Rework is an important part of the mass production process for lead-free PCB assembly, especially during the initial transition period when each link in the supply chain forms an information curve. However, due to the need for warranty, there is still a problem of rework throughout the life of the product.
It has been found that lead-free solder alloys are generally less susceptible to wetting and wicking than Sn/Pb solders. Therefore, rework of lead-free solders is more difficult, and its application in QFP is a clear example. Despite these differences, the use of flux gels, pen solders, and wick solders has been developed for lead-free solders (Sn/Ag/Cu or Sn/Ag) for different components such as discrete components, area array packages, and the like. A successful rework method (ie manual and semi-automatic). Most rework equipment for Sn/Pb can still be used in lead-free solders. The welding parameters must be adjusted to suit the higher melting temperatures and lower wettability of lead-free solders. Other precautions used in Sn/Pb rework (such as board drying if necessary) are still applicable to rework of lead-free solder. Studies have shown that reliable lead-free solder joints with appropriate particle structure and formed intermetallic compounds can be fabricated using an appropriate rework process.
Particular attention should be paid to reducing the potential for rework processes to have a negative impact on solder joints, components and PCBs. Due to the increase in soldering temperature, the Z-axis thermal expansion coefficient (CTE) between the laminate, glass fiber and Cu does not match, and a greater stress is applied to the Cu layer, which may cause cracking of Cu in the plated through hole. , resulting in the occurrence of a fault. This is a rather complicated problem because it is determined by many variables such as PCB layer count and thickness, laminate, rework temperature profile, Cu layout, and via geometry (such as aperture ratio). To determine under what conditions laminates (such as high Tg, low CTE) can replace the traditional FR4 to meet the requirements of lead-free soldering, there is still a lot of work to be done. This is not to say that lower cost materials (such as CEM, FR2, etc.) cannot be used in lead-free soldering. In fact, these methods are applied in mass production and the application is checked on a per-application basis. The impact of rework on pad and mask adhesion must be carefully evaluated.
Similarly, the impact of rework on component reliability should be carefully studied. Warpage and delamination are some of the problems that may arise. The recently released IPC-020B standard states that the focus of returning work on the rating of higher temperature components in lead-free soldering should be considered.
Electrochemical reliability is another important issue that should be considered. When the flux residue dissolves on the moisture condensed on the board, an electrochemical reaction occurs between the conductor traces under the electrical bias, resulting in a decrease in surface insulation resistance (SIR). If electromigration and dendritic growth are produced, a more serious failure occurs due to the formation of a "short circuit" between the conductor traces. Electrochemical reliability is determined by the resistance of flux residues to electromigration and dendritic growth in no-clean applications. Therefore, an SIR test or a Telecordia test based on IPC (according to TM-650 2.6.3.3) must be implemented to ensure that any reactants between reflow flux and reflow flux/wave solder and reworked flux are not given. No-clean applications pose the risk of electromigration and dendritic growth.
The problem of "component mixing" is related to the special problem of ensuring quality, especially during the transition period of this technology. A preliminary study of the effect of lead on lead-free solder on long-term reliability shows that the effect is different due to the different levels of lead in the solder joints. When the lead content is in the middle range, the impact is greatest. Because of the formation of segregation phases (eg, coarse lead particles) in the intergranular tin grain boundaries of persistent solidification, cracks begin at this location and propagate under cyclic loading. For example; it shows that 2 to 5% lead content adversely affects the fatigue life of lead-free solders, but it may not be as effective as Sn/Pb solder. Therefore, if lead-free solder is used to rework lead-containing boards, the reliability of the combination of lead-free solder and Sn/Pb solder may not affect Sn/Pb solder from a solder perspective. However, the effect of temperature on components (especially plastic components) is a concern. On the other hand, the solder joints formed by plates with Sn/Pb solder reworked lead-free solder may not be as reliable as the lead-free solder joints on other boards.
In the case of supply systems, it is critical that the soldering irons and solder materials (core solder, flux gel, etc.) used for lead-free soldering be clearly marked during the transition period.
Rework will inevitably bring about an increase in time and economic costs. Rework is only a measure to make up for it. The most important thing is to strengthen the operator's process training and try to reduce the rework.
A carbon oven is a kind of barbecue equipment using charcoal as fuel. From the baking pan, carbon bucket, oil tank, base composition, barbecue need to prepare the oven, charcoal, solid alcohol, igniter. Carbon Oven - a kind of barbecue equipment made of charcoal as raw material for smokeless barbecue. It is the most widely used oven in carbon oven. Mainly used for garden dinners, outdoor picnic.
Advantages: easy to carry, clean and sanitary
Disadvantages: one can only meet four to six people, need two or more difficult to meet a big party to use.
Practical number: 4 ~ 6 people
Floor-type carbon oven, usually on the desktop or mat above the practical. Popular in Korea and Japan and Europe and the United States, will be in some courtyard party, family dinner and outdoor travel to see the equipment to the figure, very easy to carry and clean health by the outdoor family favorite. Such as in the outdoor want to eat clean and healthy food is really difficult, if brought a carbon oven basically what are resolved. Desk-type carbon oven, this looks like a table, there are four feet. Is a very common carbon oven, the street is very much.
Advantages: You can do a lot of food at the same time to meet user needs.
Disadvantages: too large, inconvenient to carry and health problems
Number of users: should be determined
Charcoal BBQ Grill,Smokeless Charcoal BBQ Grill,Outdoor Charcoal BBQ Grill,Portable Barbeque Grill
Ningbo APG Machine(appliance)Co.,Ltd , http://www.apgelectrical.com