How to Solder double sided PCB
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.
The double-sided assembly has evolved in order to achieve higher component density. Circuit boards with double-sided tracks and component pads (land patterns) usually have copper pads on one side and vias on the other side. It’s important to place components on either side of a double-sided PCB. The solder pasting and component placement process are the same for a double-sided assembly that is totally surface mount.
THE MAIN ADVANTAGES OF DOUBLE-SIDED PCB ASSEMBLY ARE PCB SPACE SAVINGS AND LOWER PRODUCT COSTS AS COMPARED TO THE SINGLE SIDE PCB ASSEMBLY.
Double-sided SMT Reflow Process Flow
There are specific instructions and processes that must be followed to reflow soldering on both sides in every printed circuit board assembly. In most cases, electronic components are made using fully automatic machines and robots.
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.
So moving on, the board went through the reflow oven for the double-sided assembly process at the same temperature setting as the first reflow soldering process..
Double sided reflow soldering Process Flow
There are some risks involved with this process when it comes to double-sided printed circuit boards. It is warpage that is the predominant defect, while weak or failed solder joints are the secondary defect. Running the process twice increases the risk of a defective PCBA, but it does not necessarily require special equipment. The following steps can be taken by designers to prevent double-sided boards from exhibiting these defects:
Reflow Profile on Side 1 vs Side 2
Various characteristics of a circuit board assembly determine the reflow oven temperature profile, such as the size and depth of the ground plane layer, and the number and size of the components on the board. When a circuit board’s temperature profile allows reflow of solder onto adjacent surfaces, it is incapable of overheating and damaging the electrical components beyond their temperature tolerance. Reflow soldering is a conventional process that consists of four distinct stages, each of which has its own thermal profile: preheat, thermal soak (often referred to simply as soak), reflow, and cooling.
First of all, It is recommended to place the smaller components on the first side of the reflow oven because deforming the PCB when the first side passes through the reflow oven will be less. It will be easier to arrange solder paste solder-paste placement
Make a review of the temperature profile, reviewing the front backsides, top, and bottom, to ensure a maximum of 5 to 10 degrees C difference in temperature. Ensure that the boards are kept off the belts by the conveyor. Be sure that your boards are not warped or twisted when being reflowed for the second time, or for the first time, as this may affect the soldering joint.
Soldering on both sides of the PCB with vapor phase reflow has proven to be a reliable method to reflow parts. The first side should be cleaned later, after the soldering has been completed on the second side. This prevents moisture from forming on the board while soldering.
When possible, it is best to place heavy components (such as power components, large BGAs, etc.) on one side of the board (the second side). Make sure that your board is properly supported under the board if it is being reflowed a second time. Each side will have its own profile (it won’t necessarily be the same). As a result, you may have to shield the first side during the second reflow process from the peak temperatures.
Components Falling Off During Reflow
The components that hang opposite to the PCB Surface are still stuck to the surface even after the second time the reflow temperature exceeds the melting point of solder paste.
Due to the fact that the solder joints on the top side of the PCB board are inverted during double-sided assembly, this can be explained. During the second reflow, the temperature inside the oven exceeds the melting point of the solder paste.
The surface tension helps prevent the components from falling off due to gravity, but they are still held in place and do not fall due to the gravitational force. Therefore, when the flex was reflow soldered for the first time, it had already evaporated due to high temperatures.
While getting the higher temperature above the melting point of solder paste, the components still retain enough strength and holding force to prevent them from falling off into the reflow oven.
Actions to Prevent components Falling Off During Reflow
In some cases, when the secondary side of a double-sided printed circuit board assembly is reflowed with double-sided SMT, the component may fall off from the weakest of below three locations on the PCB assembly:
These actions may help you resolve the problem of components failing off while reflow soldering two sides at the same time.
The glue should be applied to the specific location of the component. Refine the temperature settings for the reflow profiles, and, if possible, reduce the second temperature setting within the window that the solder paste supplier recommends. For better results, ask your design team to redesign the PCB pads or modify the stencil apertures in affected slots. Review the component quality in relation to designing parameters and pads to determine whether there is an oxidation issue and if the component is of a different batch or manufacturer.
In this situation, the adhesive force at the liquid-solid interface at the molten solder paste surface is stronger than the cohesive force (within the molten solder paste), and the component may fall off during the second reflow through the molten solder.
Conclusion – Wrapping it up
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. Solder paste undergoes a phase change from a solid into a liquid or molten phase by reflowing at the eutectic temperature. A molten alloy demonstrating adhesion properties at this particular temperature range. Solder alloys are similarly cohesive and adhering to water. When molten solder alloys are in their liquidus state, they exhibit a characteristic called “wetting.”
Surface mount technology components or SMT are generally soldered to printed circuit boards or PCBs using reflow soldering with long industrial convection ovens. According to each assembly’s unique thermal requirements, the oven’s segments each have their own regulated temperature. By filling the holes with solder paste and inserting the component leads through the paste, reflow ovens intended for surface mount components can also be used to solder through-hole components. Although soldering multi-leaded through-hole components onto a surface-mount circuit board is common practice, wave soldering is the preferred method for attaching them.
On boards that contain a combination of SMT and plated through-hole (PTH) components, through-hole reflow, when achieved with specially designed paste stencils, can be used to eliminate the wave soldering step, potentially reducing assembly costs. Moreover, lead-free alloys such as SAC present a challenge for the temperature adjustment profile of the reflow oven and for specialized through-hole components that must be soldered manually with wire or cannot withstand the high temperatures directed at circuit boards as they roll along the conveyor. Intrusive soldering, which uses a convection oven to reflow solder paste onto through-hole components, is a method of soldering through-hole components