. They may have to leave a small hole at the top to allow the via to breathe, as air trapped inside will try to escape during soldering.

Trouble with Vias

The most common defect with vias is plating voids. The electro-deposition process for plating the via wall with a layer of copper can result in voids, gaps, or holes in the plating. The imperfection in the via may limit the amount of current it can transfer, and in worst case, may not transfer at all, if the plating is non-continuous. Usually, an electrical test by the fabricator is necessary to establish all vias are properly functioning.

Another defect is the mismatch of CTE between the copper and the dielectric material. As temperatures rise, the dielectric material may expand faster than the copper tube can, thereby parting the tube and breaking its electrical continuity. Therefore, it is very important for the fabricator to select a dielectric material with a CTE as close as possible to copper.

Vias placed in the flexing area of a flex PCB can separate from the prepreg causing a pad lift and an electrical discontinuity. It is important designers take care to not place any vias in the area where they plan the PCB will flex.

Why Use a Multi-Layer PCB?

Although a multi-layer PCB is more expensive than a single or double-layer board of the same size, the former offers several benefits. For a given circuit complexity, the multi-layer PCB has a much smaller size as compared to that a designer can achieve with a single or even a double-layer board—helping to offset the higher cost—with the main advantage being the higher assembly density the multiple layers offer.

There are other benefits of a multi-layer PCB as well, such as increased flexibility through reduced need for interconnection wiring harnesses, and improved EMI shielding with careful placements of layers for ground and power. It is easier to control impedance features in multi-layer PCBs meant for high-frequency circuits, where cross talk and skin effect is more prominent and critical.

As a result, one can find equipment with multi-layer PCBs in nearly all major industries, including home appliances, communication, commercial, industrial, aerospace, underwater, and military applications. Although rigid multi-layer PCBs are popular, flexible types are also available, and they offer additional benefits over their rigid counterparts—lower weight, higher flexibility, ability to withstand harsh environments, and more. Additionally, rigid flex multi-layer PCBs are also available, offering the benefits of both types in the same PCB.

Advantages of a Multi-Layer PCB

Compared to single or double-layer boards, multi-layer PCBs offer pronounced advantages, such as:

  • Higher Routing Density
  • Compact Size
  • Lower Overall Weight
  • Improved Design Functionality

Use of multiple layers in PCBs is advantageous as they increase the surface area available to the designer, without the associated increase in the physical size of the board. Consequently, the designer has additional freedom to include more components within a given area of the PCB and route the interconnecting traces with better control over their impedance. This not only produces higher routing density, but also reduces the overall size of the board, resulting in lower overall weight of the device, and improving its design functionality.

The method of construction of multi-layer PCBs makes them more durable compared to single and double-layer boards. Burying the copper traces deep within multiple layers allows them to withstand adverse environment much better. This makes boards with multiple layers a better choice for industrial applications that regularly undergo rough handling.

With the availability of increasingly smaller electronic components, there is a tendency towards device miniaturization, and the use of multi-layer PCBs augments this trend by providing a more comprehensive solution than single or double-layer PCBs can. As these trends are irreversible, more OEMs are increasingly using multi-layer boards in their equipment.

With the several advantages of multiple layer PCBs, it is imperative to know their disadvantages as well. Repairing PCBs with several layers is extremely difficult as several copper traces are inaccessible. Therefore, the failure of a multi-layer circuit board may turn out to be an expensive burden, sometimes necessitating a total replacement.

PCB manufacturers are improving their processes to overcome the increase in inputs and to reduce design and production times for decreasing the overall costs in producing multi-layer PCBs. With improved production techniques and better machinery, they have improved the quality of multi-layer PCBs substantially, offering better balance between size and functionality.

What are Multi-Layer PCBs?

Most electronic equipment have one or more Printed Circuit Boards (PCB) with components mounted on them. The wiring to and from these PCBs determines the basic functionality of the equipment. It is usual to expect a complex PCB within equipment meant to deliver highly involved performance. While a single layer PCB is adequate for simple equipment such as a voltage stabilizer, an audio amplifier may require a PCB with two layers. Equipment with more complicated specifications such as a modem or a computer requires PCB with multiple layers, that is, a PCB with more than two layers.

Construction of a Multi-Layer PCB

Multiple layer PCBs have three or more layers of conductive copper foil separated by layers of insulation, also called laminate or prepreg. However, a simple visual inspection of a PCB may not imply its multi-layer structure, as only the two outermost copper layers are available for external connection, with the inner copper layers remaining hidden inside. Fabricators usually transform the copper layers into thin traces according to the predefined electrical circuit. However, some of the layers may also represent a ground or power connection with a large and continuous copper area. The fabricator makes electrical interconnections between the various copper layers using plated through holes. These are tiny holes drilled through the copper and insulation layers and electroplated to make them electrically conducting.

A via connecting the outermost copper layers and some or all of the inner layers is a through via, that connecting one of the outermost layers to one or more inner layers is the blind via, while the one connecting two or more inner layers but not visible on the outermost layers is the blind via. Fabricators drill exceptionally small diameter holes using lasers to make vias, as this allows maximizing the area available for routing the traces.

As odd number of layers can be a cause of warping in PCBs, manufacturers prefer to make multiple layer boards with even number of layers. The core of a PCB is an insulating laminate layer with copper foils pasted on both its sides—forming the basic construction of a double-layer board. Fabricators make up further layers by adding a combination of prepreg insulation and copper layers on each side of the double-layer board—repeating the process for as many extra layers as defined by the design—to make a multi-layer PCB.

Depending on the electrical circuit, the designer has to define the layout of traces on each copper layer of the board, and the placement of individual vias, preferably using CAD software packages. The designer transfers the layered design output onto photographic films, which the fabricator utilizes to remove the excess metal from individual copper layers by the process of chemical etching, followed by drilling necessary holes and electroplating them to form vias. As they complete etching and drilling for each layer, the fabricator adds it on to the proper side of the multi-layer board.

Once the fabricator has placed all layers properly atop each other, application of heat and external pressure to the combination makes the insulation layers melt and bond to form a single multi-layer PCB.

Whisker Growth in Printed Circuit Boards

in whiskers are not fanciful or imaginative items, but are real and pose a serious problem for all types of electronic manufacturing. Pure tin is often used as a finish material on printed circuit boards (PCBs) to protect the exposed copper pads from tarnishing. However, pure tin spontaneously grows conductive whiskers, thin wire like growth that can form electrical paths and affect the operation of the PCB assembly.

Understanding Tin Whiskers and their Effects

First reported in the 1940s, tin whiskers are mostly invisible to the naked eye as they can be ten to hundred times thinner than a human hair. They grow to considerable lengths bridging fairly long distances between tracks and pads on the PCB. Once bridged, the whisker can short the conductors. There is no set timetable for the whiskers to commence growing. Their incubation may be fairly rapid, ranging from days, or slow, taking years.

These needle-like tin whiskers can create a short circuit between two conductors. As they are very thin, most whisker growths usually fuse or burn out when current flows through them, creating a momentary short circuit. However, in rare circumstances, rather than vanishing like a fuse link does, the whisker can form a path capable of conducting several hundred amperes. The conductive path created by whiskers generates false signals at incorrect locations, which can cause the device to operate improperly.

Sometimes, whiskers break away and fall across other traces on the PCB or between neighboring conductive components, where they can disrupt or interfere with local electrical signals. For instance, falling on MEMS, whiskers may interfere with intended mechanical functions, or diminish the transmitted light if they fall into optical systems.

As more and more electronic systems form the backbone of our manufacturing and transportation systems, our communications and financial systems, and our conventional and nuclear power plants, the problem of whisker growth in pure tin-plated electronic PCBs becomes increasingly ubiquitous.

Impact of Tin Whiskers on PCB Assembly Reliability

Manufacturers utilize tin for coating several different components used on PC board assemblies. One popular way to stabilize the tin finish is by introduction of lead. However, this method is contrary to the concept of Restriction of Hazardous Substances (RoHS), which most governments follow, as lead is a dangerous substance affecting human health. Instead of using lead, most companies now use special alloys.

Whiskers can form in different ways, some of which are:

  • From stresses on poorly formed components that do not fit together very well
  • From intermetallic formation
  • From different outside sources of stress
  • From external or internal problems causing scratches, stretching, or bending of the assembled PCB

Whiskers are not to be confused with dendrites or other such shapes in PCBs and components, as they are considerably different in both nature and function. Unless they are found and identified correctly, whiskers can pose a serious problem for a circuit board assembly. These structures of crystalline formation, whiskers most commonly occur in electroplated tin used as a finish on components and PCB traces.

Preventing or Mitigating Whisker Growth in PCBs

Growth of whiskers puts PCB assembly at considerable risk, since whiskers interfere with components, and this automatically qualifies a good product as a defective one. Although a growing tin whisker may seem harmless, it can pose a very real threat to both the product as well as to the human operator. In PCB assembly, one of the most common problems that whiskers create is a short circuit or arcing. This can cause breakdown of electrical equipment, as well as harm people from the arcing. Either way, it ultimately leads to a loss in time and money.

The impact of whisker growth on global PCB assembly results in ruined circuitry, broken equipment, and overall shoddy artisanship. Therefore, it is very important to address the issue of whisker growth. For mitigating or preventing whisker growth, the following precautions may help.

As pure tin coating is the basic reason for the growth of whiskers, avoiding the use of pure tin plating on PCBs and other components is the most obvious method. However, as this action falls in the realm of manufacturing, it is not always possible to implement at the PCB assembly level. Most manufacturing companies do utilize alloys to help stabilize the tin coating to mitigate tin whisker growth, but it is better to be cautious.

If there is a high risk of whisker growth, it may be possible to outsource the PCB/component to a contract manufacturing company to re-plate the area. To avoid tin whiskers, it is highly advisable to let the external manufacturing company strip away the current plating, and reapply newer plating.

Application of a coating or housing foam encapsulation on the whisker prone area can help to prevent problems of growth in the future. However, this method depends on several factors, including type of foam encapsulating coating used, the amount applied, and the intensity of infection of the whisker prone area. In actual practice, the foam encapsulating coating normally helps to prevent short circuits.

An alternate method is to relieve the stress on the area by using hot oil reflow, or by conducting a new reflow soldering job.

Most reliable assembly manufacturers are aware of tin whiskers, and are willing to help with any whisker growth problem. Several turnkey assembly manufacturers are also certified and make sure they use alloys in place of pure tin components for mitigating whisker formation. Of course, faulty and counterfeit components do raise the risk of causing tin whiskers, but working with US-based manufacturing and assembly companies normally ensures an overall higher standard of quality.

New Research on Preventing Tin Whiskers

New research in preventing growth of tin whiskers points to the use of an additional metal coating on the tin layer. Depositing a thin layer of nickel as an electroless metal deposition seems to be the most practical method. Although tin whiskers can penetrate most metal in days, a hundred-atom thick, about 35 µm, of nickel forms a virtually impenetrable layer. A thicker layer of nickel not only retards the growth of tin whiskers, it truly prevents their formation permanently. However, this requires a two pass electroplating process, one for depositing the tin layer, and the next for depositing the nickel layer on it.

Variables in Lead-Free Reflow for PCBs

Reflow ovens often show degrees of variability from profile to profile. This may depend on the distribution of components on the board, especially those that are slow heating, heat-sensitive, or of high mass. In general, reflow systems cannot generate one single reflow profile producing capable thermal results for all products.

For instance, a large BGA package on the PCB may not allow more than five degrees of variation near the peak of the reflow profile curve. Therefore, even while the BGA joints show good soldering, there is a probability of frying some other smaller components nearby the BGA package.

Variables during reflow can also be the result of several external factors. There may be limited control for some factors, but others could be uncontrollable. For instance, in some cases, the PCB may be non-uniform, its components may have varying thermal characteristics, or the tolerances of the process controller could be the major contributor. Even the exhaust could contribute as an external factor.

Oven loading is another major factor when creating custom reflow profile for a high-layer-count PCB. The reflow oven characteristics depend on the number of PCBs passing through it, as the total mass of the PCBs and their speed through the oven influences the rate of rise of temperature. Usually, the load capacity of the reflow oven is measured in boards per minute, and this value differs when only a single PCB is passing through as against a batch of several PCBs passing through at a time.

Customers often demand demonstrable settings of the custom reflow profile for their boards. It may be necessary to demonstrate that a given setting fulfills the requirements of a thermal profile for the board, without damaging any other component on it. Sometimes there are requirements to create documentation as evidence that a particular assembly is indeed within specifications. One of the advantages of creating custom profile for a board is it brings a total visibility to the lead-free reflow process when handling that board.

Automated Methods for Lead-Free Reflow

A reflow profiler, such as the one made by KIC, is the most popular method assemblers use for profiling groups of boards they assemble. The instrument works equally well when profiling individual boards automatically and continuously. Assemblers also use it for a cluster of boards consisting of two or three categories.

The KIC profiler has a Navigation prediction software accompanying it. This helps to drive the generated profile deeply within the specifications of the board. Typically, actual profiles need to be run on some boards that match the representative profile for that group. The process must be repeated periodically to ensure the settings remain valid. Used along with the Navigation prediction software, the KIC profiler saves much time and effort when creating lead-free reflow characteristics for high-layer-count PCBs.


Lead-free reflow of high-layer-count PCBs need not be a tiresome exercise provided it is possible to set up a custom reflow profile for a group of PCBs with similar thermal characteristics. Using modern thermal profilers makes the job economical and fast.