Defects found during PCB testing and inspection are scrapped or need to be reworked, which means loss of revenue. Defects may be caused by bare circuit boards, components, materials or other reasons. Humidity is one of the causes of circuit board failure, but it can be controlled in advance. Defects found during PCB testing and inspection are scrapped or need to be reworked, which means loss of revenue. Defects may be caused by bare circuit boards, components, materials or other reasons. Humidity is one of the causes of circuit board failure, but it can be controlled in advance.
It's not a good thing to find a defective circuit board. By this time of manufacturing process, we have added a lot of value to the circuit board components, so we can't help but eliminate these defects. The defects of the circuit board are caused by various factors. These factors may be related to components, bare circuit boards, narrow process windows, and sometimes even circuit board layout or circuit board design.
The bottom line is clear - minimize defects. Once the assembled circuit board is defective, the cost is very high.
Starting from processing raw materials, manufacturing engineers and process engineers have various methods to reduce the defects of circuit boards. In terms of components, we usually consider components and circuit boards. In terms of process, check the solder paste, template, reflow soldering temperature curve, which may be the cause of failure.
Another area where we can reduce PCB defects is humidity control, which is often overlooked.
Defects caused by high humidity
In the 1980s, people realized that the humidity sensitivity of surface mount devices may be the cause of circuit board failure. In 1990, IPC issued IPC-SM-786 "Recommended Methods for Handling Moisture Sensitive Plastic IC Packaging" to specifically address this issue. Some basic materials, such as FR-4, are sensitive to humidity. A bare circuit board absorbs more moisture than normal, which will cause dimensional instability and cause some problems when applying solder paste, adhesives and mounting components.
The visual alignment system has a limited role in reducing defects related to moisture in the printing process, because the alignment between PCB and template is the best overall adjustment method.
The coating adhesive is much better. It uses the benchmark on the circuit board to correct the entire coordinate system, reducing the overall impact of expansion and contraction on the base material.
When mounting components, the mounting machine uses the same vision system, and can also use the reference near the multi pin fine pitch devices (QFP, TQF, TSSOP) for adjustment. Correction X/Y/ θ Coordinates can help you determine the center of the mounting shaft and component, but still use the most consistent algorithm to align the pins with the pads. When the circuit board is very wet, the assembly is barely qualified. However, when using standard eutectic solder paste, due to the inherent self centering effect of reflow soldering, the components mounted in this way are still acceptable. However, the reaction of the current lead-free solder paste is different. In the liquid phase phase, the self centering effect is greatly reduced, or completely absent.
During reflow soldering, heating the wet SMD package will cause microcracks in the device package, and may also delaminate the device structure. During heating, water evaporates rapidly as the temperature rises. During the short heat preservation stage of reflow soldering temperature curve, these vapors form airflow and micro explosion occurs inside the package. This phenomenon will cause lead connection fracture and micro cracks on the substrate or lead frame.
According to IPC/JEDEC J-STD-020D 1[1]， "When an unsealed package is subjected to the high temperature of reflow soldering, the water vapor pressure inside it increases a lot. Under certain conditions, the water vapor pressure may cause the packaging material to separate from the chip and lead frame substrate inside the package, resulting in internal cracks that do not extend to the external surface of the package, connection damage, lead shortening, solder joint warping, chip warping, film fracture, or crater formation under the solder joint. In the most serious cases, Stress may cause cracks in the internal package. This phenomenon is commonly referred to as popcorn phenomenon, which is caused by stress to bulge the package, and then burst, causing cracks. During reflow soldering, SMD components are more vulnerable to this problem than jack components at higher temperatures. "
Fortunately, the provisions of this standard on humidity degree and product failure time, as well as the method of removing moisture from devices, are measurable and acceptable.
No matter what specific faults may be caused by humidity, the result is that the circuit board components cannot be used. These circuit board failures caused by the shortcomings of MSD can be prevented by some basic processing principles.
As military and other high reliability circuit board component manufacturers face the use of commercial off the shelf (COTS) components, they assume responsibility for managing moisture sensitive components (MSDs) and substrates. IPC/JEDEC re quoted J-STD-033B 1 [2,4-5], specify the proper handling, packaging, transportation, and use of wet/reflow solder sensitive surface mount devices.
For circuit board assembly manufacturers, once MSD is taken out of the moisture-proof bag (MBB), J-STD-033B 1 It can help them to develop an acceptable method to extend the storage life of MSDs. According to this standard, the storage life is defined as "the humidity sensitive components in dry packaging can be stored in unopened moisture-proof bags, and the humidity around the special inner belt of the moisture-proof bag shall not exceed the minimum time of the specified humidity."
All MBBs have a humidity indicator card (HIC) to tell you the moisture content in the moisture proof bag. The most typical HIC uses three color marks to indicate the relative humidity (RH) in MBB, and the sensitive values are 5%, 10% and 60%. When the humidity in MBB reaches the corresponding humidity value, the color mark will change. Before printing, HIC has instructions to tell the user whether it is necessary to reset the product expiration time.
According to these standards, one of the easiest ways to prolong the storage time of MSDs at normal indoor temperature is to use a desiccant moisture-proof drying cabinet.
Shook and Goodelle discussed the unique problem of handling components when assembling PCBs that use devices that are extremely sensitive to humidity [3]. Among the various treatment methods mentioned in their papers, it is a safe and effective method to store MSD in a drying cabinet to inhibit moisture, which can safely extend the storage time of MSD for a long time.
The manufacturer shall comply with J-STD-020D 1. All moisture sensitive components shall be graded and marked according to their moisture sensitivity level (MSL). In the assembly plant, this mark helps the assembly personnel determine and reset or suspend the time required for the product failure time clock.
For the original dry SMD packaging, if it is only exposed to an environment not exceeding 30 ℃ and 60% RH, a drying bag or drying cabinet can be used to dehumidify and fully dry at room temperature, which is sufficient. If a drying bag is used and the total dehumidification exposure time does not exceed 30 minutes, the original desiccant can be reused.
Suck out moisture
A recent study [6] determined the moisture absorbed from the device in a moderately low moisture proof cabinet. This test is carried out under the worst case with a super absorbent (organic hygroscopic) material.
The material with a total amount of 2000g shall be soaked in water, and the water absorption rate of the material shall be 100%, and then these materials shall be put into an ultra-low humidity moisture-proof cabinet. The material loses about 2% of its moisture from soaking to moving into the drying cabinet. The material and 98% of the moisture it carries enter the moisture proof cabinet, which has a temperature of 62 ° F and a relative humidity of 37%.
The ambient temperature and humidity of general manufacturing equipment may be the same in a year. The ambient humidity in Los Angeles and Long Beach is a good example. The average humidity in Long Beach is 80% in the morning, 54% at noon, and 79% in Los Angeles in the morning, but only drops to 65% at noon, while the distance between Long Beach and Los Angeles is just over 25 miles.
Electronic manufacturing is carried out in a factory building with controlled temperature and humidity. Try to control the ambient temperature within the range of 68 ° F-72 ° F and the relative humidity within 40-60%.
The user can reach J-STD-033B without heating under the condition that the ultra-low humidity moisture-proof cabinet absorbs water from the components 1 Required drying time. For MSL4, 5 and 5a, if the exposure time does not exceed 8 hours, the moisture-proof cabinet needs 10 × The exposure time can make the SMD dry enough, and the product storage time can be reset according to this time. This can be achieved by maintaining the relative humidity in a drying bag or moisture-proof cabinet that is not higher than 5%.
At the same time, IPC/JEDEC only approved one active heating method to dry the SMD packages that were mounted and not mounted. The minimum temperature during heating was 40 ℃ and the minimum relative humidity was 5%. The use of ultra-low humidity electronic moisture-proof box is an effective drying method, which can extend the storage time of SMD packaging and avoid the use of IPC/JEDEC approved reset methods.
Comparison between active drying and electronic moisture-proof box
The power of the high-energy and ultra-low humidity moisture-proof cabinet is about 56 watts, while the power of the small drying equipment is 2.2 kilowatts. During the 75 hour study, the electrical energy consumed by the moisture-proof cabinet is much less than that consumed by a small drying oven for 2 hours.
Ultra low humidity drying has a positive impact on PCB quality in many aspects. When the active heating system dries the components and bare circuit boards, the components and circuit boards must undergo a temperature cycle. A similar cycle is experienced in a standard reflow furnace. Active drying may cause package cracking, delamination, aluminum deformation and connection failure, which is due to the different CTE of various materials contained in package materials.
The temperature curve of active heating and dehumidification requires appropriate temperature rise heating stage and temperature drop cooling stage to minimize the thermal shock to components and circuit boards. The time required for the environment of the drying furnace to return to the room temperature limits the removal of the inventory devices from the storage room. Then, the drying furnace needs another temperature rise time to return to the drying cycle, and the inventory devices undergo another temperature cycle.
monitor
Now some manufacturing execution software (MES) systems include identification and alarm modules, which can remind users of the time limit of humidity sensitive elements. Effective use of these tools can prevent damage caused by excessive humidity, and assembly personnel can confirm whether key components are properly stored and handled in the whole process. Number of thermal cycles experienced by some suppliers SMD.
conclusion
This paper discusses a method which is often ignored but can reduce the defects of circuit boards. The key point of this method is to properly handle components and PCBs. Correcting moisture problems can reduce or eliminate defects related to moisture, and improve product quality and revenue.
As long as we spend some money on the movable ultra-low humidity moistureproof boxes and put these electronic moistureproof boxes in the proper position of the manufacturing workshop, we can control the humidity problem. When the production workshop receives the humidity sensitive devices and circuit boards, they can be put into the movable electronic moisture-proof box, sent to the storage room, and the full set of components can be brought to the production workshop during assembly. Only when necessary, move the same moistureproof boxes to the assembly production line and take out the components, so as to reduce the exposure of devices to the surrounding air, temperature and humidity