First of all, with the rapid growth of mobile phone technology, the Electronic Manufacturing Service (EMS) in mainland China sometimes meet serious labor shortage. Second, the Industry 4.0 promotes the requirement of factory automation for each EMS. Therefore, many parts which cannot adopt SMT reflow process are requested to fulfill Paste-In-Hole process (PIH process), such as the type-A USB connectors, Ethernet connector, power supply sockets, transformers, and so on. Originally, most of these kind of devices were touch-up soldered after SMT process.
Because of labor shortage, also for the purpose of saving process cost, and under the consideration of quality, many SI and EMS have requested that the parts which cannot adopt SMD process should fulfill at least PIH process, so that the soldering of all the electronic parts on circuit board can be completed after SMT process.
The change, however, should fulfill some requirements. Please refer to the article, Process Impact of Substituting SMD Parts with Paste-In-Hole Parts, to know the material requirements.
In addition, another problem occurs on the change. The larger/heavier parts on the first side may drop when second reflow because of re-melting. In fact, most companies have regulated in their DFx documents that all larger/heavier parts should be put on the same side of circuit board. However, it is more difficult to design with following the regulations because of the advance of technology and requirements mentioned above.
Is there any solution to avoid the drop of heavier parts on the first side as second reflow?
Some known solutions have been implemented. Some solutions are listed as follows for reference:
Solution 1: Dispense “Red Glue” on the side of or under the parts.
In the early period, the dispenser was required in the SMT process line because SMD parts post to dispensing can go through wave soldering. However, these is no dispenser in most of the current SMT process line. It is not recommended that dispensing is completed by operators if there is no dispenser. Since other parts may be contacted, the operation requires more manpower, longer working hour, and more difficult for quality control. Therefore, a dispenser can support better quality control.
The purpose of dispensing red glue is to paste parts on circuit boards. Therefore, the red glue should stick with parts, and then the circuit boards go through reflow process. The red glue can be solidified under high temperature and is irreversible. It means that the solidified red glue cannot be softened by heating.
If the red glue should be dispensed under a part, it should be practiced immediately after solder paste printing. Then mount a heavier and larger part on the dispensed part. Since the red glue may stick up the part, the operation is appropriate only for heavier and larger parts.
The other way is to dispense red glue on the side of parts. The operation should be practiced after solder paste printing and part mounting. Any incorrect operation may cause the risk of part dropping. Therefore, the PIH parts are adopted in general.
If dispensing red glue on the side of parts, both the red glue volume and position should be controlled precisely. After dispensing red glue on the edge of a part, the part should be pressed to fixed depth by nozzle of SMT machine in order to avoid the risk of part emergence.
Solution 2: Adopt Reflow Carriers
Reflow carriers can be designed as rib type. They can support the heavier parts so that these parts will not drop easily during the second reflow. However, the cost of each reflow carrier is expensive. In addition, the length of total arranged reflow carriers should be longer than the length of reflow oven. It means that how many reflow carriers can be operated in the reflow oven at the same time should be concerned. Considering the buffer and spares, there are about 20 or more carriers. The total cost is actually expensive.
In addition, since reflow carriers should be operated repeatedly under high temperature in the reflow oven, they are made of metal or special high-temperature-endurable plastic materials. Also, adopting a reflow carrier should cost an extra manpower to put/get boards on the carrier. That’s an other issue.
Moreover, adopting reflow carriers may cause the risk of poor soldering. Because most of the metal materials which made reflow carriers, are good at thermal conduction, so it is difficult to heat up in the oven. Therefore, the reflow carrier should go through reflow oven together when measuring the oven temperature. Also, all redundant parts of each reflow carrier should be removed, and only make sure no any distortion during reflow.
Solution 3: Adjust the Upper/Lower Oven Temperature of the Reflow Oven.
The upper/lower oven temperature can be adjusted for common reflow ovens. In the earlier time, the size of electronic parts were all 1206. The lower oven temperature was often adjusted to be 5-10°C colder than the upper one. The purpose is to avoid the drop of parts on the first side because of re-melting during the second reflow. Now, most SMT engineers have not adjusted the temperature because all parts are small, so there is no drop risk in general.
As the aforementioned, larger parts on the first side will almost drop during the second reflow. Some devices, such as connectors, are larger and heavier, so their parts will almost drop even if the upper/lower oven temperature has been adjusted.
Therefore, the adjustment of reflow temperature applies only to small parts. If almost all parts drop, the solution is invalid.
Solution 4: Adopt Touch-up Soldering (Machining Soldering or Manual Soldering)
Please compare the cost of touch-up soldering and that of part drop. At the immature time, introducing factory automation may not save the cost. The comparison results can provide further information in detail.
In addition to manual soldering, the machining soldering can also be considered to support touch-up soldering because the quality of machining soldering is more stable.