How to avoid defects in PCB Assembly?

After working so hard for a couple of months, you finally managed to buy that new smartphone you are dreaming of. Finally, you are the first one among your friends to own it. Now imagine the frustration it would bring if you find out your hard-earned gadget is defective. You cannot help but complain, venting all your frustrations on the manufacturers.

PCB-assemblyThese defects, as frustrating as it is, are often rooted from the mistakes committed during the production process. For instance, in the quick turn PCB assembly stage, a single mistake can affect the entire electronic production. However, while mistakes are part of every process, it can be very much avoided. And to avoid these defects, it is essential that one must know and understand its root causes to understand how to prevent it. Check out the following defects in PCB assembly and their root causes to prevent yourself from committing the same mistakes!

Component Shift

The misalignment of components to its target is called Component Shift. It happens during the reflow stage, wherein components can float on the molten solder, causing realignment.
Component Shift is also caused by mismatched parts to pad geometry, pulling the part towards the closest thermal mass. Moreover, misshapen and bent leads, heat sinking of asymmetrical components, and the oxidation of component may also cause this defect.

However, prevention can be done to avoid this. It includes following the required humidity and temperature and improving the accuracy of components in pick and place machines. You can also lessen the amount of movement the un-reflowed assembly sees, and use aggressive flux to improve the component or PCB’s solderability to prevent the defect.

Corrosion due to Flux Residues

Flux is often used before soldering to reduce the oxides that are forming on the copper surface. However, the problem with this is that it can cause corrosion of the surface since it contains corrosive contents such as chlorine or bromine.

To get rid of these inorganic fluxes, the “no-clean” flux, which is composed of organic acids such as adipic or citric, was introduced. This kind of flux is being marketed as a no rinse flux, since organic acids decompose upon being subjected to temperature during the soldering process. Being a no-rinse flux, some manufactures do not bother to clean it.

This organic flux however may not decompose during a wave soldering process, resulting to a flux residue. Since the flux is being shielded from the molten solder, it may not reach the correct temperature for decomposition. This flux residue is still acidic in nature and is possible to cause corrosion in the area it was trapped.
Corrosion can be prevented by removing the flux residue as you clean the wave solder pallets. Use the same solvent that is used to dilute the flux and fiber-free cloths in cleaning the residue to avoid contaminating other products with debris.

Open Solder Joints

Open Solder Joints is caused by the lack of bond between the lead and the pack, or when there is solder only at the component lead but not on the circuit board. This results to an open connection on the PCB.

Other causes include the lack or inconsistency of solder paste or solder paste deposit, or the gap between circuit board pad and components leads. In addition, corrosion or contamination at component’s lead, weak solder paste and poor reflow profile may also cause it.

To prevent this defect, make sure you do the following during the PCB assembly stage: aspect ratio is correct, lead co-planarity is investigated and operator material handling procedures are monitored, extreme environmental effects are avoided, and PCB supplier’s fabrication issues are investigated.

Solder Joints Crack

Solder joints can develop cracks due to stress-induced factors as time pass by. This crack may lead to fractures or the complete separation of the solder balls to the ball grid array (BGA) or PCB. This results to displacement which causes the pin’s permanent failure and unwanted short circuits.

PCB assemblyExperts examined trends in electronics during recent years that may lead to the cause of this defect. These trends include the use of lead-free solder and the use of compact BGA components.
Lead-free solder is known for higher brittleness and are more prone to cracks under stress than the traditional tin-lead solder. On the other hand, despite the advantages BGA possess, it is not very efficient at even distribution of stresses.

Both trends when combined increase the possibility of crack if it exceeds the maximum allowable stress.
To avoid solder joints cracks, implement a strain Gage test on the PCB to make sure these trends are within the allowed stress limits. If it exceeded, rework or redesign the fixture to reduce stress to meet the required stress level.

Whether it is a cheap smartphone or complex machinery, defects in any form is a frustrating case. And in one way or another, these can be avoided if one is equipped with the proper knowledge and right skills.

With these problems in mind, Imagineering assures its valued patrons that we have a deep understanding, knowledge and skills to prevent these defects from happening. Expect only the most efficient, cost-effective and satisfying PCB assembly services that you deserve.

 

Quick turn PCB assembly is the process were many component are used and assemble in the circuit board, here a single mistake can affect the entire electronic production.

Continue Reading