Best Practices for Assembly and Fabrication of PCBs

As an expert manufacturer and PCB assembly company, Rush PCB Inc. uses several best practices when working with industries including aerospace, consumer electronics, automotive, and many more. The best practices involve PCB design, technical aspects, and assembly issues.

Best Practices Start Early

Efficient fabrication of PCBs that high-speed circuitry utilizes is essential to ensuring results for the end user. Often, the design of the PCB layout is not thought of as a proactive step in the process. However, it requires advanced thinking and adherence to important factors to provide designs where the results lead to successful fabrication of PCBs to achieve the desired functionality. Designers need to address the practice of DFM or designing for manufacturing and including extra considerations for demands of high speed circuits early in the design stages of board layout rather than taking them on as an afterthought.

The results of poor layout show up during fabrication, assembly, and later on, as issues related to performance when putting fabricated PCBs to testing or production use. However, at that point of time, redesign or rework can be exponentially more expensive and time-consuming, requiring evaluation of circuit failures and reconfiguration of layouts of prototypes.

Material Handling

Best practices in the assembly process of any PCB assembly company start with material handling of PCBs, solder paste, and SMD components.

PCB Handling

Resin coated foils, prepreg, and core materials are susceptible to damage while handling. They need handling by their edges by operators using clean latex or nitrite gloves. Prepreg needs storing on a flat surface in a cool dry environment, preferably at less than 23°C and lower than 50% relative humidity.

If the room temperature of the PCB assembly services is significantly higher than the storage temperature, the prepreg needs to be acclimatized to the ambient temperature, prior to starting assembly. During acclimatization, the prepreg must remain in its sealed package for the stabilization period to prevent any moisture condensing on it. Any unused prepreg must be returned to their package bags and resealed. Therefore, it is best to package PCBs in brick counts that closely emulate run quantities. Prepregs must not be folded.

Some moisture is inevitably absorbed into the PCB material during the time the fabrication process is completed and start of exposure to the assembly soldering. Removal of this absorbed residual moisture may need baking the PCBs at 105-125°C for 4-6 hours.

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Solder Paste Handling

As solder paste is a shelf-life dependent item, it should be put directly into a storage refrigerator of the PCB assembly services on delivery, and stored as FIFO or first in first out manner, preferably with refrigerator temperature below 10°C. Preferably store solder past in lots, and ensure older lots are used first for optimal material management.

Manufacturers usually print the manufacturing date on each label and include a use by date for best performance of PCB assembly in UK. This must be strictly followed. Prior to use, equilibrate the solder paste to the environmental conditions where it will be used. For a jar or cartridge of solder paste, it is best to remove from refrigeration one day prior to use. This allows the solder paste plenty of time to equilibrate in the environment. However, this is not recommended for syringes.

Never expose solder paste to heat greater than 25°C for bringing it up to temperature fast. However, temperature-controlled water bath at around 25°C may be used. Whenever removing a container from refrigeration, label it with the date of removal for monitoring exposure.

Although homogenizing solder paste prior to use may not always be required, if necessary, stirring with a plastic spatula is recommended. Solder paste removed from the stencil must always be stored in a separate jar, rather than reintroducing it into fresh paste, as this can result in process inconsistency. Do not return solder paste to the refrigerator after opening the container, as this can cause condensation and compromise performance.

SMD Components Handling

While storing SMD components, it is essential to ensure they are kept in conditions that prevent moisture ingress and avoid electrostatic charge build up to prevent any damage.

While storing incoming SMD material, PCB assembly in UK such as Rush PCB Inc., use an ERP system help to keep track of information such as delivery date, order number, and material data. If unused material is returned to the stores, the ERP system can keep track of the used components, rejections, and damaged SMDs.


Best Practices for Screen Printing

Consistent stencil printing requires proper board support, typically provided bch as y vacuum tooling. Adequate paste must be used to enable a generous bead to roll freely when the squeegee moves. The squeegee pressure must be adequate to ensure a clean sweep without leaving paste on the stencil after a pass.

Enable proper gasketing to align the apertures with the pads properly. Ensure levelness of the board surface, and solder mask definition must not detract from contact between the stencil and the surface of the board.

Occasionally wipe the underside of the stencil to remove any excess paste. Although wipe frequency is recommended with the product data sheet, it also depends on the process optimization and proper gasketing.

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Best Practices for Reflow Soldering

For best results, the reflow soldering profile should be broken down into four phases—preheat, pre-reflow, reflow, and cooling.

The preheat phase allows preconditioning the PCB assembly prior to the actual reflow. It removes flux volatiles while reducing thermal shock to the assembly.

The pre-reflow phase uses the flux activator to remove any existing surface oxide from the PCB pad finishes, component leads, and any oxides on the powder particles within the solder paste. Basically, it prepares the surfaces to be joined during reflow. This phase also involves a temperature soak, allowing the thermal gradient across the PCB assembly to equilibrate prior to reflow.

The actual reflow of the solder alloy allows the creation of a suitable electrical and mechanical bond. Formation of an optimum bond involves two critical parameters—the peak temperature, generally 20-30°C above the liquidus temperature of the alloy, and the time-above-liquidus, typically 30-90 seconds to form the effective intermetallics.

To form a reliable mechanical bond, the grain structure should be fine, which can be formed via the cooling phase. A rapid cooling rate while transitioning from liquidus to solidus can stress the joint; therefore, a cooling rate of 4°C/second is preferable.

Best Practices for Handling PCB Assemblies

ESD is one of the major causes of failure of assembled PCBs. Therefor proper electrical grounding of worktables and operators is necessary. Worktables must have electrically conducting mats and workers must wear anti-static clothing, while being grounded with discharge straps on the wrists or ankles.

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