The move to lead-free solders has its own challenges and hidden problems

Oct. 1, 2004
The details of lead-free soldering, and the impact of the transition to lead-free solders, are often lost in the seemingly endless discussions about costs, markets, and reliability.

The details of lead-free soldering, and the impact of the transition to lead-free solders, are often lost in the seemingly endless discussions about costs, markets, and reliability.

There are two chief methods by which assemblers attach components to printed circuit boards — through-hole mounting and surface mounting. Through-hole mounting is the older of the two methods, although their histories are intertwined.

Through-hole mounting

Until the mid-1980s, through-hole mounting was the primary way of populating a production-quality board with components. Although through-hole mounting places all of the components on the top side of the board, soldering takes place on the bottom side by a process called wave soldering. All components — ICs, capacitors, resistors, etc. — have wires that extend downward through holes drilled in the board.

To do the soldering, assemblers transport the board slightly above a tilted reservoir filled with hot liquid solder. Pumps continually force the molten solder over a dam, and the bottom side of the board moves upstream through this wave. Liquid solder makes contact with the entire bottom surface of the board, but sticks only to unmasked areas such as the bottom protruding wire connections at the bottom of the drilled holes where the solder helps provide interconnections.

Although assemblers use through-hole mounting on fewer boards than they use surface mounting, through-hole is still widely used, and a few lead-free wave-solder systems are in operation. It requires extra process steps: for some components, the wires must be at 90 degrees for insertion, and after wave-soldering, assemblers must clip off the tips of the wires. Despite these drawbacks, through-hole soldering is a reliable process, and has the advantage of little heating of the components themselves, which are located on the top of the board away from the soldering operation.

Surface mounting

During the 1960s, engineers developed film hybrids primarily for use in high-reliability military applications. Assemblers built these devices mainly on ceramic or glass substrates and necessitated the use of surface mounting components due to the cost and difficulty of producing holes in these types of substrate materials.

This stimulated a demand for the leadless components, in which end caps or pads replaced the wire leads. The demand for miniaturization in the commercial field led designers to place a few surface-mounting-type capacitive and resistive components on the bottom side of the board, which adhered to the underside. The wave reflowed the components during the wave-soldering process. As further developments took place, these components migrated to the top of the board and became the first surface-mounted components in the commercial field. As other component types became available, surface mounting largely replaced through-hole mounting.

In today's surface mounting, assemblers first print or dispense solder onto the board to form the pads. There may be hundreds or thousands of pads, since a single IC can have in excess of 100 pads. Next, assemblers place the components precisely on the pads, which places the leadouts of the component in contact with the pads, but not yet bonded to them.

The board, with components in place, then enters the reflow oven. The temperature of the whole assembly slowly increases until it is just below the temperature at which the solder will melt. There may be a dwell zone where the temperature holds just below the melting point to permit the various regions of the board to reach the same temperature. This is an important consideration because large, thick components heat up more slowly than small, thin components. This presoak dwell zone helps to minimize the time-temperature envelope necessary to melt and reflow all the joints involved in the final zone.

When the board has reached a uniform thermal state in presoak, the temperature increases above the melting point of the solder for a precisely defined time period. The goal in this "reflow zone" step is to have all of the pads melt and flow around their corresponding pins. Assemblers select a temperature that will permit all pads to melt, but which will avoid the high temperatures that can damage components.

Control of time and temperature is also critical, because oxidation of the tin-lead solder, and other metal elements can begin in a normal atmosphere; even minor oxidation can interfere with adhesion and conductivity. For this reason, some reflow processes happen in a pure-nitrogen atmosphere, where minimal oxygen is available to combine with the metals.

At the end of the prescribed time period, the board exits from the reflow zone and begins a gradual cooling. Each board has its own particular reflow profile involving specific temperature ranges and specific times, all of which have been carefully worked out to achieve uniform soldering and to maintain high product reliability.

The various phases of the reflow process are essentially the same for lead and lead-free solders, except that the temperatures necessary for lead-free solders are significantly higher. During all stages of reflow, various regions of the board experience somewhat different temperatures. The entire four-stage reflow process typically takes around five minutes, which does not permit complete thermal equilibrium over all regions of the board and in the interior of components having different thicknesses and different volumes.

The critical temperature is 260 degrees Celsius, established by more than 40 years of practical experience. As the board and components approach this temperature, damage is more likely to occur. Lead solder has the advantage that its highest reflow temperature is about 220°C — well below the critical 260°C threshold. Designing reflow processes for lead-free is inherently more difficult because temperatures are inevitably closer to the 260°C danger point.

The challenge for engineers is to use materials and processes that will achieve well-bonded solder joints in lead-free solders without incurring damage from the higher temperatures. The highest-temperature portions of the reflow profiles for conventional lead solder and for the most widely used lead-free solders.

As temperatures approach the critical the 260°C point, various components can be damaged in different ways.

Plastic-packaged ICs

High temperatures can damage plastic-packaged ICs in two ways. Damage can be caused by differences in the coefficient of thermal expansion among the various materials in the IC package. As the temperature increases, different materials expand — in three dimensions — at different rates. The influence of thermal expansion is less critical in the solder joint that forms between the pins and the pads on the board, because the liquid state of the solder will accommodate some expansion. Yet within the IC package itself there are differences in the rate of thermal expansion between the die, the lead frame, the epoxy, and other elements. Cracks form when heating stresses become high enough.

A crack resulting from thermal expansion might be in a location where it will break a wire inside the package, or crack the die, or in some other way cause an immediate electrical failure. Later electrical testing should catch this type of failure, and it can be remedied — although at some expense — by sending the board to rework and replacing the IC. Many cracks, however, do not initially cause an electrical failure. Instead they remain in place and grow during the service life of the board; the normal powering up and powering down of a system involves more opportunities for thermal expansion. As these cracks grow they become a real reliability threat because they can cause unpredictable numbers of field failures.

Humidity that flashes into steam at high temperatures also can cause damage within the IC package. The volume of water involved is minuscule, but the increase in volume that occurs when water becomes steam is so great that some form of internal damage to the component is likely to occur. One form is the infamous "popcorn crack," so called because its formation sounds much like a single kernel of corn popping. Popcorn cracks generally originate below the die and form a circular crack around the die and extending upward or downward, often reaching the surface of the package. They are so disruptive that they nearly always cause electrical failure of the component.

Humidity within the package can also cause delaminations between various material interfaces in the package. For example, the molding compound is supposed to bond firmly to the face of the die, but a heat-generated delamination can form at this interface and can expand during service. Eventually the delamination may reach and break wire bonds.

A delamination can also cause an electrical failure through corrosion. The thin gap is a natural collecting point for humidity and contaminants that gradually permeate the plastic package during service. The delamination can then become a miniature electrolytic cell that begins the process of corrosion.

Effects of high temperatures

The problem that both of these failure agents pose — differences in coefficient of thermal expansion and expanding humidity — is the likelihood of damage as reflow temperatures increase.

For engineers, the switch to lead-free solders means the peak reflow temperature will increase from the familiar 220°C for conventional lead solder to around 250°C for tin-silver, and to around 260°C for tin-silver-copper. At the same time, engineers must consider the imperfectly known characteristics of new materials specifically designed for the lead-free environment. Will the new finish layer on the lead frame adhere well to the epoxy? Will the epoxy stick to the die face?

Damage to passive components

Electrolytic capacitors are inherently intolerant of high temperatures because of the chemical process the capacitors rely on for their capacitive characteristics. Only tantalum capacitors, and recently niobium capacitors, can withstand temperatures above 260°C.

Aluminum electrolytics, the most volume-efficient members of the electrolytic family, are rated to withstand 260°C for only 10 seconds, a period that the lead-free reflow process is likely to exceed. The capacitance permanently alters when the reflow process exceeds this limit; sometimes the internal expansion can rupture the component's enclosure. Multilayer ceramic capacitors (MLCs) have a tendency to delaminate at the higher temperatures. During heating, the solder terminations at each end of the capacitor melt, while the ceramic layers simply expand. During cooling, however, the solder terminations solidify while the ceramic layers are still cooling and contracting, a condition that creates stresses that lead to delaminations among the layers.

Excess temperatures also can damage magnetic components such as ferrites. Each magnetic component has its own curie point, above which the magnetic alignment properties of the component are effectively changed. A typical ferrite might thus be limited by its classification for exposure to 235°C, give or take five degrees, for a period of about two seconds. There are also limitations on termistors, varistors, and other passive components that contain plastic elements. For these parts, manufacturers typically specify exposure to 260°C for not more than 10 seconds.

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