Solution to Prevent Component Drops During Second Time Reflow Soldering

Solution to the Drop of First-Side Part during the Second Reflow Soldering

Due to the rapid growth of mobile phone technology, the Electronic Manufacturing Service (EMS) in mainland China sometimes faces serious labor shortages. Additionally, Industry 4.0 promotes the need for factory automation in EMS. As a result, certain parts that cannot undergo SMT reflow process must fulfill the Paste-In-Hole (PIH) process, including type-A USB connectors, Ethernet connectors, power supply sockets, transformers, and others. Previously, most of these devices were touch-up soldered after SMT processing.

Due to labor shortage and a desire to save costs without compromising quality, many SI and EMS now require that parts which cannot be processed through SMD be subjected to the PIH process, allowing for the soldering of all electronic parts on the circuit board to be completed after SMT process. However, these changes must meet certain requirements as outlined in the article What Process Impact of Substituting SMDs with Paste-In-Hole components?

Is there a solution to prevent heavier components from dropping during the second time reflow on the first side of the PCB? This issue can arise due to re-melting, and despite regulations in DFx documents, it can be challenging to design with larger/heavier parts on the same side. Are there any solutions to address this problem?

There are various solutions to prevent the drop of heavier parts during the second reflow process.

Solution 1: Dispense “Red Glue” on the side of or under the parts.

In the past, a dispenser was necessary in the SMT production line because SMD parts that underwent dispensing could go through wave soldering. However, most modern SMT lines do not have a dispenser. If there is no auto-dispenser available, it is not advisable for operators to perform the dispensing process. This is because it would require more manpower, longer working hours, and would be more difficult to control the quality due to the risk of other parts being accidentally contacted. Thus, having a dispenser would greatly aid in improving quality control.

The purpose of using red glue in the SMT production line is to paste parts onto PCBs. The glue should stick to the parts, and then the boards go through a reflow process. Once exposed to high temperatures, the red glue solidifies and cannot be softened by heating.

When dispensing red glue under a part, it should be done immediately after solder paste printing and before mounting a heavier or larger part on top. This method is only appropriate for heavier or larger parts, as the red glue may stick up the part.

Alternatively, red glue can be dispensed on the side of parts after solder paste printing and part mounting. However, any incorrect operation may result in the risk of parts dropping. Therefore, PIH parts are generally adopted to mitigate this risk.

When dispensing red glue on the side of parts, both the volume and position of the glue should be precisely controlled. After dispensing glue on the edge of a part, the part should be pressed to a fixed depth by the nozzle of the SMT machine to avoid the risk of part emergence.

Solution 2: Use Reflow Carriers

合成石過爐托盤(durostone reflow carrier) 合成石過爐托盤(durostone reflow carrier)

Reflow carriers can be designed with rib structures to support heavier parts and prevent them from dropping during the second time reflow process. However, each reflow carrier is expensive, and the total length of carriers required should be longer than the length of the reflow oven. This means that the number of carriers that can be used simultaneously in the reflow oven needs to be considered. Typically, around 20 or more carriers are needed, including buffers and spares, making the total cost expensive.

Furthermore, reflow carriers are made of metal or special high-temperature-endurable plastic materials to withstand repeated high-temperature use in the oven. Using reflow carriers also requires additional manpower to load and unload boards onto the carriers.

However, there is a risk of poor soldering when using reflow carriers because the metal materials used in most carriers conduct heat too well, making it difficult to heat up the boards in the oven. Therefore, each reflow carrier needs to be tested together in the oven to ensure that they are heated up correctly. All unnecessary parts of each reflow carrier must also be removed to prevent any distortion during reflow.

Solution 3: Suggests Adjusting The Upper/Lower Temperature of The Reflow Oven

Suggests adjusting the upper/lower temperature of the reflow oven, which is a common feature available in most reflow ovens. In the past, when the electronic parts were larger in size (1206), engineers adjusted the lower oven temperature to be 5-10°C colder than the upper one to prevent parts from dropping during the second time reflow. However, as the size of parts has decreased, this adjustment is no longer necessary and is not commonly practiced.

For larger and heavier parts such as connectors, adjusting the temperature alone may not be enough to prevent parts from dropping during the second reflow. This solution is only effective for smaller parts, and if almost all parts are dropping, then this solution is not valid.

Solution 4: Mixing High and Low Temperature Solder Paste

I don’t particularly recommend this solution because solder paste with a low melting point often has weaker solder joints. Prior to usage, it should undergo rigorous reliability evaluation. Also, having different solder pastes with varying melting points within a factory can risk misapplication.

The idea is to use high-temperature solder paste for the first side reflow and low-temperature solder paste for the second side reflow. This prevents the reflowing of solder on the first side components during the second reflow. For example, you might use SAC305 solder paste (melting point 217°C) for the first side reflow, and Sn42Bi58 low-temperature solder paste (melting point 138°C) for the second side reflow. The reflow profile for the second side should not exceed 220°C (preferably below 200°C). This way, the solder joints on the first side with SAC305 solder paste will not remelt during the second reflow.

Solution 5: Consider Touch-up Soldering (Machining Soldering or Manual Soldering)

The cost of touch-up soldering should be compared to the cost of part drop to determine its feasibility. It’s important to keep in mind that introducing factory automation may not always lead to cost savings in the early stages. Therefore, a detailed cost analysis should be conducted before adopting touch-up soldering.

Both manual soldering and machining soldering can be considered for touch-up soldering. Machining soldering is preferred due to its more stable quality compared to manual soldering.

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