Reflow soldering guidelines for surface mounted power modules

Transcription

1 Design Note 017 Reflow soldering guidelines for surface mounted power modules Introduction Ericsson surface mounted power modules are adapted to the ever-increasing demands of high manufacturability and low cost in modern high volume and prototype production. Due to the automatic handling enabled by the surface mount technology, throughput is increased, leadtimes decreased and the working time needed is decreased. Many different customers buy surface mounted power modules for a wide range of applications. These guidlines have been created to enable the most efficient use of power modules in production processes. The guidelines are general. Since printed circuit boards (PCBs) and production processes differ from application to application, the guidlines must be evaluated individually for every application to enable the most effective production. Basics about the products The Ericsson surface mounted power modules basically come in two versions; one that has an open-frame solution and one that has an over-moulded rugged design. Open-framee Over-moulded Substrate Ceramic Ceramic Case Leadframe Moulding material Lead finish Ni with a flash of Pd Ni with a flash of Pd Melting temp., internal solder 220 C 1) Intended for leadfree soldering C 2) Conventional 220 C 1) During lead-free reflow of the module to a PCB, the solder within the module remelts. Small components stick by wetting forces, larger by gluing. 2) During reflow, the temperature inside the module is lower than on the leads Both types of modules are manufactured in surface mount technology on a ceramic substrate. The ceramic substrate enables the use of thick fi lm printed resistors that are limiting the number of discrete components used. Reducing the number of components gives the modules their high reliability, low package heights and small surface area. The ceramic substrate enables good heat dissipation. The use of a ceramic substrate also prevents warping during soldering, which prevents loss of contact in the solder joints. e

2 Over-moulded module Semiconductor grade epoxy is used as casing for the over-moulded module. The epoxy protects the ceramic substrate from being damaged mechanically, enables good cooling even if the air flow is low in the final application, protects components inside the module from temperature variations and finally, the epoxy contributes to good coplanarity which is important for reliable, high-yield production processes. The moulding process also gives a low-price packaging. Open-frame module The open-frame module was the fi rst true surface mount module ever to meet the requirements of lead-free soldering when it was released in October The open-frame module consists of a ceramic substrate with surface mounted components, welded to a metal leadframe. The leadframe protects the ceramic substrate from mechanical damage. The connection between leadframe and substrate enables good cooling. The low number of parts gives a low-cost product with high reliability. Lead finish Regarding the leads, the fi rst plating is nickel (Ni) and the second (outer) plating is a fl ash of palladium (Pd). The key role of the palladium is to protect the underlying nickel plating from oxidation during the soldering process and during storage. During the soldering of the modules to PCB, the palladium dissolves into the solder. The solder joint is actually made to the underlying nickel plating which presents itself as an unoxidized surface ready to accept solder. Nickel makes durable solder joints with all types of commercial available types of solder pastes. Due to the low amount of palladium dissolved into solder there is no risk for changing the melting temperature of the solder joint. Since the palladium needs some time to dissolve into the solder, a lower limit for time spent above a certain temperature is needed. Upper limit for time at a certain temperature during reflow is set by the risk for damaging the module. See datasheets for each product for further information. Printed circuit board design A PCB is generally consisting of two parts: Copper and laminate. The laminate consists of glassfibre and epoxy. The copper is a very good conductor of heat, but the laminate can almost be considered as a thermal insulator. Due to the relitively large size and high mass during soldering, power modules usually are among the coolest components on the board. More effective transfer of the heat to the module can increase the process window for reflow soldering. During reflow, heat is transferred to the solderpaste and the components by the medium used (infrared radiation, nitrogen, air or vapourized liquid) and by the copper in the board. Guidelines for PCB design More copper in the board enables more heat to reach the solderpaste and the leads and thus gives a higher temperature where the solder joint will form. More copper is though demanding more energy from the reflow oven. In electronics, thermal vias (a hole, the walls of the hole covered with metal, connecting copperlayers to each other) is sometimes used to cool down components that are known to become hot during operation. When soldering power modules, thermal vias can be used in an opposite way; to heat up the area around the module during reflow. Adding thermal vias in the area close to the power module can increase the temperature of the leads significantly. If the vias are placed close to or within the solder pads, lacquer may be used to cover the vias in order to prevent solder paste to be sucked into the holes. Thermal vias can be used in the area beneath the power module, which rarely is used for other purposes. If introducing thermal vias, the most effective is if the vias can go all the way from the backside of the PCB to the frontside. That has though to be in conjunction with the electrical design of the board. If thermal vias are used, applying more heat from below can increase the temperature of the solder joint even more. Two or more power modules, or other thermally heavy components, can be difficult to heat up properly if they are placed too close to each other. Design tools developed to revise the design of PCB do sometimes not take the thermal properties of components into consideration. Recommendations by such programs can hence suggest improper distances between power modules from a soldering point of view. As a rule of thumb, please provide at least 30mm of spacing between power modules or other larger components (depending on the application and process). Reflow soldering Size of apertures, stencil thickness, etc Sufficient volume of solder paste is needed to create a durable solder joint between component and PCB. Since a PCB generally flexes during reflow, the volume of solder paste must compensate for the flexing to avoid inadequate solder joints. For power modules, a relative large area of the lead is connecting to the PCB. Extra precaution has to be taken into consideration when deciding which stencil thickness to use for solder printing. If components with various sizes are used on the same PCB, it might be advisable to use a stepped profi le. Different soldering 2 EN/LZT R1A Ericsson Microelectronics AB, June 2002

3 patterns are recommended depending on stencil thickness. For soldering pattern for the individual products, please see the product datasheets. Solder paste Solder pastes are used in reflow soldering. The solder paste is applied by screen printing or dispensing. The solder paste is generally made up of small solder balls with a diameter of µm, rheology modifiers and flux. The metal content is generally wt %. The solderballs should be spherical and have the same dimension through the paste. The smaller the solder balls the better the printability. But the smaller the solder balls are, the higher is the oxide content due to the increased surface area/volume. Therefore, the choice of the size for the solder balls is always a compromise. The flux system consists of colofonium (natural; resin) or modified colofonium (artificial; rosin), solvents, activators and stabilisators. Resin or rosin is used to remove and prevent oxidization of metal. Solvents with different boiling temperatures are used to give the solder paste different lifetime on the screen or PCB, or to give flexibility with regards to different temperature profi les. Activators are used to remove oxides on metal surfaces. Stabilisators are used to prevent chemical reactions at room temperature, in order to enlarge the possible storage time. The size of the solderballs together with the fluxsystem is adapted to fit the actual application and type of soldering. When soldering power modules, no special requirements apply regarding the choice of solder paste. Types of reflow soldering The main types of reflow soldering are infrared, convection and condensation soldering. They differ mainly by their heat transfer mechanism. The four phases of reflow soldering In the first phase, the preheat zone, the PCB is heated to an elevated temperature. The solvent in the solder paste evaporates and the flux starts reducing metal oxides. Too rapid heating can cause the solvent to boil; giving solder balls or, components may be damaged due to thermal influence. In the second phase, the soak zone, the purpose is to achieve an even thermal distribution before reaching the reflow zone. The resin and some of the activators reduces metal oxides. Solvents are evaporated as the temperature rises. If the temperature rises too rapid, the flux will break down and give grey solder joints or solder balls. In the third phase, the reflow zone, the solder balls in the solder paste are melting and the surface tension is taking over. The activators are removing the oxide, and the flux is preventing the recreation of oxides. If the temperature is too low, the solder joint will not be strong enough. If the temperature is too high, the flux content of the solderpaste may be too low due to increasing oxidization with increasing temperature. In the fourth phase, the cooling zone, the PCB should be cooled down as quickly as possible in order to get small metal grains and hence a strong solder joint. Too slow cooling will decrease the strength and give the joint a rough surface. For some types of components, for instance ceramic capacitors, too rapid cooling may damage the component. Due to the high mass of power modules, the heat will dissipate depending on the characteristics of the module. Therefore, too rapid cooling will not damage the module. Max. temperature at reference point Preheat zone Time Soak zone Reflow zone t lim Cooling zone Figure 1. Schematic of the four zones of reflow soldering. Infrared Convection Condensation Heat transfer mechanism Infrared radiation Preheated nitrogen or air Vapourized liquid Advantage Gives an even heating Very effective heat transfer. Uniform heat distribution. Inert atmosphere. Disadvantege Non-uniform heating due to differences in heat absorption and reflection Low heating rate with low gas flow High cost due to expensive fluids Suitability when soldering power modules Practically impossible due to large differences in absorption Good Excellent, with controlled profile EN/LZT Ericsson Microelectronics AB, June

4 Temperature profile When soldering power modules, the upper limit in time and temperature is set by the risk of damaging the module. Achieving a durable solder joint sets the lower limit in time and temperature. For soldering profi les/data, please see the datasheets for each product. Cleaning and drying All commercially available washing liquids can be used for both open-frame modules and over-moulded modules. Especially for the open-framee module, it must be assured that the rinsing and drying process is sufficient. Pick & place Due to the relative large size and high mass of power modules it is not advisable to use fast chipshooters when assembling. Surface mounted power modules can be assembled using equipment intended for high precision and/or high flexibility. The orientation of the module can be verified, depending on what type of vision system used, by the asymmetries in the module body. An example of an Ericsson over-moulded module has an asymmetry when viewed from beneath. Figure 2. Assymetry in an over-moulded module An example of an Ericsson open-frame module has an asymmetry when viewed from beneath or above. There is a small difference in the distance between the leads Rework and multiple soldering of modules When a power module is soldered for the fi rst time, the palladium dissolves into the solder, revealing the nickel that makes contact with the solder. If a module, for some reason, needs to be removed, the solder residues are covering the nickel in the leads, protecting the nickel from oxidization. When the module is mounted again and soldered, the solder residues on the leads will keep on protecting the nickel from oxidization during the soldering process. Note that the solder residues should not be removed from the leads. During soldering, components inside the modules are exposed to an extra heat treatment. Heat treatments are degrading the parameters of some components. Therefore, the power modules should not be soldered more than twice in a reflow profi le. Rework and correction soldering with soldering iron can be done more times. Removal of modules for analysis If a module for some reason will be sent back to the supplier for analysis, it is recommended to cut off the leads by the solder joint or to use a soldering iron and melt the solder around each lead one by one. The use of a hot air nozzle must be avoided. Possible overheating of the module will spoil the subsequent analysis Figure 3. An open-frame module viewed from above. Note the difference in distance at pin 1 and pin 24, the marking next to pin 1 can also be used for verifying correct orientation of the module. Testboards A layout drawing for a testboard is available at The drawing is intended for creation of testboards for soldering of open-frame modules and over-moulded modules. The testboards can be used to evaluate the influence of thermal vias and different placements and orientations of the modules. Since PCBs are designed differently, it is recommended to create a testboard which is as similar as the actual board as possible with regards to number of copper layers, distribution of copper etc. 4 EN/LZT R1A Ericsson Microelectronics AB, June 2002

5 Information given is believed to be accurate and reliable. No responsibility is assumed for the consequences of its use nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Ericsson Microelectronics. Specifications subject to change without notice. Ericsson Microelectronics SE KISTA, Sweden Phone: For local sales contacts, please refer to our website or call Int: , Fax: The latest and most complete information can be found on our website Design Note EN/LZT R1A Ericsson Microelectronics AB, June 2002 EN/LZT Ericsson Microelectronics AB, June