PCBs (printed circuit boards) are widely applied in modern electronics industry, including laptop charger. Laptop charger features small volume and simple internal design with less material applied. As one of essential elements in electronics industry, PCB plays a key role as a stage carrying components.
PCBs applied by laptop charger are double-sided. As far as high-volume double-sided PCBs are concerned, if components on circuit boards are unsuitably designed, incompatible with design specs, or PCB boards suffer from issues such as oxidation, open circuits, short circuits and solder mask pull-off, soldering quality and performance will be affected including unqualified solder paste printing, PCB open circuits, short circuits and solder mask pull-off. This article will provide some solutions based on the above issues to further improve the quality of PCBs for laptop chargers.
Improving Solutions for PCB Solder Paste Printing
Unqualified solder paste printing can be classified into unqualified solder paste printing on transformer pins and unqualified solder paste printing due to PCB oxidation.
Unqualified Solder Paste Printing on Transformer Pins
For the analysis on solder paste printing on transformer pins, analysis should be carried out in terms of temperature during solder paste printing. In accordance with practical operation record, preheating temperature is over 100°C. The first wave soldering temperature is between 230°C and 260°C while the second wave soldering temperature is over 150°C. Both wave soldering temperatures are lower than 260°C with printing time between 3 and 6 seconds, compatible with printing requirement. Therefore, it can be concluded that both printing temperature and printing time are qualified. Next comes the analysis on transformer pin specs. The length of black jump wire is in the range from 22mm to 25mm, leading to unqualified solder paste printing. Therefore, black jump wire should be modified to the range from 20mm to 23mm. As a result, defective rate of solder paste printing will be reduced to 0.
Unqualified Solder Paste Printing due to PCB Oxidation
Unqualified solder paste printing due to PCB oxidation results from the following reasons.
a. Package issues leading to unqualified solder paste printing due to PCB oxidation mainly include:
① PCBs are placed at manufacturer for quite a long time with insufficient sealing. In other words, PCB boards are not preserved with incomplete vacuum packaging.
① PCBs are preserved in warehouse for a long time and without administration in terms of constant temperature and humidity.
b. PCB manufacturers pay little attention to OSP (organic solderability preservatives) administration and don't carry out SPC (statistics process control) at regular intervals. Up to now, control range lies in 0.35μm±0.1μm but practical measured value is approximately 0.34μm that is a lower limiting value.
c. Drying technology is unqualifiedly implemented and sponge stick to absorb water absorbs too much water so that water on the surface of PCBs cannot be totally eliminated. Furthermore, drying temperature is regulated to be at 80°C but the practical measurement temperature is 75.6°C so that complete drying fails to be achieved, leading PCB pad to be oxidized after going through high-temperature oven.
d. Operators may touch PCB boards directly with hands so that some contaminants are left on the surface of circuit board.
Improving solutions catering to the above three issues settlement will be listed in this paragraph. For vacuum packaging, manufacturers have to package PCBs with vacuum package, or they would be returned. Furthermore, once vacuum package is opened, PCBs have to go through solder paste printing within 7 days and PCBs after SMT (surface mount technology) assembly have to be stored in the environment with constant temperature and humidity that should be inspected at regular intervals. During the process of manufacturing, OSP should be inspected every two hours and SPC should be carried out as well. Plus, drying technology control should be strengthened and water-absorbing sponge should be changed every week and washed every 4 hours. Practical temperature has to be compatible with setting temperature. In addition, PCBs mustn't be touched with naked hands, which should be informed to operators and workers.
After such a series of measures, unqualified solder paste printing due to PCB oxidation will dramatically go down from previous 0.0089% to 0.001%.
Improving Solutions for PCB Open Circuits
PCB open circuits are classified into two categories: via open circuits and copper foil open circuits.
Solutions for Via Open Circuits
Open circuits on vias feature far more complexity than ordinary technological issues because they are concerned with numerous manufacturing processes including substrate material, lamination, drilling and copper plating etc. Furthermore, this type of defects can never be found until SMT assembly. As a result, the essential solution to this issue lies in its prevention from the root.
As via open circuit is analyzed, it can be found that copper is evenly distributed at top and bottom side of via while no copper is available at one end of via. With solder mask peeled off, circuit is found to be open.
Analysis is then implemented on PCB drilling, electroplating, dry film, etching, electrical test and product inspection. Via open circuits are generated possibly owing to the following reasons: drilling burs are not totally eliminated; cleaning isn't complete before electroplating; dry film is generated at a too high speed; dry film is insufficiently clean; backlight inspection isn't carried out on each via on PCB after etching; there's no clear identification marks between qualified and unqualified products during electrical test; no inspections are done to via copper.
Improving solutions have to be implemented to defeat all the above defects. To prevent drilling burr from being incomplete cleaning, operators need to get it polished together with manual polishing. In order to totally clean the products before electroplating, burrs at via opening should be polished off. To stop dry film from rapid drying, film pressing speed should be modified to 2.0m/s.
Solutions for Copper Foil Open Circuits
Copper foil open circuits usually take place during etching engineering, testing engineering and electroplating engineering. As a result, solutions should be carried out from the three aspects.
Etching engineering should be carefully monitored and analyzed. Once dirty etching is found, operators need to make it cleaned. If drying film gets broken during cleaning, open circuits tend to be generated. Because first article isn't inspected or approved during etching, incomplete etching won't be regulated or respected with copper foil open circuits flowing into the next engineering.
Based on observation and analysis on testing engineering, qualified and unqualified samples are not well monitored. If unqualified samples are placed in chaos, operators will possibly lead to unqualified products' exposure to the market.
When it comes to electroplating engineering, electric clippers feature bad conductivity and electroplating copper features thinness, easily leading open circuits to occur.
Solutions can be achieved conforming to the following instructions:
a. First article should be inspected and approved for etching products that mustn't be manufactured until first article is approved.
b. Modification measures should be regulated catering to unclean etching, including manual repair at board edge and secondary operation within board.
c. For left copper reaching unqualified board edge etching in less than 10%, manual repair should be done.
d. Cleaning should be implemented along traces and water cleaning should be instantly carried out on broken film.
e. Qualified and unqualified products should be marked with distinct identifications.
f. Maintenance times should be improved for clippers with once a week improved to once a day so that excellent conductivity can be guaranteed.
Improving Solutions for PCB Short Circuits
PCB short circuits are such a type of serious defect, possibly causing destructive results that they should be stopped as much as possible.
Simple test should be implemented on unqualified loop of PCB short circuits, which indicates that short circuits usually focus on oval slotted hole. Because copper has been plated on the internal wall of slotted hole, numerical-control drilling is applied to participate in manufacturing. Since drilling equipment is leveraged for a long time, drilling isn't so complete.
To solve this problem, slotted hole should be enlarged and compensation settings should be done to chamfer in accordance with different numerical-control equipment to stop PCB short circuits from taking place.
Improving Solutions for PCB Solder Mask Pull Off
The reasons for PCB solder mask pull off include:
a. PCB suffers from high temperature and long time during reflow soldering;
b. Solder resist features insufficient adhesion or PCBs suffer from badly-performed pretreatment;
Based on the two reasons, improving solutions should be analyzed from the two perspectives. In accordance with the first reason, temperature curves for different reflow soldering ovens belonging to different manufacturing lines are analyzed so no issues are really available on reflow soldering temperature curve. As a result, the first reason can be neglected.
When it comes to the second reason, however, solutions are achievable:
a. Once equipment suffers from breakdown, manufacturing should be stopped at once;
b. Operators should be aware that inspection should be implemented again during the breakdown of equipment so as to avoid unqualifications in later manufacturing stages.
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