High-temperature Environments Create New SMT Challenges

Estimated reading time: 8 minutes

By Gary Kerbow

Through-hole components are becoming more obsolete as the demand for surface mount grows. Surface mount soldering, however, is a challenge in high-temperature environments. Laser soldering can best meet this challenge in several situations.

Surface mount components used in products that must work in high-temperature, hostile environments face processing challenges similar to those found when changing from standard solder paste to lead-free.

Figure 1. A typical reflow profile as indicated by a paste manufacturer for a 200° to 220°C solder paste formula.

The Sn/Ag/Cu solder, with or without the many different additional alloy percentages that currently are being tested, require a much higher reflow temperature than standard paste. Liquidus for 63Sn/37Pb, the typical paste formula, is 183°C. Some lead-free formulas have melting points of 200° to 220°C and higher. Add to that the typical 30° peak rise over melt point recommended by most paste manufacturers, and this translates into ±250° to 260°C, which must be held at liquidus for a specified dwell time (Figure 1).

To protect components, various sealing compounds are being developed, including new packaging alloys. Because this is the movement of the future and almost every manufacturer is at least experimenting with lead-free solder, component developers and board manufacturers are moving as fast as they can to accommodate the environments in which their products will have to survive. This will benefit all electronics manufacturers eventually, however, some industries currently face the double challenge of high-temperature processing and high-temperature working environments for their final products.

High-temperature Environment ChallengesThe oil industry, with its down-hole logging instrumentation, is one industry facing this double challenge. They have long been selecting components for heat durability. All components use through-hole and high melting point (HMP) solder (93Pb/5Sn/1.5Ag) with a liquidus of 295° to 305°C. HMP solder is not easy to work with to achieve high-quality solder joints, requiring skilled operators and the best equipment, but it produced the best results for solder joint integrity. The move to surface mount components over the last few years, however, has changed options radically.

Figure 2. This board shows an example of component types used in a typical simple SMT assembly for down-hole logging products.

Typical down-hole logging products can range from 1.5 to 4.5" in diameter and tool strings can be more than 20 ft. long, filled with sophisticated printed circuit board (PCB) assemblies of 12 layers or more. Components run the gamut from the latest microprocessor and digital signal processing chips to simple discrete devices (Figure 2). Pressure down hole can be up to 20,000 psi and "sour gas" (hydrogen sulfide) is common. Components are selected for temperature resistance and boards are composed of several temperature-resistant materials, with polyamide being the most widely used. This whole board structure is placed in a stainless steel protective housing and sealed with O rings. The housing is thick enough to protect the boards from the high pressure and the O rings seal out caustic gases. The housing does not, however, protect from the heat which is ±200°C on average, with an internal tooling temperature rise of 15 to 20°, bringing the temperature up to 215° to 220°C.

Typically, these tools do not remain in the hole for long. They are sent into the environment and withdrawn at a prescribed steady speed. This technique is used for readings taken on a wide range of measurements. The graphs produced indicate whether there is oil, then exactly where and how much there is. Many problems have been encountered in this hostile environment, but the HMP solder rarely has been the cause.

Today, finding the needed through-hole component selection is becoming virtually impossible, as they are just not being made anymore. Even with manufacturers conducting full life buyouts when they do find the components, the required instrument sophistication demands surface mount assemblies. The benefits of surface mount include higher density packaging, a wider selection of leading edge component technology and virtually unlimited instrument capabilities. The only drawback lies in effectively attaching the components.

With a melting point of 305°C (+30° ramp up for reflow), HMP is extremely difficult to use for mass reflow. Although some have had limited success reflowing simple discrete devices, most boards and components cannot tolerate mass reflow at that temperature. There is a tradeoff between effective wetting of surface mount device (SMD) pads and the integrity of the components themselves. This is both an immediate and a long run threat because there always is a chance that unseen thermal damage could cause early field failure.

This same challenge is faced when using lead-free solder formulas, but with a twist: what can be considered a high-temperature solder for mass reflow, like SN96 (96%Sn, 4%Ag) with a melting point of 221°C, is just at the tolerable point for most components at reflow. But down hole, the internal temperature of 220°C causes many pads to reflow, causing components to actually fall off the boards during test.

Other paste formulas tried with higher melting points of 240° to 255°C fared much better down hole, but at mass reflow of 270° to 285°C, some components will not make it through the reflow process. Subjecting components to 220°C down hole is not the same as subjecting them to 270°C or higher, held at liquidus peak for reflow. This brings the formula dangerously close to the upper limits of the components and PCB materials.

After failures in post-assembly test, in which boards virtually fell apart using solder with a 221°C melting point, it was determined that not all board configurations should be processed the same way. There are certain instruments that could, because of the application down hole, be assembled effectively using the SN96 or equivalent paste formulas. They endure reflow successfully and maintain integrity in the field.

Figure 3. An example of a successful profile where components have similar thermal mass configurations.

There also are a large section of boards that require a much higher temperature solder formula. Trying to reflow the SMT process with these temperatures does not work. When the larger components are heated enough to get the ideal profile for the solder paste, it exceeds the temperature specifications for the PCB itself. Many of the other components will not survive either (Figures 3 and 4).

Figure 4. An example of an unsuccessful profile where components have a wide variation in thermal mass configurations.

The Laser Soldering SolutionEventually, the decision was made to stop compromising and look for a completely different alternative to go back to a process in which there was no limitation on the solder to be used, including the original HMP. Experiments in this uncharted territory began with various forms of single point, also known as selective soldering.

A compromise was necessary here too, however, because any type of single point soldering cannot meet the speed of mass reflow for overall throughput. Some throughput speed has to be sacrificed for high quality. In this industry, quality is not an option it is imperative. Most runs are highly mixed and relatively low quantity per type batch. These conditions, therefore, make the throughput sacrifice a very reasonable compromise.

Hot iron selective soldering was ruled out because of pressure on the boards and component leads. Micro flame soldering produced too wide a halo of high ambient heat that tends to radiate, making it risky for very fine-pitched pads. Laser soldering offered the best conditions. Like micro flame soldering technology, it is noncontact. Laser soldering offers stability and easy adjustment of the energy source. The actual laser intensity can be varied with precision. Unlike flame technology, intensely high laser heat can be withdrawn in a second, leaving no measurable ambient heat. Additionally, a single laser setup typically can be used for wire or reflow soldering. There still are some mixed-technology boards being assembled in this industry, so the flexibility can be a real benefit.

Laser, or light soldering, offers exceptional life ranging from 10 to 30,000 hours. The laser control extends to power, frequency, time and focus. Coupled with a high-speed optical pyrometer, completely closed loop soldering may be achieved. Diode lasers are becoming available with up to 60 W of power and a beam down to 0.5 mm. The systems tend to be very tolerant of Z-axis variations, which are a particular problem with hot iron soldering.

Figure 5. Laser soldering of a typical board assembly.

The particular laser now being used is a 30 W continuous diode, which operates in the infrared band and has a standard red laser for aiming. The laser is controlled completely by software and can pinpoint intense heat to a very small prescribed area. Because the polyamide boards are very unforgiving and can be burned badly by concentrated heat to any single point, the laser must be integrated through a fully robotic or automated system. The fact that the laser beam itself is microscopic and can spot very small areas is a moot point if it is not programmable to detect only the right areas, in this case the pads of fine-pitch SMDs. A four-axis programmable (x, y, z, Q) machine is the type being used today in a series of experiments for multiple types of SMT board assemblies (Figure 5).The technology promises great uniformity because the amount of energy delivered to a pad can be controlled to within a few percent points. The heat remains localized to the pads and does not excessively heat the component bodies. The type of instant heat that reflows the solder at the individual pad also disappears as fast, which allows for very fast pad cooling. This delivers better mechanical properties to the solder connection.Laser selective soldering is an effective alternative for SMT boards that will see heavy use in hostile environments such as automotive, aerospace and other high-temperature atmospheres. Also, when it is best to avoid physical contact with parts, and where any formula of high melt solder is used, selective laser soldering can be a good alternative to reflow because it opens up the solder spectrum to include virtually any formula that is in use or that will be developed in the future basically anywhere highly localized heating is required and adjacent parts may be particularly susceptible to thermal damage. Laser soldering may not be the ideal solution for many high-volume production applications where the basic mass reflow speed is essential. It may, however, be an effective alternative for manufacturers experimenting with lead-free solder formulas, particularly where high-mix boards are produced. The quality is highly controllable and balances the lower production speed with defect-free product batches. Laser soldering is not meant to replace reflow, nor is it right for all applications, but is a very viable addition to the assembly line of particularly difficult SMT applications.

GARY KERBOW, president, may be contacted at Systronix Inc., 3924 Bluebonnet, Stafford, TX 77477; (281) 240-0515.