No engineer expects defects to occur to his/her PCBs (Printed Circuit Boards). However, some commonly-seen PCB design issues are hardly figured out sometimes due to environmental aspects, improper applications of PCB board or even pure accidents. Thus, engineers should stop accidents from taking place to PCB but it’s more significant for them to take instant measures when being confronted with those problems.
Defect#1: Shorts on PCB
PCB short is one of the most common issues leading PCB boards to fail and there are a lot of causes for it. The causes will be discussed in the order of significance and emergency solutions will be given.
• Emergency measure#1. The foremost cause for short on PCB lies in improper design of pad. To stop pad from causing PCB short, the shape of pad can be designed to be oval instead of circle so that the distance between points can be enlarged and short can be avoided.
• Emergency measure#2. The incorrect placement direction of components can also make PCB suffer from short. Under such condition, component placement direction should be adjusted correct to stop short from taking place.
• Emergency measure#3. Another cause for PCB short is bent pins on SMT (Surface Mount Technology) components. To effectively solve this issue, solder joint should be 2mm away from circuitry.
• Other causes. Apart from the leading causes for PCB short mentioned above, some causes can never be neglected, including too large substrate hole, too low soldering temperature, bad solderability of board, unworkable solder mask, board surface pollution etc.
Defect#2: Dark or particle solder joints on PCB
• Emergency measure#1. Dark or particle solder joints on PCB mostly result from polluted melted tin and oxide much participation in melted tin, which makes solder joints suffer from much brittleness.
• Emergency measure#2. Another cause for this defect lies in the solder paste used in PCBA manufacturing. If solder paste contains too many impurities, solder joints tend to become dark in color or particle-shaped. Under such a case, solder paste should be modified or pure tin should be applied.
Defect#3: Golden yellow solder joints on PCB
• Emergency measure. Generally speaking, normal solder joints on PCB perform in silver grey. If solder joints on PCB become golden yellow, it mostly results from too high temperature. To solve this problem, the temperature in oven should be decreased.
Defect#4: Badly-performed PCB
When a well-designed PCB performs bad after its fabrication, it is mostly the result of environment.
• Emergency measure#1. The first environmental cause for board breakdown lies in extreme temperature or uncertain change in temperature. Besides, high humidity or high vibration also could lead board to badly performed or even failure. For example, temperature change possibly arouse PCB to be deformed so that solder joints may be destroyed.
• Emergency measure#2. The moisture in the air perhaps leads copper to be oxidized or eroded and the exposed copper lines, solder joints, pad or components will be unable to normally work.
• Emergency measure#3. If circuit board and components are covered with so much dust, air flowing and cooling will be affected, leading PCB to be overheated and degraded.
Defect#5: Opens on PCB
• Emergency measure. When lines are cut or solder paste maintains only on pad instead of component lines, opens will possibly take place. Additionally, opens can be also caused during manufacturing process or soldering process. The cause for line breaking lies in board deformation, falling or mechanical deformation. Similarly, chemical causes or humidity can also lead solder or metal components to be abraded, which may lead component lines to be broken.
Defect#6: Looseness or misplacement of components
• Emergency measure. In the process of reflow soldering, small components may be floated on melted solder and totally far away from target solder joints. Looseness or misplacement of components on PCBs possibly derives from insufficient board support, improper reflow soldering setting, solder paste or operation errors.
Defect#7: Soldering defects
• Emergency measure#1. External interference may keep solder moving before sodification, which is similar to cold soldering. This defect can be overcome by reheating for correction and solder joints should be far away from external interference when cooling.
• Emergency measure#2. Cold soldering is also a leading soldering defect that usually takes place. Cold soldering usually happens when solder isn’t correctly melted, leading surface to roughness and unreliable connection. Excess solder stops itself from being totally melted, which is also a cause for colde soldering. The emergency measure to defeat this defect is to reheat connection so as to eliminate excess solder.
• Emergency measure#3. The third defect that is met in soldering is bridging that refers to the fact that solder meets to make two lines connected. Bridging may cause unexpected connection, shorts, component breakdown or routing burning down when high current flows through.
• Emergency measure#4. The fourth defect concerning soldering in PCB is insufficient wettability of pins or leads, which results from excess or insufficient solder. Additionally, pad may be raised higher due to overheating or rough soldering.