Exploring Double-Sided SMT Reflow Soldering and Key Aspects of Component Placement

Exploring Double-Sided SMT Reflow Soldering and Key Aspects of Component Placement

In the realm of PCB assembly, the go-to technology in SMT (Surface Mount Technology) is what we call “Full Board Reflow Soldering.” While there are other methods for assembling PCBs, this “Full Board Reflow Soldering” can be divided into two flavors: “Single-Sided Board SMT Reflow” and “Double-Sided Board SMT Reflow.” These days, single-sided reflow seems to be taking a back seat because double-sided reflow lets us save precious space on the PCB, making our products more compact. That’s why you’ll find most boards in the market are crafted using the double-sided reflow technique.

(By the way, if space isn’t an issue, the single-sided board process could cut down one SMT cycle. When you weigh material costs against SMT labor costs, single-sided might just be the more cost-effective route.)

But, of course, there’s a catch with the “Double-Sided Reflow Soldering” process—it requires two trips through the reflow oven, bringing about some process limitations. A common challenge is that, when the board revisits the reflow oven for the second time, components on the first side might fall off , thanks to the remelting of solder and the pull of gravity. This becomes particularly tricky when the board reaches the high-temperature reflow zone. In this article, we’ll unravel the nuances of component placement in the double-sided reflow process.

(By the way, have you ever wondered why most of the smaller components soldered on the first side stay put during the second trip through the reflow oven? Why is it usually the weightier components that pose a risk of tumbling off?)

Which SMD Components Should Go on the First Side for Reflow Soldering?

In simple terms, it’s a good idea to place smaller and find pitch components on the first side when using reflow soldering. When the printed circuit board goes through the reflow oven for the first time, it undergoes less bending, and the accuracy of solder paste application is better. This makes it a favorable environment for installing smaller components.

Moreover, smaller components are usually lighter, reducing the chance of them falling off during the second pass through the reflow oven due to gravity. When these components, initially placed on the first side, are flipped to the bottom during assembly of the second side, they face downward. This minimizes the risk of components dropping off as the board enters the high-temperature reflow zone.

Additionally, components on the first side of the board must endure two reflow cycles. Therefore, they need to withstand the high temperatures of reflow soldering multiple times. For example, resistors and capacitors are often designed to endure reflow temperatures at least three times. This design consideration accounts for situations where a board might undergo reflow soldering again, like for maintenance or repair purposes.

Which SMD Components Belong on the Second Side for Reflow Soldering? This is crucial.

  1. Size and Weight Matter:
    Place large or heavier components on the second side to prevent them from falling into the reflow oven during the second pass.

  2. Consider LGA and BGA Components:
    It’s recommended to put Land Grid Array (LGA) and Ball Grid Array (BGA) components on the second side. This avoids unnecessary re-melting of solder during the second pass, reducing the risk of soldering issues. For fine pitch BGAs, placing them on the first side might be considered.

    However, the debate continues on whether BGAs should go on the first or second side. Placing them on the second side avoids re-melting risks but may cause more PCB deformation, impacting solder joint quality. Therefore, for fine-pitch BGAs, putting them on the first side could be an option. Yet, if the PCB is significantly deformed, placing such components on the second side may affect solder paste printing accuracy.

    PCB deformation not only affects the soldering quality of components but can also lead to solder cracking issue with BGAs during the integration of the assembly into the final product. During PCB assembly, it is assumed that  the PCBA remains free from deformation. However , if deformation occurs, there is an attempt to restore the distorted PCBA to its original shape, resulting in solder fractures that were previously securely joined. Utilizing reflow carrier  tools to minimize board deformation is a commonly considered solution.

  3. Handle Temperature-Sensitive Components with Care:
    Components sensitive to high temperatures should be placed on the second side to avoid damage from prolonged exposure to reflow temperatures.

  4. Watch Out for PIH/PIP Components:
    Paste-In-Hole(PIH)/Plated-In-Pad(PIP) components should generally go on the second side unless their pin length exceeds the board thickness. If not, the protruding pins may interfere with the stencil on the second side, causing abnormal solder paste printing issues.

  5. Special Attention for Components with Internal Soldering:
    Components with internal soldering operations, like connectors with LED lights, need special attention to temperature resistance for multiple reflow cycles. If they can’t handle the heat, they should be placed on the second side during assembly.

  6. Be Mindful of Warping:
    When components are placed on the second side, the PCB has already been through one reflow cycle, causing some warping. This makes it challenging to control solder paste printing, increasing the risk of defects. So, avoid placing tiny components like 0201-sized and fine-pitch components on the second side. For BGAs, opt for those with a larger ball diameter to minimize potential issues.

Looking at the pictures of the SD card PCBA presented at the start of this article, you should be able to easily figure out which side is designated for the initial component placement and reflow soldering (first side) and which side is intended for the subsequent component placement and reflow (second side).

For more insights, check out:

Moreover, when it comes to the large-scale production of assembling electronic components onto PCBs, there are various manufacturing approaches. However, the choice of each method is typically determined during the initial PCB design phase. This is because how components are positioned on the board directly impacts the soldering sequence and quality during assembly, while the routing indirectly affects the entire process.

Presently, PCB soldering processes generally fall into two categories: full board soldering and localized soldering. Full board soldering includes methods like reflow soldering and wave soldering. Localized soldering methods for circuit boards encompass carrier wave soldering, selective mask soldering, and non-contact laser soldering.

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