Faced with obsolescing components and attracted by SMT benefits such as smaller board size, expanded functionality in the same space, and increased reliability, many product manufacturers decide to convert existing board designs from PTH to SMT technology. Whether the redesign is to be undertaken in-house or outsourced, PTH to SMT conversion is not necessarily a straightforward process. By carefully considering the starting point and design goals along with potential pitfalls, manufacturers and designers can set realistic expectations as to the time, effort and budget required for a successful conversion.

CAD Conversion Options
When a PTH design is three years old or more, it’s almost a given that the CAD software used in creating the design has been updated, is obsolete or is no longer supported. This leaves the engineer/designer with three choices: (1) convert the schematic and PCB layout files into a format usable for current CAD software, (2) revive the version of the CAD software originally used to generate the design, or (3) start from scratch, creating the schematic and SMT board layout using current CAD software.

Option One: It makes perfect sense to take advantage of the most recent CAD capabilities for designing schematics and board layouts. However, it doesn’t always make sense to convert older CAD files into a format that enables full use of current CAD software. Nor is it always possible. Despite the fact that many CAD packages attempt to support upward compatibility, converting the original CAD files may require many hours of effort to establish a current layout database or schematic. When a totally different CAD package is to be used, available conversion utilities and services are often limited and yield design files that require additional clean-up.

Option Two: Although reviving an older version of CAD software may appear to be the “easiest” solution, this approach will not take advantage advanced new CAD software capabilities that help optimize designs and reduce design time. Furthermore, current computer hardware, peripherals and system software may present compatibility issues that prohibit or restrict the use of software designed to run on hardware that is now obsolete. For example, software optimized for use on a 386-based PC may have speed-dependent features that simply won’t run on a Pentium-based PC. In addition, if the design is to be supported for any length of time, this approach may amount to delaying the inevitable. The result is paying twice -- once for the update using old software and then again for starting over as future design modifications become necessary.

Option Three: Many times the most efficient approach to converting old CAD files for an existing PTH design is to start from scratch in current CAD software. The time required to establish new files often is equal to or less than the time required for updating databases and/or cleaning up artifacts from old, converted files. This approach also assures the ability to make full use of current CAD software capabilities.

Component “Swapping”

In an ideal world, updating the Bill of Materials for a PTH to SMT conversion would involve a straightforward “package swap.” Indeed, there is a common misconception that most PTH components can be replaced one-for-one with corresponding SMT components. If it only were that simple.

While exact SMT counterparts exist for PTH components such as resistors, capacitors, diodes and signal transistors, simple “swapping” does not work for most die-packaged components. When die-packaged components such as IC devices are part of a board, the engineer/designer must carefully consult the data books and review the circuit design to find SMT components with comparable circuitry.

More often than not, pin-out designations and pin counts need to be changed in an SMT conversion. Not only do pin counts and pin-out designations differ for a specific component, but there are pin-out differences between different manufacturers. Here are a few examples: the SMT counterpart of an 8 pin DIP IC device may be a 16 pin SOIC; two different manufacturers have different pin-out designations for the same device; the suitable replacement for a 40 pin PTH DIP package is a 44 pin PLCC SMT package. In the last case, the extra pins are typically for additional power and ground connections that must be reviewed and implemented in revised schematics.

In replacing PTH components with SMT components, the design impact of power rating/thermal dissipation requirements and circuit requirements need to be taken into account. For example, the mass of a large DIP package may have been great enough to absorb heat generated by driving a stepper motor in the original PTH design. When that DIP package is replaced with an SOIC of much lower mass and the power requirements of the stepper motor remain, the design may need to be modified. Even the replacement of a passive component such as an 1/8 watt resistor with its 0805 equivalent rated at 1/10 watt can require design modifications to meet power rating requirements.

A Logical Approach to Circuit Design

Many conversion projects include additional goals of performance enhancements and/or design improvements. In the conversion process, the engineer/designer must be cautious when weighing desired improvements against the performance of the original design, especially if the converted design must remain compatible with other existing equipment.

We do not always recall, or have access to all the information that influenced the original design. . Factors such as time-to-market constraints, project budgets, target product cost, experience of the original designers, design tools available, and circuit component availability may have influenced design choices in ways that aren't clear after the fact.

Typically, a designer studies the old design and thinks about how to convert and possibly reduce the amount of circuitry in a design. Depending on the nature of the design, there may be options for the converted circuit that didn't exist or weren't viable at the time of the original design effort. Care must be taken when examining these new options, however, because performance improvements in components can have unforeseen negative effects.

In the case of logic design, options may range from replacing an obsolete logic family with functional equivalents to consolidating a complex circuit into a programmable logic device (PLD). In either case, the timing behavior of the original circuit and the requirements of the circuits it interacts with must be very carefully considered. Subtle changes in timing can cause big headaches once you get to the testing stage of the design. Such changes can completely (or worse, intermittently) change the behavior of asynchronous logic circuits, or cause race conditions or runt pulses that weren't a problem in the original design. It's also possible that hardware changes will have implications for system firmware, or vice-versa. (Figure 1.)

Figure 1. In this example, 7400 series parts
are replaced with two alternatives: 74HC and
74AC parts. Uncertainties in timing are
shown as gray blocks. Where the original circuit
produces two pulses, the replacement
circuits may not produce both pulses reliably
(indicated by the solid black blocks in place of
a pulse). Even when pulses are reliably produced
in the replacement circuits, their timing
relative to the input signal “clkA” is different
from the original, which may complicate
interface with existing circuits.
















In the case of analog design, simply replacing an old op amp with a newer part can have hidden consequences. If the new amp is significantly faster than the old one, it may react to high-speed signals that the old part effectively filtered out. Depending on the circuit, the result could range from increased ringing to instability.

These concerns may seem overwhelming and make a project manager hesitate to promote such a design conversion. However, if circumstances require that the conversion be undertaken, it can be done successfully provided the design details are given the attention they require.

Space, Cost and Layout Design

Another common misconception of SMT conversions is that, without exception, an SMT board will be smaller than its PTH equivalent. This is not always the case, however. Nor is it always the primary objective. A number of layout factors influence the final size of an SMT board.

Unlike PTH, SMT does not enable ready access to components pins from any layer of a multi-layer board. Instead, SMT parts are connected between layers using vias (circuit feed through) which require a little board space. Routing requirements for even the smallest SMT components takes more space than the parts size might lead you to suspect.

For example, there may not be a significant gain real estate gain by converting an 1/8 watt TH resistor to a SMD1206 package that requires additional vias for signal connections to other layers. Buried vias, smaller packages (R0402) and other space saving techniques can be applied, but manufacturing and bare board costs may increase.

The tight pin spacing of SMT components generally does not allow routing through the space between pins -- a practice that is sometimes used to save space in PTH designs. As a result, routing demands for an SMT component may not save space over its PTH counterpart. For example, connections for data and/or address bus lines can often routed between the pins of a through-hole memory device, but not through the pins of its SMT replacement -- typically a fine pitch PLCC or SOL. (Figure 2.)

Physical constraints and enclosure size also affect final board size. For example, if the enclosure design is to remain unchanged, its LED readout must be designed to fit the enclosure and match original industrial design parameters. For example, a surface mounted LED may need to be designed to fit the space of an original T1-3/4 TH LED and needs to maintain the same level of visibility the end user.

Agency and Industry Compliance

Over time, government and industry compliance requirements change and expand. This means that an existing design may comply with older standards, but does not hold up to current standards. Once approved, most products are “grandfathered” through the years, remaining on the market despite changes in standards. When a product design is updated or revised, however, the product is expected to comply with current standards and will need to be resubmitted for approval. For this reason, current regulations and standards must be taken into account in a PTH to SMT conversion.

Most government standards have been developed to assure the safety, efficacy and reliability of products. Above and beyond government standards, industrial consortia often devise standards that promote product compatibility among different manufacturers and within an industry segment.

Hundreds of standards for product design and performance exist throughout the world. Major North American and European organizations impacting electronic-based products include: the Canadian Standards Association (CSA), European Community (CE Mark), FCC (Federal Communications Commission), FDA (Federal Drug Administration), IEC (International Electro-technical Commission), International Standards Organization (ISO), Underwriters Laboratories (UL), and German Safety Standard commission (VDE).

Which standards apply depends upon the product and its intended marketplace. For example, an electronic medical device to be marketed in the U.S. and Europe may require FDA compliance, a CE Mark, and FCC compliance. An elevator control mechanism to be used throughout the U.S. will be expected to meet specific UL safety standards, EMC (electromagnetic compatibility) standards, as well as national and regional building codes. A printed circuit board to be used in automobiles sold in Canada must meet Society of Automotive Engineers (SAE), CSA, and EMC standards.

Within the past 5 years, most agencies have added compliance requirements for electromagnetic immunity and electrostatic discharge (ESD) protection. Even for products not subject to electromagnetic immunity and/or ESD protection standards, changes in component technologies can lead to a design that is more susceptible to these phenomena. For example, the smaller size of today’s semiconductors may render the circuitry more susceptible to ESD during fabrication. And, faster logic devices and processors of today may respond to electrical noise pulses that may not have affected older, slower circuitry. Design modifications may be needed to ensure reliable performance and agency compliance.

Standards that have been changed and/or strengthened in recent years include creepage distances, electromagnetic compatibility, and safety requirements. It is also important to note that many established standards have remained, but are being interpreted differently or more tightly controlled than in the past. By identifying which specific standards apply at the start of the PTH to SMT conversion process, electronic engineers and designers can be sure to deliver a compliant design.

Setting Realistic Expectations

No matter how much effort has gone into estimating the amount of time and effort that will be involved, expect the unexpected! The unexpected may present itself at any point--or more likely several points--in the conversion process: updating original CAD files, replacing components that don’t allow direct conversion, dealing with design performance issues resulting from component replacements, and/or in complying with current agency standards. Or, the unexpected may present itself in researching product forecasts and part availability. Budget and/or scheduling nightmares can be averted by allowing for potential setbacks. Equipped with sufficient design tools, knowledge, and experience, the engineer/designer can successfully convert a board design and take full advantage of everything SMT and current CAD software has to offer.