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Tracking the Flow of Solder Defects
December 31, 1969 |Estimated reading time: 8 minutes
By Zulki Khan, Nexlogic Technologies
Soldering defects can occur from reflow, wave, or hand soldering. However, the most prone to defects is reflow soldering for SMT. Reflow soldering is more complex compared to other forms of soldering. Reflow soldering requires the development and use of a correct thermal profile. Most often, the usual industry practice is to develop a thermal profile recipe for small, medium, and large boards. Depending on the assembly job, one of these three profiles is selected for reflow soldering, and assembly is performed using this scheme. But this is not the proper method, and can result in an unusually higher numbers of solder defects open solders, cold solders, or a combination of the two.
The correct way is to create an ideal thermal profile by dialing in the accurate recipe for a specific project. The correct thermal profile depends on a number of factors, including board complexity and density; number of components populating the board; whether the board includes SMT or thru-hole components; number of layers, including embedded ground planes; board size; and other factors. If the PCB has a number of power and ground planes, it will take considerably higher temperature cycles at different oven zones before the board is ready to be reflowed.
The perfect thermal profile takes on a different complexion when a hybrid board with eutectic and lead-free components is involved. Special care must be given in these cases, and a perfect thermal profile can first be developed based on lead-free components, PCB materials, solder paste, and surface finishes. Eutectic components are then added at the end of the reflow process. Alternatively, the thermal profile can also be developed based on eutectic components and then later, after reflow, lead-free components can be added to the PCB. In lead-free assembly, it also is especially important to pre-heat reflow oven temperatures properly so that soldering is properly performed and defects can be avoided. This is crucial when BGAs are involved.
Component types thru-hole or SMT are factors that determine the proper thermal profile. When components are placed using the SMT machines, thermal couples are used to localize heating zones, creating an accurate thermal profile. A thermal couple is a device that is attached to a specific area on the board, and provides real-time temperature readings as the board goes through reflow cycles in the oven. An accurate thermal profile is critical to avoid soldering defects. If the perfect profile isn't dialed in, it may be too cold and, consequently, can cause cold solder joints. If the profile is too hot, then solder bridging may occur between surface mount component leads. If there are BGAs and the profile is too hot, it will create shorts in the BGA package. When these and other defects occur, a considerable amount of time is consumed in rework operations, and PCB assembly costs increase.
From another perspective, if a perfect thermal profile isn't dialed in, a marginal product will result. Even after passing stringent tests, a marginal PCB may fail in the field and come back to the EMS provider with solder joint problems that must be reworked. Running a thermal profile tailored for a lead-free PCB and using it on a eutectic board can have devastating effects because an extreme amount of heat is placed on the components, thereby damaging them.
Conversely, if an ideal thermal profile is not created, then the appropriate amount of heat is not applied, resulting in surface mount components that have not reflowed properly. Care must be taken to consider the PCB material used when developing and dialing in a thermal profile. Epoxy-based material such as FR-5, Teflon-based materials, or polyimide, have different materials requiring different temperature cycles to reflow components.
Stencil PrintingProper stencil printing is also very important to avoid solder defects. A critical aspect is stencil thickness, which varies depending on component density. The stencil must be cut at the correct thickness and with the correct aperture sizes or decodes. It determines the amount of solder paste to be dispensed on the surface mount pad. Formulating the right thickness comes with experience and there's no logical or textbook-based formula to determine with proper stencil thickness. The exact amount of paste must be dispensed on surface mount pads to avoid creating bridges or cold solder joints.
Stencil printing is the first assembly process that involves dispensing solder paste onto SMT components. Solder paste has a tack life the time lag between the solder paste deposition to the reflow and has few hours before the solder dries up and does not reflow properly. Therefore, those components must be placed using a pick-and-place machine within a specific period of time to reflow them properly and avoid solder defects. If too much paste is placed on the surface mount pad, the thermal profile must be re-adjusted, and conveyor belt speed and/or temperatures must be changed in different reflow oven zones to allow the extra paste to cure properly. Placing too much paste can create shorts between fine-pitch leads of a surface mount device (SMD). Conversely, if there is not enough paste on the surface mount component, proper wetting will not occur, resulting in cold solder joints. This creates a disconnect between components and the board, resulting in a void or open between two otherwise-connected points.
WettingIt is important to balance wetting, which means having sufficient flux to spread evenly on the entire pad surface throughout the PCB. Wetting balance is detected by a force meter. Force is measured as solder wets up component legs; wetting force is determined accordingly across the PCB to ensure that the amount of wetting is balanced. Using advanced 3-D solder paste inspection (SPI) tools for paste-height inspection and automated optical inspection (AOI) machines is critical to avoid solder defects. In the case of paste-height inspection, the tool measures the distance from the SMT pad on the board to the top of the printed solder paste (Figure 1). It uses laser technology to measure the amount of solder paste deposited on a component's pad. On prototype runs, a good a rule of thumb for paste-height measurement is to use a small sample size of 510 boards to check solder paste deposition. This way, the amount of solder paste can be inspected effectively. If it's a large-volume run, such as 200300 boards, sample sizes of 2030 are appropriate to determine the amount of paste being dispensed to avoid solder defects. AOI, on the other hand, checks for voids, opens, and shorts. These and other tools are necessary in an EMS provider's arsenal to consistently produce high-reliability PCBs.
Tricky BGA SolderingBGA-associated solder defects are tricky due to shrinking BGA sizes and because ball collapses cannot be seen by the human eye or through a magnifying glass. X-ray machines are essential to inspect BGA assemblies. A key reason for special inspection is to ensure that there is no damage to the BGA. A poor-performing BGA is taken off the board, the PCB surface area is cleaned, and then the re-balled BGA is placed back on the board. If this process is repeated two or three times, chances are that other components in the periphery of that BGA will be damaged, or the board surface finish may be damaged as a result of being exposed to multiple reflow heat cycles. Excessive heat also creates solder mask delamination (Figure 2).
A number of solder defects are emerging as a result of shrinking BGA ball size and pitch. For example, an open solder ball (non-collapsed) can occur due to insufficient heat during reflow. Five years ago, 1.0-mm pitch devices were standard. Three years ago, it shrunk to 0.8 mm, and today, it is 0.5- and 0.4-mm-pitch BGAs. Assembling a PCB populated with 0.5-mm-pitch BGA packages or less is extremely challenging, and requires considerable assembly control and precision. Tolerance must be tight; using the right type of flux is critical because the method of activating the flux determines how reliable the BGA solder joints will be.
Dispensing and controlling the amount of flux and solder paste on the BGA balls plays a vital role in avoiding solder defects. To correct a BGA solder defect, it is best to use a dedicated BGA rework station. The routine here is to treat the correction as a separate process, dial in the perfect thermal profile for that BGA, and then replace the BGA.
Nitrogen ReflowUsing nitrogen reflow for lead-free component assembly wets the surface mount pads considerably better than hot-air reflow, which helps avoid solder defects. It also creates a shiny appearance so that quality assurance (QA) technicians can detect damaged or improper connections. Nitrogen reflow is the preferred method for lead-free projects that must meet RoHS compliance. However, using a nitrogen profile is more expensive than hot-air reflow because nitrogen generation can add significant cost to an EMS provider's bottom line. Additionally, empty nitrogen tanks must be replaced every two weeks or so. However, nitrogen reflow significantly contributes to error-free PCB products, and can ease inspection and the QA process. An EMS provider incurs these added costs, but provides the OEM an environment that minimizes or eliminates solder defects, especially for the RoHS-compliant products.
Solder PotA solder pot may be the basis for creating solder defects. Occasionally, a solder pot may be used to perform quick fixes when time is an issue, or in circumstances when only a handful of thru-hole components must be reflowed. The problem is that there are no controls associated with using a solder pot; there are no mechanical or electronic ways to control the amount of time that heat is applied to thru-hole components. This is based largely on a technician's judgment and expertise. If a technician is not sufficiently experienced, he or she can damage a thru-hole component or peripheral components by subjecting them to the solder pot heat for too long.
ConclusionIn most cases, it is best to avoid using a solder pot, if possible. However, sometimes this unavoidable because thru-hole components might be lead-free, requiring the use of a lead-free-specific solder pot. In such cases, experienced personnel must control the amount of heat dispensed to the components to avoid solder defects. SMT
Zulki Khan, president and founder, Nexlogic Technologies, Inc., may be contacted at (408) 436-8150; info@nexlogic.com.