When the Flex Printed Circuit Board loaded with components first enters the re-flow oven, it is at ambient temperature around 18-20ºC. Initially, the preheat section of Flex Printed Circuit Board will increase the temperature of the assembly. The rate of temperature rise will depend on the oven settings and the thermal demand of the Flex Printed Circuit Board and pallet. Some component suppliers limit the component specifications to a rise temperature of 3-4º per second.
During preheat, any volatile material in the paste can be driven off, and the initial cleaning action of the flux contained in the paste of Flex Printed Circuit Board can be started. Some materials do not activate until they reach substantially higher temperatures. During preheat of Flex Printed Circuit Board, every effort is made not to create too much of a temperature differential on the board surface as it may be difficult for this to be overcome during the soak period. A temperature rise of Flex Printed Circuit Board in the initial preheat zone can be between 80-150ºC.
Flex printed circuit board Convection reflow is the most common reflow soldering process in the industry; machines have been used successfully for reflow of tin-lead or, more recently, of lead-free for many years of Flex printed circuit board assemble. Both air and nitrogen systems have proven to be very successful when correctly set up and maintained.
Basically, a reflow oven of flex printed circuit board consists of multiple heating zones with a mesh conveyor for small-volume or pin conveyor for inline high-volume transportation of the product through the zones. Each zone temperature of flex printed circuit board is set to achieve a temperature rise on the assembly prior to final reflow of the solder paste to form the interconnection between the component termination and the flex printed circuit board pads. Generally, the higher the convection rates in the zones, the further reduced the delta T across the surface of the assembly.
Controlling the zone temperatures, convection rates and conveyor speeds of flex printed circuit board allows an engineer to define a process profile.
Flex circuit board Vapour phase soldering (VPS), also referred to as condensation soldering, has been around in the industry for many years and was one of the only two serious options during the early introduction of surface mount technology.
The first reflow system of flex circuit board used by many engineers for SMT was VPS, as they preferred the simplicity of the process and process set-up over the problems of accurate loading, board belt positioning, profiling and overheating flux residues on infrared IR. The introduction of convection reflow of flex circuit board, which is still the most significant part of the marketplace, was like VPS without the fluid when compared to IR reflow.
A well-designed flex circuit board worked very well and gave high soldering yields. A poor design, particularly on passive components, would amplify the number of lifted and tombstoned components. All vapour phase systems can show a difference in component lift due to the fundamental nature of the process—fluid on a surface. As the vapour condenses on the surface of the flex circuit board and turns to liquid, component movement can occur. Is this a reason to dismiss VPS? No—it’s often just an excuse to stick with the poor design. A recent lead-free trial comparing VPS with convection showed an increased number of lifting defects with VPS of flex circuit board. If you only looked at the total PPM levels, you might say that VPS was the cause, but the defects were all on one component size; hence the need for good documentation of flex circuit board during design reviews and NPI at contract assembly companies.
Flex Circuit Board Reflow Soldering
There are three methods used for reflow soldering of flex circuit board and surface mount components. They are:
Hot plate – conduction
Vapor phase reflow
Hot plate reflow
Hot plate reflow is probably one of the oldest processes, often referred to as brown or black belt reflow, where flex circuit boards are reflowed by conduction. A linked chain conveyor would drag pallets holding the flexible circuit boards over a series of heated plates at different temperatures to achieve reflow. A profile could be defined by individually setting the hot plates—not an ideal control, but cost-effective in the early days of SMT.
A simple and quite elegant manual soldering process for a few small surface-mount non-fine pitch packages on polyimide flex circuit board would be a static hot plate. Positioned on either side of the hot plate would be a preparation plate and a cooling plate. Operators would place a solder-leveled circuit on a hot plate after first fluxing the pads with a liquid flux.
The surface mount components would be placed manually on the pads and then the circuit slid onto the hot plate; when reflow of the joints had taken place, the circuit would be moved from the hot plate to the cooling plate.
Often, operators would flux and move the circuit onto the hot plate, then start to place the components to increase throughput.
When assemble flex circuit board prototype, Tin-lead-based solder has long been the most common method for interconnecting components to flex circuit board. Of the many different types of tin-lead solder available, Eutectic Sn63Pb37 solder (M.P. 183ºC) is perhaps the most commonly used solder. Other solders are of value for flex circuit board assembly, especially where lower melting-point base materials, such as polyester, are used in the flex circuit board construction. Indium-tin solder (In52Sn48 [M.P. 117ºC]) and bismuth-tin solder (Bi57Sn43 [M.P. 138ºC]) have both been employed for such purposes.
More recently, the tin-silver-copper family of lead-free solder alloys has been used with melting points between 217ºC and 221ºC, suitable only with polyi-mide flex circuit board materials.
In a reflow process, the solder is melted using a reflow oven. IR, forced air convention and vapor phase are all candidate processes of flex circuit board assemble. Care should be taken in profiling any flex circuit board assembly process because of their much lower thermal mass.
In the case of flex circuit board wave soldering, care needs to be taken over the selection of the adhesive and the polyimide cover layer, as often the adhesion forces can be lower after curing. Trials should be considered using the flexible substrate of choice and adhesive. Shear force measurements of components like SOT23 will be in excess of 600-800grams.
There are several process options available for making electrical interconnection between the component and the Flex Circuit Board. The choice of joining method is linked hierarchically—firstly to the electrical performance and reliability expectations of the finished product and secondly to the materials used in making the flex circuit board. Following are some of the most common methods in use.
Conductive Adhesive Attach to flex circuit board
Conductive adhesives are a popular method of assembling components to certain types of flex circuit boards. The adhesives normally consist of UV- or thermally-cured epoxies, which are filled with silver particles. The material can be stenciled or dispensed point by point onto the selected component lands prior to assembly. However, care needs to be taken over the application of these flex circuit board, as any dynamic flexing may still require the component and joints to be supported, often with another material.
Flex circuit board assemble include printed solder paste onto it and place electronic components on it, Placement of components onto a flex circuit board is best performed automatically, though manual placement can be used for prototype or low-volume production depending on complexity. The efficacy of modern automated equipment and manufacturing management practices continues to reduce the low-volume crossover point.
Component density on the flex circuit board is another factor requiring consideration, for as component densities increase, manual placement becomes more difficult. As the carrier pallet supports the flexible through each stage in the process, there are no issues during placement: The machine just sees the flex circuit board as a rigid board for assembly.
Non-conductive, high-temperature adhesives are useful in holding components in place while the soldering operation takes place. Only small dots of adhesive are required for wave soldering applications. They can be applied by point dispensing, stenciling or pin transfer. SMT adhesive would normally be used only when the components are to be reflow-soldered; it has in the past been used when conducting simultaneous double-sided reflow.
Flex circuit board smt assemble is different from rigid circuit board assemble, Surface mount assembly of flex circuit board is extremely difficult without proper tooling and fixturing. There are a number of different tools and fixtures required. A solder paste stencil is the most common way to apply solder paste on the surface mount lands. There are no real changes in stencil design or aperture modifications for flex circuit board as the surface is basically rigid when printed. Stencil thicknesses used are generally in line with rigid board technology: 0.005-6″ thick, produced by laser or electroforming nickel. A new solder paste jetting technology is an option for New Product Introduction NPI and small- to medium-production runs of flex circuit board, thus eliminating the stencil requirements. Development over the last couple of years with different paste suppliers makes it a valued option.
Normally, a vacuum system is used to hold the flex circuit board flat during the screen printing of solder paste. Direct vacuum fixture on reel-to-reel lines is used for special applications but less so solder paste printing of flex circuit board. Reel-to-reel assembly is illustrated in below. This is frequently used in combination with special fixtures such as described earlier. The individual fixtures or pallets can be made from a variety of materials such as glass epoxy or anodized aluminum. The fixture of flex circuit board provides a stable base rfor processing, thus allowing more common soldering process profiles to be run.
Flex pcb board wave solder fixtures are designed to carry the flex pcb board assembly over a molten solder wave while keeping the flex pcb board and components stable. There are several possible solutions to this problem. For example, the flex pcb board can be left attached to the stiffener assembly in panel form. This method is relatively common. It can be a cost-effective solution, provided there are not an excessive number of defective parts in the panel and the use of the stiffener is beneficial at other stages during assembly and test.
Another possible method for fixturing flex pcb board for wave solder is either individual holes are drilled or much larger openings are provided in a carrier plate that allows the component leads and plated through holes to be properly accessed by the solder wave. There are design rules available from many solder pallet/fixture manufacturers for both wave and selective soldering. Basically, the thickness of the flex pcb board material between the wave and the flexible should be the minimum to support the flexible and to make sure that the pallet does not flex during soldering. The thicker the material, the more it will displace the wave during contact, requiring larger clearances. The flex pcb board must be held flat in the pallet and not allowed to rise, as this will lead to flux and solder getting under the flex pcb board and onto the pallet, leading to a manual cleaning step each time a pallet is processed.
If baking of the flex pcb board is required, then a typical temperature would be above 100ºC to dry out the flex pcb board. The upper temperature may be 110-125ºC; it’s all dependent on the time when the flex pcb board reach temperature.
Plain flex pcb board have very little mass; flexi-rigid boards will have more mass and require more time to dry out. To minimize the temperature and time so as not to impact the solderability of the surface coating, the moisture content levels need to be established.
For component preparation of flex pcb board, it has been suggested that the optimum component lead angle for leaded surface-mounted components is 60º +/-5º. That angle can be opened up to 45-65ºfor existing designs where the component body has been reduced in width. The purpose of the 55º to 65º lead angle is to provide added strain relief in x, y, and z axis during thermal cycling beyond what the flex pcb board can offer intrinsically. To achieve high reliability, the heel fillet of the solder joint should be adequate. Component lead angles greater than 65-70º improve the chances of this. While component lead co-planarity is important to good assembly, one shouldn’t force leads into co-planarity by using a thermode or other method to drive component leads of flex pcb board into the solder. It is better to reform the leads off line. In practice, there are few issues with the components, provided the design of the footprint is correct, the terminations are solderable, and they all meet the minimum requirements of the soldering process—i.e., they can stand up to convection, vapour phase reflow and rework temperatures for tin-lead or lead-free alloys. Typical reflow temperatures for tin-lead are 210-225ºC; for lead-free they will be 230-250ºC. If parts of flex pcb board are to be subjected to wave solder on the base of the board, then the tin-lead can be between 240-250ºC and lead-free 255-270ºC.