MAGAZINE ARTICLE
Challenging Conventional Wisdom Can Optimize Solder Reflow
By Marc Peo, President, Heller Industries
Even though surface mount assembly has a relatively short history as a manufacturing technology, it has acquired a "heritage" of widely accepted assumptions about various stages of the process. However, advances occur so rapidly in this field that yesterday’s assumptions are continually subjected to the challenges posed by shifting paradigms on today’s production line.
In solder reflow, relying on commonly accepted practices can lead engineers to overlook some relatively simple factors that are critical to optimizing the solder reflow process. An examination of six of these situations reveals that challenging the conventional wisdom about solder reflow can contribute significantly to improved yields.
Conventional Wisdom #1: Figure 1 represents the "ideal" reflow profile. It is characterized by a 2 to 4ºC/second ramp-up to a dwell zone of 30-90 seconds at a temperature of approximately 150ºC, followed by a second ramp-up to approximately 210ºC.
Figure 1 "Standard" Reflow Profile
New paradigm #1: This graph does represent a satisfactory reflow profile for certain pastes and some ovens, but it is by no means suitable for all situations. Applications for which this conventional profile are appropriate include: a) pastes that require a dwell time at 150-160ºC to allow for flux activation and b) IR-based ovens that create large temperature differentials (D T) on the surface of the PCB. In the latter case, the time spent in the dwell zone reduces the D T by allowing lower-temperature areas to "catch up" to the higher-temperature areas, at which point the entire board achieves near equilibrium.
Today, however, many of the no-clean pastes that are being used with greater frequency actually require a reflow profile without a dwell zone. The typical form for such a profile, shown in Figure 2, is commonly referred to as a "tent" or "straight ramp" profile. For no-clean pastes, which have less active fluxes, excessive dwell time serves only to deplete flux prior to reflow.
Figure 2 "Tent" Profile
A second advantage of using a tent profile is that it speeds the reflow process, since residence time in the oven is reduced by the length of time formerly required for the dwell zone. As a result, when typical four- to five-minute profiles can be completed in three to four minutes, throughput increases of 20-25% can be realized. As an alternative, a shorter oven can be used to achieve the same throughput, conserving valuable factory floor space.
Conventional Wisdom #2: Reflowing double-sided boards in a full convection oven should be avoided, since it can cause components to be blown off the bottom side of the PCB.
New paradigm #2: All full convection reflow ovens on the market today are engineered to account for the process requirements of double-sided reflow. The development of improved controls for air velocity within ovens, combined with the surface tension of solder paste, holds bottomside components in place and eliminates the possibility of blowing components off during double-sided reflow.
Additionally, after eutectic solder has been reflowed once, the alloying that occurs at the solder joint during reflow effectively increases the melting point. At this higher melting point, the alloy no longer has eutectic properties, making it uncommon for bottomside components to be released during a second run through the reflow oven.
In some applications, components as large as 68-pin PLCCs can be suspended on the bottomside purely by the surface tension of the solder. A formula for secondary side mounting can be used to determine a component’s candidacy for bottomside attachment:
Weight of components in grams
_____________________________
Total pad mating area in square inches
Grams per square inch must be £ 30 for secondary side mounting
To handle components larger than 68-pin PLCCs, relays and other high-mass/low-lead-count packages, most PCB assemblers use epoxy to anchor these components. In other cases, solders with differential melting points can be used. For example, if the bottomside is reflowed first using a high melt-point solder (with a melting point greater than 183ºC), then the topside can be reflowed using a eutectic solder (with a melting point of 183ºC). In this procedure, there is no chance for bottomside components to reflow a second time. Thus, communicating process issues back to the design stage, and developing strategies to deal with them, is an important factor in resolving process challenges.
Conventional Wisdom #3: Tombstoning is caused by accelerated heating, when exceptionally high ramp-up rates — 6 to 7ºC/second — cause the solvents in the paste to boil, creating bubbles that can literally pop components out of place.
New paradigm #3: This was a possibility, until recent years. The development of non-water-soluble solvents, which have higher boiling points, has eliminated the bubbling issue. Nor do water-soluble solvents attract water that can boil off as heating progresses. Instead, incidents of tombstoning may be caused by several factors that can occur at various stages of the assembly process. These include inaccurate or asymmetric component placement, excessive pad size and uneven solder paste deposition.
Inaccurate component placement : As shown in Figure 3, the greater wetting force that is exerted on one end of an incorrectly placed component will pull the component up on end, creating a tombstoning effect.
Figure 3 Inaccurate Placement
Excessive pad size: When a solder paste pad is too large in relation to the size of the component terminal, the larger amount of paste relative to the surface area of the terminal can also result in tombstoning, as shown in Figure 4.
Figure 4 Ratio of Pad Size to Termination Size
Uneven solder paste deposition: When there is an excess of paste in the solder pad at one end of a component, the greater wetting force that is created during reflow can also lead to tombstoning.
Conventional Wisdom #4: Using nitrogen improves reflow processing versus air.
New paradigm #4: Using nitrogen does improve reflow results: it creates solder joints that are shinier, improves wetting angles and increases the "margin for error/process window." In addition, certain applications such as ultra-low-residue fluxes require nitrogen, due to their lower levels of activity.
However, in applications where nitrogen is not stipulated, many assemblers have found that nitrogen has been used as a band-aid to compensate for situations that can be fixed by careful process control. Poor soldering can result from many causes: improper deposition of solder paste, use of an inappropriate solder paste, solder paste that has been on the stencil too long, inaccurate component placement or an improper reflow profile. While nitrogen reflow may mask the effects of these problems, a simple process adjustment may be all that is required to improve reflow results.
Most important, making process control adjustments carries no ongoing costs, while nitrogen usage certainly does — there are usage, facilities and set-up charges involved in nitrogen processing. Therefore, it is necessary to review the costs versus the benefits of nitrogen before committing to the process. During the review, the issue of nitrogen consumption becomes paramount. Obviously, reflow ovens that consume less nitrogen will cost less to operate, as shown in Figure 5, and are easier to justify when they are in fact required.
Figure 5 Cost of Nitrogen vs. Consumption
Conventional Wisdom #5: When running nitrogen, the lower the PPM level of oxygen, the better the results.
New paradigm #5: Field testing has shown virtually no difference in reflow results from oxygen levels ranging from 15PPM to 100PPM. Wetting angles and joint strength have been found to be basically identical. In certain cases, it has even been reported that wetting forces at very low PPM levels may actually be too strong, creating yet another cause of tombstoning on small parts. Since running at a higher PPM level makes no difference to the overall process, and running at the very lowest levels consumes significantly greater amounts of nitrogen, this is certainly an area in which cost savings can be realized, as shown in Figure 6.
Figure 6 Nitrogen ($/hr) graph
Certain ovens offer a closed-loop nitrogen controller that allows users to minimize nitrogen consumption by maintaining a specified PPM level. These systems regulate nitrogen flow automatically and compensate for changes in board load, ambient air conditions and other factors. Therefore, these ovens not only save money for users, but also ensure consistent process results by maintaining PPM levels within an acceptable tolerance band.
Conventional Wisdom #6: In nitrogen processing, the cooling zones cause flux condensation, resulting in more frequent maintenance and increased downtime.
New paradigm #6: While the temperature of the water in the heat exchanger does cause flux condensation to occur, several systems are now available to address this issue.
Flux burn-off: Certain oven manufacturers offer a periodic burn-off system, which heats up automatically to vaporize flux as it accumulates. The flux turns into a fine ash and is exhausted from the oven with no ill effect to the process, the oven or the environment.
Flux filtration: A patented system is available to trap and remove flux prior to its entry into the heat exchanger. As a result, very little flux enters the cooling chamber to condense on the heat exchanger. The flux filters are easy to remove and replace with clean filters, even while the oven is running, eliminating the need for downtime to perform this maintenance procedure.
Waterless cooling: One system eliminates recirculated water heat exchangers, and thus removes the possibility of condensation inside the oven. In this enclosed system, the flux-laden gas is diverted from the heating zones and the flux is precipitated out using a cyclonic separation system and air-to-air heat exchanger. The cooled, flux-free gas is re-introduced into the cooling zone, while the flux is collected outside the oven chamber, allowing preventive maintenance to be performed while the oven is running and eliminating the need for PM downtime.
A review of these commonly accepted reflow practices indicates that the surface mount process often defies generalization. In many cases, each process is unique and requires individually designed methods to satisfy its own requirements. And, of course, there is no substitute for rigorous process control.
However, manufacturers are not without resources to assist them in fine-tuning their processes. To take advantage of rapid developments in advanced technology, assemblers should work in partnership with all their vendors, including suppliers of placement equipment, screen printers, solder paste, conveyors and reflow ovens. The sales and technical support staffs of these companies have experienced literally thousands of applications. Their expertise can provide valuable process input, at no cost to manufacturers.
When vendor consultation is combined with a manufacturer’s own engineering resources to challenge commonly accepted practices and optimize process control over each step in an assembly line, the stage is set for creating a highly successful operation with high productivity levels.