Circuit board chip package welding methods and steps
Today's SMT products are becoming increasingly complex, with smaller and smaller electronic components, thinner and smaller wires, and rapid development of new components. Following the BGA, CSPs and FCs have also entered a practical phase, making SMA's quality inspection technology more and more complicated. While the complexity of SMA is increasing, the development of electronic science and technology, especially the rapid development of computer, optical and image processing technologies, also provides the technical basis for developing and adapting to the needs of SMA detection. More and more applications of SMT are being introduced Various automatic test methods: component testing, PCB light board testing, automatic optical testing, X-ray testing, SMA online testing, non-vector testing and functional testing. What method (or a combination of several methods) should be used depends on the performance, type and quantity of the product. Not all SMAs require advanced test equipment to evaluate, and frequently used results form the order in which they are tested: Connectivity Test-An online test-A functional test. In many cases, testing of components and PCB boards, especially after printing solder pastes, to enhance the source monitoring of SMT production can lead to a significant reduction in failure rates. This chapter mainly introduces the evaluation of welding quality, including the quality requirements of various components and solder joint defects of the various performance, and finally SMT production of welding defects often analyzed and put forward the relevant solutions.
First, the connectivity test
Manual visual inspection (plus auxiliary magnifying glass)
In digital circuits, the basic requirement that the product being soldered works is that the interconnection pattern is intact; the components are well soldered and not leaky; the solder joints are void free and bridge free.
In the SMT large-scale production, people often use the naked eye or auxiliary magnifying glass, microscopy, basically to meet the components except BGA and CSP solder joints observed. Ideally, there should be no shadow magnifying glass and a large center distance microscope.
Check, you can also use metal pin or bamboo toothpick, with the appropriate force and speed across the QFP pin, rely on the touch and visual judgments to be integrated, in particular whether the IC pin Weld or bridge inspection, Has a good effect. With magnifying glass and microscope artificial visual test method has the flexibility, but also the most basic means of detection. IPC-A-610B solder joint acceptance criteria, basically visual. Now with the IPC-A-610B standard, the appearance of the solder joint / PC quality are as follows.
Excellent solder joints appearance
Good solder joint appearance should generally meet the following requirements:
(1) a good degree of wetting;
(2) The solder spread evenly and continuously on the solder joint surface, and the thinner the solder layer is near the solder joint, the contact angle should generally be less than 30 °. For the solder joint with smaller pad edge, a concave meniscus should be seen. Solder metal surface is not allowed to have a barrier layer of solder and other contaminants, such as the barrier layer, character maps, frame, etc .;
(3) Solder joints at the solder layer should be moderate, to avoid too much or too little;
(4) The position of the solder joint must be accurate, the end / pin of the component should be at the center of the pad, and the transversal phenomenon should not appear in the width and length direction;
(5) The surface of the solder joint should be continuous and smooth, and the solder joints formed by the reflow should have a bright appearance.
In principle, the above requirements apply to all solder joints regardless of the method by which they are soldered, and wherever they are on the PCB, should make them feel uniform, smooth and full.
Second, defect classification
Welding defects can be divided into major defects, minor defects and surface defects. Defects that disable the SMA function are referred to as primary defects; secondary defects are those where the wetting between solder joints is good without loss of SMA function but with possible defects that affect the life of the product; and surface defects are those that do not affect the product Function and life expectancy.
Often, major defects must be repaired, minor defects and surface defects need to be repaired, depending on the extent of the defect and the intended use of the product. Often, electronic products fall into three broad categories: consumer devices such as TVs and VCDs; specialized devices such as measuring instruments and communications; and highly reliable devices such as spacecraft and pacemakers.
Depending on the IPC-A-610B standard and the nature of your own product, different manufacturing divisions may decide whether to repair a secondary defect or a surface defect, or whether a surface defect requires a specific appearance or has not been accurately identified Give repair.
1. Common main defects
(1) bridge / bridge
Solder joints between unwanted metal parts can cause a short circuit. Various defects will occur when the solder joints, there must be repaired, as shown.
(2) monument
Drawbridges, also known as Drawbridging, Manhattan, and Tombstones, are common pitfalls in SMT production and appear mainly on lightweight RC elements as shown.
(3) Dislocation
The position of the component moves open state, all kinds of component pins will happen.
(4) Solder paste is not melted
SMA reflow oven after welding, solder pins appear on the components did not melt the phenomenon, a variety of components will happen.
(5) Suction / wicking phenomenon
Solder is not wetted at the element leads, but rises through the pins to the junction of the element and the body. The oil in the lamp rises to the top of the wick, common in QFPs and SOICs.
2. Common secondary defects
Pad wetting effect is acceptable, will not make SMA loss of function, but will affect their life expectancy. Production testing based on the quality of welding developed 1,2,3 standards, different grades of welding quality determines the level of the product.
For chip components, good solder joints should be smooth, bright and continuous, and should be thinned as far as the edges. The bottom of the front is not exposed and does not show sharp protrusions. Component position does not deviate from the components without cracks, nicks and damage, port electrode no leaching phenomenon. For more detailed technical standards can refer to other information.
Chip on board (COB), the semiconductor chip is mounted on a printed circuit board, the chip is electrically connected to the substrate by a wire stitch method, and the chip is electrically connected to the substrate by a wire stitch method and covered with a resin to ensure reliability . Although COB is the simplest bare chip placement technology, its packaging density is far less than that of TAB and rewinding technologies.
Chip On Chip (COB) The process involves first covering the surface of the substrate with a thermally conductive epoxy (typically silver-doped epoxy) to cover the mounting point of the wafer, placing the wafer directly on the surface of the substrate, heat-treating the silicon The sheet is firmly fixed to the substrate and then the electrical connection is established directly between the wafer and the substrate by wire bonding.
Compared with other packaging technologies, COB technology at low prices (only with the same chip about 1/3), saving space, technology is mature. However, any new technology can not be perfect immediately after its appearance. COB technology also has the disadvantages of requiring reprovision welding machine and encapsulation machine, sometimes keeping pace with the environment, and stricter and inability to maintain the PCB environment.
The placement of some chip-on-board (CoB) improves IC signal performance because they remove most or all of the package, which means that most or all of the parasitic devices are removed. However, with these technologies, there may be some performance issues. In all of these designs, the substrate may not be well connected to VCC or ground due to the leadframe die or BGA markings. Possible problems include CTE problems and poor substrate connections.
COB main welding method:
(1) heat pressure welding
Use of heat and pressure to weld the wire and the pad together. The principle is through the heating and pressurizing, so that the weld zone (such as AI) plastic deformation and destruction of the interface at the pressure of the oxide layer, so that the atomic attraction to achieve the purpose of "bonding", in addition, the two metal interface is not Flat heating and pressing can make the upper and lower metal inlaid with each other. This technology is generally used for glass chip COG.
(2) ultrasonic welding
Ultrasonic welding is the use of ultrasonic energy generated by the transducer, the transducer in the UHF magnetic field induction, the rapid expansion and contraction of elastic vibration, so that the corresponding vibration chopper, while the chopper knife to impose a certain pressure, so the chopper in the Under the combined action of these two forces, the AI wire is rapidly rubbed on the surface of the metallized layer of the soldered area such as (AI film) to cause plastic deformation on the surface of the AI wire and the AI film. Such deformation also destroys the interface of the AI layer Oxide layer, so that two pure metal surface in close contact to achieve the bonding between atoms, thus forming the welding. The main welding materials for aluminum welding head, generally wedge.
(3) gold wire welding
Ball bonding in wire bonding is the most representative of the welding technology, because now the semiconductor package two, transistor package are used AU wire ball. And it is easy to operate, flexible, solid solder joints (AU wire diameter of 25UM welding strength is generally 0.07 ~ 0.09N / point), and no direction, the welding speed can be as high as 15 points / second or more. Gold wire welding is also called hot (pressure) (super) acoustic welding the main bonding material for the gold (AU) wire welding head is spherical so the ball welding.
COB packaging process
The first step: expansion crystal. Expansion machine manufacturers to provide the entire LED wafer film uniform expansion, so that the film attached to the surface in close arrangement of the LED crystal opened, easy to stimulate the crystal.
The second step: Beijiao. Expand the crystal expansion ring on the silver paste has been scraping the gum machine surface, the back of the silver paste.
Point silver paste. Suitable for bulk LED chips. The use of dispenser will be the right amount of silver paste in the PCB printed circuit board.
The third step: the crystal silver splay ring ready into the crystal box, by the operator under the microscope with the LED chip stabbed in the PCB printed circuit board.
The fourth step: the crystal will be pricked PCB printed circuit board into the thermal cycle oven at a constant temperature for a period of time until the silver paste cured out (not a long time, or LED chip coating baked yellow, that oxidation, to the bonding Cause difficulties). If there is LED chip bonding, you need more than a few steps; if only IC chip bonding to cancel the above steps.
Step five: sticky chip. Use a dispenser on the PCB printed circuit board IC location on the amount of red plastic (or vinyl), and then anti-static equipment (vacuum pen or child) will be on the IC bare red or vinyl on the right.
The sixth step: drying. The sticky die into the heat cycle oven on a large flat heating plate at a constant temperature for some time, you can also natural curing (a long time).
Step Seven: bonding (wire). Aluminum wire bonders are used to bridge the wafer (LED die or IC chip) with the corresponding aluminum wire on the PCB board, ie inner lead wire bonding of the COB.
Eighth step: pre-test. The use of special testing tools (according to different uses of COB have different equipment, is the simple high-precision power supply) COB detection board will be substandard board re-repair.
Step Nine: Dispensing. Dispensing machine will be deployed a good amount of AB glue to the good bonding of the LED die, IC is packaged with vinyl, and then the appearance of the package according to customer requirements.
The tenth step: curing. Will seal plastic PCB printed circuit board into the heat cycle oven thermostat still, according to the requirements can be set different drying time.
Step 11: Post-test. The package of PCB printed circuit board reuse special testing tools for electrical performance testing, good or bad distinction between good and bad.