Double-sided SMT reflow (Printed Circuit Board Assembly) is becoming more and more popular and getting complex day by day. So it is very crucial to know How to solder double-sided PCB to improve quality and save cost.
Due to demand for compact size and lite weight of electronic products like camera, smartphone, tablets, and laptops it forced the electronic product Designers to designers to dense and double side mountable PCB Assembly.
Double sided SMT Reflow Process Flow
In every Printed circuit board Assembly a certain set of instructions and processes are followed. Normally the fabrication of electronic components was done through robot bases and fully automatics machines.
The cost of one line set up is may up to 10 cores based on machine selections. The whole Printed Circuit board assembly process can be divided into the below steps. When a single side assembly completed the same process was followed for the secondary side fabrication process and all steps were replicated to produce a double side assembled board.
Now come to the point, during the double-sided assembly process the board was passed through the reflow oven on the almost same set of temperature settings that were used in the first-time reflow soldering process.
Components Falling Off During Reflow
Even after the second-time reflow soldering temperature exceeds the melting point of solder paste, the components that are hanging opposite to PCB Surface remain stuck-on PCB Surface and do not fall off in the reflow oven.
The reason for this can be described as during double-sided assemblies, the solder joints on the top side of the PCB board are inverted and re-flowed again. During the second reflow time, the temperature inside the oven reaches more than the melting points of the solder paste.
But still, components are held in place and do not fall off due to the surface tension, which helps to prevent the components from falling off due to the gravitational force. In other simple words, we can say that the flex was already evaporated due to high temperature during the first time reflow soldering process.
Even getting the higher temperature more than the melting point of solder paste the components still having sufficient strength and the holding force to hold and prevent them from falling off into the reflow oven.
Actions to Prevent components Falling Off During Reflow
In some cases double-sided SMT reflow the secondary side of a double-sided printed circuit board assembly, it was found that once the solder is molten, the component may fall off from the weakest of below three locations on PCB Assembly:
Below actions are recommended can be used to overcome components failing off problem during double-sided reflow soldering
- Apply some adhesive glue on the required particular specific components location.
- Re Fine-tune the reflow profile temperature settings and if possible try to reduce the temperature of the second reflow profile setting within the window recommended by the solder paste supplier.
- Request your design team to Redesign the PCB pads or modify stencil apertures of affected slots to get better results.
- Re-Check the component quality of view of designing parameters and pads for an oxidation-related issue and if the possible component of the different batch or manufacturers.
Now we can understand that, if the molten solder Paste should wet the pad, the adhesive force at the liquid-solid interface is strong than the cohesive force (within the molten solder paste), so the molten solder should be the weakest location and the component may fall off within the molten solder during the second reflow passing through.
In this article, we learned How to solder double-sided PCB. Because double-sided SMT reflow is highly in demand due to cost-saving and compact PCBA size of advanced technology products.