Understanding the critical process steps in design, fabrication and assembly can increase the robustness of the PTH and improve reliability.

PCB real estate, along with components, vias, and annular ring sizes are shrinking dramatically. Five or 10 years ago, a via hole of 15 to 20 mils with a 5 to 10 mil annular ring was the norm. Now, via sizes ranging from 7 mils down to 4 mils on highly complex boards are increasingly more common. As through-hole size (via) shrinks, so does the annular ring, posing major challenges to fabrication and assembly manufacturers in the process.

In earlier PCB generations, two to three mils of play (or wander) within a 10 mil hole was common. Currently, precision laser drilling allows about 0.5 mil wander, (sometimes less) as smaller drills have reduced via size to 4 or 5 mils. To illustrate, this means that one mil of wander represents 25% of the hole size when a four mil via is created.

Consequently, tighter fabrication and assembly controls and tolerances, state-of-the-art equipment and trained technicians must be utilized at design, fabrication and assembly to assure high plated through-hole (PTH) reliability. Laser drilling equipment must be up-to-date, highly precise and accurate to ensure that vias and annular rings remain intact as a PCB undergoes the multiple heat and reflow cycles during the different phases of fabrication and assembly.

Sufficient clearance is necessary to avoid creating break out on internal layers. This condition is typically caused by miscalculations of drill size, pad size, incorrect annular ring size or over-etching at fabrication, resulting in weak or broken continuity.

Lead-free design, fabrication, and assembly are especially vulnerable areas requiring careful attention to assure PTH reliability. Experienced EMS providers fully understand that proper focus must be placed on selecting the correct PCB material, ensuring proper thermal profile characteristics, and choosing the right flux as part of assembly precautions in order to insure long term PTH reliability.

Currently, the lead-free assembly industry is experiencing an anomaly called “hot tear,” which occurs with the use of SAC305 lead-free solder. As shown in Figure 1, hot tear is a crack created in a through-hole component joint. SAC305 manufacturers are now investigating this problem.

With lead-free projects, PTH reliability is also dependent upon the right choice of flux at PCB assembly levels. Contract manufacturers (CMs) are now dealing with a very narrow window of flux activity at a considerably higher temperature levels. At these higher temperatures, error tolerances are greatly reduced. Without the correct solder and flux composition and proper thermal profile, PTH reliability is severely jeopardized.

At Design

A major function in DfM of the PCB is to determine accurate calculations for through-holes and plated slots. These calculations apply to determining via diameter in relation to annular ring size; another is setting correct via spacing and consistently maintaining that spacing for good signal integrity.

Accurately calculating drill hole and annular ring sizes is critical. A larger hole diameter requires a larger annular ring. Miscalculation in annular ring design creates long-term reliability issues, especially at rework levels. Experienced and disciplined designers refer to IPC610D and IPC221A charts to obtain PTH and annular ring definitions as a basis for making precise calculations.

The designer must also carefully select the proper type of copper foil when designing a PCB. Choices for copper foil are typically one-half, one, or two ounces. When one-ounce copper foil is used, each square inch of the board will be layered with one ounce of copper. However, in an analog PCB application designed for higher current, a two ounce copper foil may be required to insure the capacity for proper current flow. The choice of the correct internal copper foil is an important consideration in PCB long-term, reliability.

The copper deposited on the hole wall through the plating process is also critical in the PTH reliability equation. If insufficient copper is deposited on the via wall long-term reliability can be affected. The problems created by thin copper can include fractures or cracks in the barrel of the via, leading to poor solder joints and intermittent or broken continuity in the current flow. Recently the IPC reduced it’s the PTH thickness requirement to a minimum of 0.8 mils. In some cases this may not be sufficient copper for a particular design. If designers require more copper in the PTH they will need to specify it on the drawing or the fabricator can use the IPC guideline.

Connectors and sockets require special design attention. Performing the right PTH calculations are particularly critical because connectors and sockets demand greater PTH precision, especially for press fit connectors. If drill sizes and annular rings are calculated poorly, there is the possibility of creating a through-hole that is too small to accept the connector. And while it may be possible to maneuver a component lead into a too-small plated-through hole, such actions often increase the probability of cracked vias in the middle of the wall, creating an unreliable connection. This problem can either immediately surface during test, or become a latent or intermittent defect, causing reliability issues for products in the field.

Reliability becomes a major issue when via-in-pad technology is used. In some manufacturing schemes, via-in-pad uses different materials and processes than are used in conventional through-hole via. In some cases, a conductive material such as epoxy or silver is used to fill the via. For via-in-pad, a PCB fabricator must fully understand how these materials will differ from the conventional via creation to maintain the board within its own specific tolerances. This is a challenge because the technology is still new, there are few fabricators that have the necessary trained staff, experience, and equipment to provide the high level of PTH reliability required today from this technology type.

Plated through holes using SMA connectors in high-speed designs require simulation modeling also known as via modeling. This software assures there are no reflections or discontinuities along along the electrical signal paths to assure certain transition points are smooth. This is especially important when there is a cable or connector used in the board. Therefore, the optimal design requires that through hole components and vias are designed with proper transition points, eliminating reflections, discontinuities, or sudden changes in signal impedance requirements.

Rework cycles should also be considered at PTH design. This is especially critical to press fit connector reliability, as the connector will not tolerate warp or movement of the board (see Figure 2) and is more prone to defects during rework cycles. Extremely accurate PTH design is required for these connectors, as after a mistake in the PTH process is made, reworking a press-fit connection is extremely difficult. If the via is too small, the connector won’t insert into the board; if too big, an intermittent connection is created.

One of the few rework options involves cutting and adding jumpers. As shown in Figure 3, an incorrect PTH calculation resulted in a too-small PTH. The via broke off the board, and an unsightly jumper wire is used to recreate the connection. Besides the aesthetic considerations, this repair method lacks the level of reliability of a properly designed via.

At Fabrication

When considering drill and pad sizes, a ± 3 mils tolerance is typical for vias up to a 10 mil size. Some are tighter, where tolerances could be +3 and -0 for a 7 mil or smaller sized via. There are specific guidelines for drill hole and pad sizes. For instance, a 10 mil drilled hole must have at least a 22 mil annular ring, allowing at least a 6 mil annular ring on each side. When space permits, a larger annual ring, such as 26 to 30 mil would be preferable, to create a more solid pad joint able to withstand several component extractions and placements, as well as being able to undergo multiple reflow cycles without incurring problems.

Aspect ratio, which refers to the comparison of board thickness to the smallest via created on that board, must also be carefully reviewed, as it plays an important role in PTH reliability. The greater the aspect ratio, the more difficult it is to fabricate the board and to assure that it’s plated through holes will maintain reliability throughout the product life. A 10:1 or 12:1 aspect ratio is standard, but there are now 30:1 ratio PCBs constructed for specialized applications. This 30:1 aspect ratio board requires more precise pad-to-hole ratio calculations for PTH reliability. It’s important to maintain robust drill sizes and annular rings for high aspect-ratio boards to avoid half-moons and breakouts.

The different board fabrication cycles need to be followed and precisely implemented. This includes a number of precise process sequences that must be at specific temperatures for defined periods of time. There is always the possibility that a quick turn order will have a few corners cut. The OEM must be sure that the PCB fabricator doesn’t shorten the bake cycles in order to meet tight shipment schedules. This can severely compromise long-term product reliability. Such shortcuts could result in numerous problems, from insufficient material cure and moisture absorption to incomplete stabilization of the processing materials.

In addition, if preventative maintenance is not regularly conducted on the various chemistries used plate and process the PCBs, they it may become contaminated or may stop performing as required. This contamination, as in the case of a contaminated copper electroplating bath, can cause vias to be more susceptible to cracking during the reflow cycles in the assembly stage.

A cross-sectional inspection and analysis, as well as other relevant testing, are often required to pinpoint hairline cracks in the via’s barrel (Figure 4). These defects may not be readily apparent and may pass a non-stressed electrical test at the PCB fabrication facility and through the final quality control without notice.

Two tests are used to further insure PTH reliability. The first is a PTH Life Curve; the second is a Current Induced Thermal Cycle Test (CITC). The PTH Life Curve test plots a curve that determines when and how a PTH fails during accelerated thermal cycling. CITC inspects and tests the different board materials, finishes, and via construction. Both tests determine through-hole longevity before they begin to show fatigue characteristics and other reliability issues that might not otherwise become apparent until the product is in the field.

At Assembly

Among the major requirements for maintaining high PTH reliability is a well organized, trained, disciplined, and comprehensive incoming audit and inspection for fabricated boards entering the assembly process. The focus here is on via stability and reliability. Inspectors should insure that sample vias, annular rings and holes are thoroughly plated and within tolerances as specified in the fabrication design documentation. This includes insuring that holes designed to be non-plated aren’t mistakenly plated through.

The board is also subjected to different thermal cycles at reflow and wave solder stages. Reflow incorporates multiple heat zones, gradually increasing board temperature then holding it at a peak level, then reducing it, so that the board experiences fatigue as a result of repetitive temperature cycling. Here, the Z-axis expansion of the via barrel alloy needs to match the expansion of the board material composition. Alloy composition is different than the board’s material composition, and this mismatch creates fatigue that can cause cracks in the via barrel when subjected to heat cycling.

Temperature spikes between critical zones within reflow ovens also have the potential of creating temperature fatigue at PTH and vias. This fatigue ultimately damages through-hole components when they undergo excessive wave soldering cycles.

Special attention must also be given to via size expansion and contraction since the board is exposed to varying temperature ranges. This is especially important for micro vias with thin plating on the via barrels. Via expansion and contraction – which is inevitable in the assembly process – will take its toll on PTH reliability unless all of the previously outlined design, fabrication and assembly guidelines are carefully followed. PCD&F

Zulki Khan is president and founder of Nexlogic Technologies; This email address is being protected from spambots. You need JavaScript enabled to view it.

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