Effects of stencils and thermal profile are reviewed.

Stencil quality and the major elements comprising it play a pivotal role in ensuring solder paste is properly deposited on a PCB’s surface mount pads. Experience in defining and cutting perfect stencils is critical because there are virtually no textbook solutions on how best to make stencils. When properly combined, stencil quality and other associated factors comprise a critical step before a thermal profile is applied to reflow soldering.

Creating the ideal thermal profile is equally important. In many instances, the common practice is to develop three basic thermal profile recipes: one for small PCBs, another for medium-sized ones, and a third for large boards. On the other hand, the perfect thermal profile meets a particular PCB’s requirements, and needs to be created separately, one part number at a time.

Suffice it to say, a poorly devised thermal profile can have devastating (read: expensive) repercussions.

Reflow soldering is a superset of a 100% accurate thermal profile. It is the most common means to solder a surface-mounted component to a PCB. The steps involve applying solder paste, placing components on the PCB, and reflowing solder in a conveyor oven. Reflow melts powder particles in the paste, joining the surfaces through the solidified solder to create a strong metallurgical bond.

Stencil quality. A stencil dispenses solder paste on surface mount pads. Stencil thickness, aperture sizes and frames (or non-frames) are factors that contribute to paste dispensing and ultimately the solder quality on the PCB.

Thickness can vary from 0.003" to 0.010" and determines the amount of paste dispensed on the pads. Larger SMT component packages (1206, 0805, 0603) require an 0.008" thick stencil, which deposits a larger quantity of paste. Smaller package sizes (0403, 0201) require a thinner stencil, about 0.005", and hence a smaller amount of paste on the PCB to accommodate the smaller pads. Even thinner 0.003" to 0.004" stencils are used for 0201 and 01005 packages. In these instances, very small amounts of paste are used, usually a small drop.

Using the wrong stencil thickness creates havoc on a PCB’s performance. For example, an 0.008" thick stencil used with 0102 components will result in an inordinate amount of deposited paste. During reflow, capacitors, resistors and inductors are prone to shorts because of the excessive paste. Conversely, using a thinner (0.004") stencil on 1206 or 0805 packages could result in cold solder or voids during reflow because of an insufficient amount of paste applied to the pads.

First article inspection will minimize or eliminate stencil thickness issues. Pre- and post-reflow quality assurance (QA) inspections on the first article are vital for catching and correcting these and other discrepancies. Those can include, for example, component damage or tilted or twisted components as a result of higher-than-typical oven temperature or airflow.

Wrong aperture sizes are another root cause of opens and shorts. Therefore, it is important to perform a design-for-manufacture (DfM) check after a post-layout database is converted to Gerber files, which creates the stencil apertures decodes. Gerber files are a non-intelligent combination of lines through X-Y coordinates. Consequently, sometimes there’s a chance of a software glitch that wrongly defines X-Y coordinates or aperture sizes. This causes opens and shorts at Gerber levels. A DfM check is of utmost importance at the CAM level, then, before PCB files are released to fabrication.

Jeopardizing the image is an aperture-related issue to consider. This occurs when careful attention isn’t paid to apertures. Creating a wrong aperture opening can define too large an image, creating a short or too thin an image, creating an open. Also, an aperture defines trace thickness, which affects board impedance. If trace thickness is properly defined, perfect impedance is achievable.

Non-framed stencils can pose even more problems. Without rigid framing, a stencil is not uniform. Nonuniformity can prevent a stencil from dispensing solder paste evenly on all surface mount pads. Again, during reflow, too little paste creates voids or cold solders; too much causes shorts or opens. A framed stencil typically provides the crucial uniformity to prevent these problems (Figure 1).


PCBs populated with BGA and CSPs require extraordinary care during reflow. The mistake designers make is to place vias next to BGA pads or around BGA peripheries. This hinders perfect soldering because solder is sucked into vias during reflow, rather than being consumed by the BGA balls. That occurs when vias are not properly masked or tented.

The better approach is to mask or tent BGA vias during PCB design. Even when this step is omitted, a DfM check mapped at the planning stages or CAM level can catch this. Otherwise, reflow becomes a major challenge and undetected vias with solder paste traces can cause highly evasive, intermittent BGA connections.

Thermal profile. A 100% accurate thermal profile goes hand-in-hand with reflow soldering. The perfect thermal profile precisely follows the curves and specifications set forth by the solder paste manufacturer (Figure 2). A typical reflow oven has seven to eight thermal zones, although a larger oven can have more. A thermal profile is composed of three specific cycles before it cools. One is preheat, in which a board is processed in two initial zones to heat it to a specific level. Second is soak time, in which two to three zones soak the solder paste and activate the flux. Last is ramp for the reflow at peak temperature. The board is kept under specific temperature cycles for these time zones before it is cooled. Figure 3 shows an example of careful thermal profile monitoring; the computer screenshot shows 170°C in zones 3 and 4 and 195°C in zone 5.



As mentioned, a common practice is to deploy one of three types of thermal profiles, depending on PCB size and largely based on guesstimates. Consequences include a high probability of cold solder joints or damaged components. The result: extra rework, assembly costs and product delays. The number and specific factors that affect a thermal profile’s accuracy include:

• Number of layers.
• PCB thickness.
• PCB dimensions.
• Number of planes.
• Type of PCB material.
• Component types.
• Air pressure applied.

Take, for example, the PCB thickness. A 0.062 board will have different thermal characteristics associated with the different zones compared to a 0.093 or 0.125 board. The thicker board will take more time to heat the board’s different segments. Likewise, layer count is important. The stackup structure containing the number of power and ground planes determines the amount of heat the board needs to absorb in any given cycle to create a perfect thermal reflow.

Component types are also an important consideration for generating a perfect thermal profile. If the board is mostly populated with through-hole components and a few surface mount ones, it will require less heat than a board largely populated with SMT parts. Moreover, a fully populated SMT board with CSPs, BGAs and 0201s will have a vastly different thermal profile than a PCB populated with through-hole and SM components. Further, the reflow oven air pressure must be carefully monitored to avoid blowing away smaller devices such as 0201s and 01005s.

Aside from these key factors, planning an accurate thermal profile includes avoiding thermal shock by ensuring gradual temperature increases or decreases between different oven zones, carefully monitoring soak time and peak temperatures, and following solder paste manufacturer specifications. Solder paste manufacturers provide a specification chart associated with a particular paste. It provides detailed information about ideal temperature ranges in different oven zones. Included are preheat, soak, reflow and cool-down cycles during the final stages of reflow. If an assembler follows these charts carefully, it can nail the thermal profile. But if these specifications aren’t followed to the letter, the subsequent poorly developed thermal profile could result in voids and dewetting if too hot, and non-wetting and solder joint fractures if too cold.

Proper thermal zone management is another major contributor to perfect thermal profile accuracy. Ovens must be well calibrated and routinely maintained. Specific segments must be lubricated on a regular basis, and manufacturer technicians must perform annual maintenance to calibrate the ovens. An improperly operating heater in a zone can apply the wrong temperature in a specific zone, thereby creating a defective and potentially unreliable product.

Finally, efficient reflow soldering demands a highly effective exhaust facility. A reflow oven manufacturer’s specifications detail the correct cubic feet per minute (cfm) exhaust. It needs to be measured on a regular basis, and if needed, calibrated. If an oven’s exhaust is lower or higher than spec, the temperature within different zones is jeopardized.

If hot air stays in a zone longer than is required, the board is subsequently heated more than what is specified for a given zone. If hot air is not present for a sufficient time, the board remains cold and proper reflow not performed. It may change flux activity at the solder paste level because the air may be too hot or cold. Correct air pressure is also important to avoid sucking out or altering location of such small devices as 0201s or 01005s. A good rule-of-thumb is to routinely check exhaust levels and the cfm to ensure proper calibration. By implementing these major principles and practices, perfect reflow soldering is on the way to being ensured.

Zulki Khan is president and founder of NexLogic Technologies Inc. (nexlogic.com); zk@nexlogic.com.

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