PCBA Tech - Solutions to avoid during PCBA secondary reflow soldering

With the rapid development of mobile phone technology, PCBA EMS (Electronic Assembly Factory) has reported serious shortages from time to time. Secondly, Industry 4.0 has made EMS factories increasingly demand for automation. Therefore, many of them are not necessarily able to pass the SMT process. Parts are also required to comply with the PIH (Paste-In-Hole) process, such as Type A USB connectors, Ethernet connectors, power sockets, transformers, etc., which are relatively bulky parts. Most of these parts are touch-up and hand-soldered after the SMT process.

Due to the shortage of labor, and to save subsequent process costs, on the other hand, quality considerations, many system factories and EMS companies are now beginning to require those parts that cannot be converted to SMD processes, at least to meet the PIH process. It is required that all the electronic parts on the circuit board can complete all the soldering processes of the circuit board as long as the SMT process is completed.

However, there are requirements for changing parts to SMD or PIH process. Please refer to this article [What is the difference and impact of changing SMD parts to Paste-In-Hole process?] to understand it. Material requirements.

In addition, changing all electronic parts to SMT process will encounter a new problem, that is, when the second side of the circuit board is passed through the reflow oven, the original parts that have been tinned on the first side will drop if there are heavier parts. In fact, most companies will define in their design specifications (DFx) that heavier parts must be designed on the same side of the circuit board, but with the evolution of technology and the various requirements mentioned above, RD It seems that it is becoming increasingly unable to comply with this rule.

Is there any way in the PCB factory process to prevent the heavy parts on the first side from falling when the second side is over the furnace?

I believe that most of the PCB engineers have already implemented the several methods that Shenzhen Honglijie currently knows. Here are just for reference to some friends who have not met:

Method 1: Put red glue under or beside the part

Related reading: What is the PCB "red glue" process? When should red glue be used? What are the restrictions?

In fact, in the early PCB lines, the glue dispenser is a necessary equipment, because the SMD parts that have been glued can be used for wave soldering, but most of the SMT lines now have almost no such equipment. If you don’t have a dispenser, you have to use manual to dispense glue. I don’t recommend manual work, because manual work consumes manpower and man-hours, and quality is difficult to control, because you will encounter it if you are not careful. For other parts that have been mounted, the quality of the dispenser is better to be controlled if there is a machine.

The purpose of red glue is to stick the parts on the circuit board, so the red glue must be applied on the circuit board and stick to the parts, and then pass through the reflow oven, and use the high temperature of the reflow oven to solidify the red glue. Red glue is irreversible glue and cannot be softened by heating.

If the red glue is to be spotted under the parts, the glue dispensing operation must be applied immediately after the solder paste is printed on the circuit board, and then the heavier parts should be covered on it. It should be noted that there is a risk that the red glue dots will prop up the parts under the parts, so generally heavy and large parts will work in this way.

Another type of dispensing operation will be on the side of the part. This can only be done after the solder paste is printed and the part is placed in a fixed position. If you are not careful, there is a risk of touching the part, so it is generally used for PIH parts .

If you use a machine to dispense glue on the side, you must precisely control the amount of glue and the position of the glue, place the glue on the edge of the part, and then use the nozzle of the placement machine to gently press the part to a fixed depth to ensure that the part does not float. risks of.

Method 2: Use a furnace carrier/carrier

The furnace carrier can be designed such that the ribs just support the position of the heavier parts, so that the heavier parts are not easy to fall during the second reflow soldering. However, the cost of the furnace carrier is not cheap, and the total number of carriers must be greater than the length of the reflow oven. That is to say, the number of boards in the reflow oven must be calculated at the same time, and the delay must be added. There are not 30 buffers and spare parts in total, but there are also 20. There may be more, which is expensive.

In addition, because the furnace carrier needs to withstand the high temperature of repeated reflow soldering many times, it is generally made of metal material or special high-temperature resistant plastic. There is also a special reminder that using Carrier will require an extra labor cost. Putting the board on the carrier requires labor, and the recycling and reuse of the vehicle also requires labor.

Secondly, the use of the carrier may cause the risk of deterioration of the tin melting condition, because the furnace carrier is usually made of metal, and the large area is easy to absorb heat, which will cause the risk of difficult temperature rise, so when adjusting the furnace temperature, it must be It must be measured together with the furnace carrier, and the carrier should try to steal the unused meat, as long as the carrier is sufficiently supported and not deformed as a principle.

Method three: adjust the temperature difference between the upper and lower furnaces of the reflow oven

Reflow soldering furnaces can usually control the upper and lower furnace temperatures. Early electronic parts are still in the age of 1206. When adjusting the reflow soldering furnace, the lower furnace temperature bar is usually 5-10°C less than the upper furnace temperature., The purpose is to hope that when the second side is reflowed, the parts that have been soldered on the first side will not fall due to remelting the tin, but now most SMT engineers do not adjust the furnace temperature in this way, because the parts are very small. There is no risk of falling.

With the above requirements, it is certain that the large parts on the first side fall during reflow soldering on the second side, but if it is a large and heavy part of the connector, even if the temperature difference between the upper and lower furnaces is adjusted, the part still cannot reach the part. Not required to drop.

Therefore, adjusting the upper and lower temperature difference of the reflow oven is only useful for small parts, that is, it is effective when it is found that some parts will fall, and some parts will not fall. If all parts are dropped, this method is invalid.

Method four, go back and re-welding after use (machine welding, manual welding)

Calculate the cost of re-soldering after the PCB board and the cost of the parts falling. Compare that cost-effective. Sometimes blindly pursuing automation when the time is not ripe may not save costs. It is better to compare and calculate whether Cost-effective.

In addition to manual welding for post-re-welding, robot welding can also be considered. After all, the quality of manual welding is more doubtful.