PCB Reflow Soldering in SMT Assembly Processes

PCB Reflow Soldering Overview

The main method for attaching surface mount parts to printed circuit boards during PCB Assembly is Reflow Soldering. This process ensures accuracy and dependability in connecting parts to the board. It is a vital step in SMT Assembly. Reflow Soldering uses a series of controlled heating and cooling steps. These steps melt the solder paste. They create strong and lasting joints between parts and the PCB.

Reflow Soldering Process

Reflow Soldering is a multi-step process. It ensures high-quality solder joints. Each step plays an important role in getting the desired results.

Initial Heating / Preheat

The first heating step warms the Assembly and solder to a set temperature at a specific speed. This speed is called the ramp rate. This phase slowly raises the temperature of the PCB and its parts. It prevents thermal shock. A common ramp rate is 2°C per second or less, as noted in the solder paste datasheet. Heating too fast can cause problems like solder splattering or beading. These issues harm electrical dependability.

Thermal Soak

The thermal soak step lasts 1 to 2 minutes. It brings all parts to the exact temperature needed for Reflow Soldering in the next step. This phase balances temperatures across the board. It ensures even heat distribution. Doing this step correctly activates the flux in the solder paste. It also prevents issues like graping or head-in-pillow (HIP).

Reflow

The Reflow step heats the oven, board, parts, and solder to the highest temperature in the process. The solder paste melts during this phase. It forms strong metal bonds between component leads and PCB pads. The temperature must go above the solder paste’s melting point. However, it must stay below the highest tolerance of any part on the board. Time above liquidus (TAL), usually 30 seconds to 1 minute, is key for proper wetting. This avoids damage.

Cooling

The final step cools the reflowed board at a steady rate. Cooling hardens the molten solder into strong joints. It reduces thermal stress. Controlled rates between 2°C and 4°C per second are best. Faster cooling creates finer grain structures in metals. This improves mechanical strength.

Nitrogen Reflow Soldering

A Reflow Soldering environment with nitrogen is often suggested or required by Assembly rules. It reduces surface oxidation. Oxidation can affect wettability. It causes issues like HIP. Nitrogen environments reduce these problems by removing oxygen during Reflow Soldering. This improves joint dependability. However, nitrogen Reflow Soldering has higher costs. It may also cause issues like increased tombstone effects or solder wicking.

SMT Oven Reflow Profile / Thermal Requirements

It’s important for designers, engineers, and oven operators to set the right SMT oven reflow profile and thermal needs for the PCB and parts. This avoids damage from overheating. Key factors include line speed, process speed, oven length, and temperature zones. These are tailored to specific materials and part types.

Reflow profiles are created by running test assemblies with thermocouples through reflow ovens. These tests ensure temperatures are enough for good joint formation. They also prevent damage. Most production-level assemblies use a conveyor-type convection oven. It supports boards along their edges for double-sided soldering.

Advanced ovens have automatic width adjustments for different board sizes. They also have special convection fans for precise control. Options include infrared ovens for radiation heating or vapor phase ovens. These offer energy efficiency and oxygen-free environments.

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Reflow Ovens

Reflow ovens are key to the Reflow Soldering process. They provide a controlled thermal environment needed for high-quality solder joints. The commercial-level Reflow Soldering process uses a reflow oven with a conveyor belt. The belt moves the PCB through various heat zones. These ovens ensure accuracy by keeping steady temperatures across multiple zones. Each zone is tailored to a specific stage of the reflow profile.

Reflow Oven Types

There are several types of reflow ovens. Each has unique heating methods and uses. Infrared ovens use ceramic infrared heaters to transfer heat via radiation. Convection ovens are more common in production settings. They heat air within the oven using convection and conduction. Some board needs require an oxygen-free environment to reduce oxidation. These ovens reflow PCBs using nitrogen. Vapor phase soldering ovens are another option. They use a liquid medium for thermal energy transfer. This method saves energy. It reduces overheating risks. It also prevents oxidation due to its oxygen-free environment.

Reflow Soldering for Plated Through-Hole Components (Intrusive Soldering)

Reflow Soldering is mainly linked to SMT. However, it can also be used for plated through-hole (PTH) parts in a process called intrusive soldering. Reflow Soldering can be used for plated through-hole parts, though wave soldering is usually preferred. This method is helpful when assembling boards with both SMT and PTH parts. It removes the need for separate wave soldering processes. This lowers overall Assembly costs. However, it needs careful consultation with experienced technicians to ensure the best results.

Reflow Soldering Common Defects & Issues

Reflow Soldering can face several issues if not carefully controlled. Knowing these problems and their fixes is key for reliable Assembly results.

Head-in-Pillow Defects

The head-in-pillow defect happens when the solder ball on a component package—Ball-Grid Array (BGA), Chip-Scale Package (CSP), or Package-on-Package (PoP)—and the solder paste on the board both melt but don’t join into one solder mass. This issue often comes from weak flux activity or misaligned parts during placement. X-ray inspection or other testing methods are needed to find this problem.

Graping

The graping issue occurs when solder particles on the solder mass don’t fully join. It is often linked to smaller parts and lead-free solders. Choosing the right solder powder with proper distribution ranges and oxidation barriers can reduce this defect.

Tombstone Phenomenon

Tombstoning happens when one end of a surface mount part lifts from the PCB. It stands upright, like a tombstone. Uneven wetting forces or uneven pad designs often cause this issue. Automated optical inspection (AOI) testing after Reflow Soldering helps find and fix such defects.

Solder Wicking / Solder Drainage

Solder wicking happens when a component terminal has a much lower melting point than the pad it is soldered to. This temperature difference causes the molten solder to flow away from its intended spot. Adjustments in reflow profiles and material compatibility are needed to prevent this problem.

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