Make SMT less common process problems

2020-12-14 11:24:10 hongling

Make SMT less common process problems

Most companies are now using surface mount technology, while moving towards ball grid array (BGA), chip scale packaging (CSP) and even flip chip assembly. However, some companies are still using through-hole technology. The use of through-hole technology is not necessarily related to cost or experience-it may simply be that the product does not require miniaturization. Many companies continue to use traditional through-hole components and will continue to use these parts in hybrid technology products. This article will look at some process issues that are not common enough. It is hoped that the traditional component assembly problems and their practical solutions will help provide insight into what might go wrong in today's manufacturing.

  The destruction of components by static electricity

   Static electricity is an objective natural phenomenon, produced in many ways, such as contact, friction, rushing, etc. The basic process of its production can be summarized as: contact → charge → transfer → formation of an even electric layer → charge separation.

  The static electricity on the equipment or the human body can reach up to tens of thousands of volts or even hundreds of thousands of volts, and it often reaches hundreds to thousands of volts under normal operating conditions. The human body can carry thousands of volts or even tens of thousands of volts of static electricity due to its own actions and contact-separation, friction, or induction with other objects. Static electricity is the result of the loss of balance between positive and negative charges in a local area. It is a kind of electric energy, which has the characteristics of high potential, low power, small current and short action time.

  The main measures of static electricity control include: static electricity leakage and dissipation, static electricity neutralization, static electricity shielding and grounding, humidification, etc. The breakdown of components caused by electrostatic discharge is the most common and serious electrostatic hazard in the electronics industry. It is divided into hard breakdown and soft breakdown. Hard breakdown is a one-time cause of dielectric breakdown, burnout or permanent failure of components; soft breakdown is caused by deterioration of device performance or decreased parameter indexes.

   The transfer and storage of electrostatic sensitive components and printed circuit boards between processes in the production process must use anti-static feeding boxes, component boxes, turnover boxes, turnover trays, etc. To prevent the accumulation of static electricity from causing harm. Electrostatic sensitive components and printed circuit boards must be packaged as finished products with anti-static shielding bags, packaging bags, packaging boxes, strips, baskets, etc., to avoid electrostatic damage during transportation.

In the production process of electronic products, the components and components of finished products often come into contact with, separate from equipment and tools, and generate static electricity due to friction. Anti-static cushions, turnover carts, maintenance kits, tools, work chairs (stools), etc. must be used, and Through proper grounding, static electricity can be discharged quickly. Friction electrification and human body static electricity are the two major sources of hazards in the electronics and microelectronics industries, but the generation of static electricity is not the hazard. The hazard lies in the accumulation of static electricity and the resulting electrostatic charge discharge, so it must be controlled.

   An object with static electricity will form an electrostatic field around it, which will produce mechanical effects, discharge effects and electrostatic induction effects. Due to the mechanical effect of static electricity, floating dust particles in the air will be adsorbed on silicon chips and other electronic components, which will seriously affect the quality of electronic products. Therefore, anti-static measures must be taken to purify the working space. The walls, ceilings, and floors of the clean room should be made of anti-static, non-dust materials, and a series of electrostatic protection measures should be taken for the operators, workpieces and appliances.

   In order to understand the static electricity in the production process, determine the impact of static electricity in the production process, and inspect the quality of electrostatic protection products and equipment, it is necessary to measure static electricity and related parameters. The measurement of static electricity mainly refers to the measurement of static voltage, material resistance, ground resistance, static off-period, static electricity quantity, static elimination performance, and surface density of cloth charge.

Electrostatic protection is a systematic project. Omissions or mistakes in any link will cause the failure of electrostatic protection. It must be guarded from time to time, and everyone should guard against it.

   We use optical photographs and scanning electron microscopy (SEM, scanning electron microscopy) to see the electrostatic breakdown on the surface of a silicon wafer. Electrostatic discharge, introduced to a pin, causes a change in the working state of the component, leading to system failure. The simulation of electrostatic discharge in the laboratory can also show the failures that occur on the chip surface in real time.

   Static electricity may be a problem, and the solution is an effective control policy. The wrist strap is the most important defense initially.

Dendrite growth

  Dendrites occur when voltage and humidity are applied and some ionizable products appear. The voltage must always be on a circuit, but the moisture content will depend on the application and environment. The ionizable material may come from the surface of the printed circuit board (PCB) due to poor cleaning during assembly or during the blank board manufacturing stage.

   If you want to investigate such defects, do not touch the board or components. Before all the evidence of the cause of failure is destroyed, the defect is photographed and studied. Contamination can often come from the soldering process or the flux used. Another possibility is the general operational fouling during assembly. The most common cause of defects in the industry is self-flux residue.

   In the above example, the failure occurred after the component was repaired. This particular phone unit was reworked by a third-party company using high-activity flux, unlike the low-activity materials used during the original manufacturing.

Cracked pad

   When components or wires must be installed as a second stage assembly, C-shaped pads are usually used. Examples include heavy-duty components, wire weaving, or components that cannot meet welding requirements. In some cases, quality personnel do not know the cause of the rupture, thinking it is a PCB corrosion problem.

   The photo above is a design trap, not a PCB defect. There are two cracks on the pad, but only one needs to prevent soldering and generally prevent the direction of the soldering process.

Solder balls 

  Solder ball is a problem for any engineer who introduces no-clean technology. To help control the problem, he must reduce the number of different circuit board suppliers his company uses. In this way, he will reduce the different types of solder mask used on his board and help isolate the main problem-the solder mask.

  The solder balls may be caused by many process problems during assembly, but if the solder mask does not allow the solder balls to stick, the problem is solved. If the solder mask type does not allow the solder balls to stick to the surface, then this opens the process window for the engineer. The most common cause of solder balls is the exhaust generated from the flux on the surface of the wave crest. When the board is processed from the wave crest, the solder is ejected from the surface of the tin pot.

Welding point of IC seat

   Solder shorts between integrated circuit (IC) pins are not so common, but they can happen. Generally short circuits are the result of high process problems. This kind of problem may come from wireless technology and must be considered for future process assembly.

  Using tin/lead terminals on the socket pins and/or IC pins increases the possibility of short circuits. The parts are almost fused together. The problem will get worse if you change the tin/lead thickness on the contact surface. If we use all lead-free, there will be less fusible coating on the pins and socket pins, and the problem can be avoided. This problem can also be avoided by not preloading the IC.

Solder joint failure

  The reliability of single-sided solder joints is determined by the number of solders, the ratio of holes to pins, and the size of the land. The example above shows a failed solder joint with a relatively small solder joint cross section.

   The hole-to-lead ratio in this example is large, resulting in weak solder joint strength. As the distance from the pin to the edge of the hole increases, the thickness of the solder joint on the cross-section decreases. If any mechanical stress is applied to the solder joints, or if the solder joints are exposed to temperature cycling, the result will be similar to the example shown. Yes, you can add more solder, but this will only extend the life span-it will not eliminate the problem. This type of failure may also occur due to improper handling of already weak solder joints.

Incomplete welding fillet

   An example of incomplete welding fillet on a single panel. This defect occurs due to many reasons. Incomplete solder fillets are caused by improper hole-to-lead ratios, steep conveyor angles, excessive peak temperatures, and contamination on the edges of the pads. The photo shows a clear example of an improper hole-to-pin ratio, which makes the mass soldering of this connection difficult to achieve. The design rule for the pin-to-hole ratio is that the pin size plus at least 0.010" (0.25mm). Adding 0.015" (0.38mm) holes can also get satisfactory solder joints during soldering. A problem that is often forgotten is that as the pin-to-hole ratio increases, the size of the solder joint decreases, which is affecting the strength and reliability of the solder joint.

   The example above also shows deburring on copper pads. During drilling or punching, the copper on the board surface has been inclined in certain areas, making welding difficult. If the rosin is applied to the edge of the pad from either the substrate or the junction between the substrate and the copper pad.