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In previous discussions, Workingbear introduced the “wave soldering” process, which was used in early day of PCBA soldering. SMT assembly, on the other hand, emerged later. Can wave soldering be used for Surface Mount Devices (SMDs) after SMT placement? Workingbear noticed that many people still seem confused about the relationship between wave soldering and SMDs. So, this article aims to explain the key process of making SMDs compatible with wave soldering.
Before we delve into the article, let’s clarify the difference between SMD and SMT. Sometimes you might hear people use SMT, and other times they might refer to SMD. These terms are sometimes used interchangeably, but there are some basic distinctions:
SMD: Surface Mount Device So this term represents components that can be used with SMT process or technology.
SMT: Surface Mount Technology So this term refers to a technique of soldering electronic components onto the surface of a printed circuit board.
Generally, wave soldering is primarily used for Through-Hole Devices (THDs) employing traditional Through-Hole Technology (THT). THDs are placed above the printed circuit board (PCB), with their soldering leads exposed through Plating Through Holes (PTHs) on the underside of the board. During soldering, the underside of the PCB glides over the molten solder wave like a barge, allowing the molten solder to adhere to the component leads, the inner walls of the PCB’s through-holes, and the annual pads, thus completing the soldering process.
Can SMDs go through the wave soldering process? Won’t SMDs placed on the bottom side of the PCB fall into the solder pot during wave soldering?
Typically, if SMDs are to undergo the wave soldering process, they must first be mounted to the PCB underside with red glue and then cured using an oven (usually directly using a reflow oven). The heat resistance of this red glue must exceed the temperature of the wave solder pot. Otherwise, SMDs will become brittle or melt when exposed to the high temperature of the wave solder pot. Of course, if the red glue process fails to hold or due to other factors, components might fall into the wave solder pot. Therefore, wave solder pots usually need to be shut down after some time to remove residues of fallen components. Otherwise, over time, these components left in the solder pot can contaminate the solder, leading to poor wave soldering quality.
Another important point to note is that not all SMD components can undergo wave soldering, which Workingbear will explain further later.
Can solder paste replace red glue when SMD components pass through wave soldering?
Someone once asked Workingbear, “If solder paste is applied during the SMT process to hold the SMDs before going through the reflow oven, can we skip applying red glue when passing through the wave solder pot?” This question puzzled Workingbear for a while at first, but later, it was understandable, considering that many friends might not have seen or understood the principles of wave soldering. So, Workingbear’s answer is, “No,” because the composition of solder paste is almost identical to the solder composition in the wave soldering, so its melting temperature is almost the same. In other words, if SMD components are only held by solder paste, they will melt along with the solder during wave soldering. If a higher temperature resistant red glue is not used to hold them, SMD components will directly fall into the solder pot.
Can SMDs Run Wave Soldering with dual-process of Solder Paste and Red Glue?
Is it possible to use both solder paste printing and red glue adhesive dual processes for wave soldering? This dual-process approach is indeed feasible. Typically, it’s done to reduce the occurrence of solder bridges and non-wetting during wave soldering. Wave soldering processes often create shadow effects behind large components, making it difficult for solder to reach the solder points or components underneath, resulting in poor soldering.
However, implementing this method requires an additional step, which increases costs. Additionally, there’s a risk of solder paste residue remaining under the red glue, potentially causing occasional quality issues. During solder paste application, sometimes excess paste can smear around the openings of the stencil, and if red glue is applied directly over these areas, the solder paste may not be completely removed by the wave soldering process, leading to solder residues causing electrical shorts or electrochemical migration over time due to moisture and potential differences. Furthermore, solder paste residue can also affect the adhesion of the red glue.
Which SMD Components Can Be Wave Soldered?
Components like BGA, connectors, transformers, QFNs, LGAs, etc., generally cannot undergo wave soldering because solder cannot penetrate beneath the components (as in the case of BGA, QFN, and LGA) to form solder joints. Additionally, some components may have gaps that allow solder to penetrate, causing short circuits or damaging the components (as with connectors and transformers).
Based on my experience, small chips larger than 0603, SOT, and IC components like SOP or SOIC with two rows of pins facing outward, can go through wave soldered. QFPs with four rows of pins facing outward can also be used to a certain extent, but fine-pitch QFPs are prone to short circuits. Small chips smaller than 0402 and ICs with pins bent inward (like PLCC) or under the body are not recommended for wave soldering, as they can cause solder shorting or non-wetting. (I have seen successful attempts of using 0402 components with red glue for wave soldering, with key considerations being the proper amount of red glue and suitable wave soldering conditions.)
Therefore, in wave soldering processes, components unsuitable for wave soldering are usually concentrated on one side of the PCB, while the other side only accommodates components suitable for wave soldering.
Can QFN and BGA Components Be Placed on the Second Side for Wave Soldering? (Using Selective Mask Wave Soldering)
Sometimes, due to PCB design limitations, QFNs, BGAs, and similar components have to be placed on the second side of the board, posing a challenge for process engineers. However, such Surface Mound Devices that cannot undergo wave soldering can indeed be placed on the second side of the PCB. We typically employ a process called “selective mask wave soldering”, where a wave soldering carrier or template shields these components from the solder wave. This prevents solder from making contact with components unsuitable for wave soldering.
However, there are specific conditions for using selective mask wave soldering, such as component height restrictions and the need to allocate space for the soldering carrier during component placement on the PCB. Additionally, the cost of the carrier is a factor to consider. For more details, refer to the article on selective mask wave soldering usage conditions.
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