RELIABLE SOLDERING FOR HIGH AND MIXED VOLUME SELECTIVE SOLDERING PROCESSES

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1 Originally published in the Proceedings of SMTA International, Rosemont, IL, September 28-October 2, RELIABLE SOLDERING FOR HIGH AND MIXED VOLUME SELECTIVE SOLDERING PROCESSES Gerjan Diepstraten Vitronics Soltec B.V. Oosterhout, Netherlands ABSTRACT Although the number of Through Hole (THT) connections the electronics assemblies is decreasing due to miniaturization, selective soldering is still a growing market. More and more Surface Mount Devices (SMD) technology is being used and reflow soldering is becoming the main stream solder method. Nevertheless there will remain components that require through hole connections for strength or because they simply can t withstand the high temperatures of a lead-free reflow process. Selective soldering is very effective method to solder the THT components. There are two mainstream selective solder applications: Drag soldering (point to point) Dip soldering (stamp) Both applications have a dropjet flux device to apply very small amounts of flux on the areas that are soldered. Despite these small amounts the remaining flux residues might cause electro-migration if not de-activated completely during the assembly process. This drives engineers to implement different flux formulations to avoid claims. Special in the automotive applications there are examples in the field that bridges and corrosion are caused by the growth of dendrites due to flux residues in combination with temperature, humidity and a current. The solder joints can be much more reliable when an inert flux is used in combination with a controlled soldering process. Key words: Selective soldering, reliability, flux ELECTRO-MIGRATION In a humid environment with an applied electric field flux residues that are not inert are a potential risk for electromigration. This process starts when a thin film of water has been formed and a potential is applied between oppositely charged electrodes. Metal ions migrate towards the negative charged cathode and generate a bridge. The risk for electro-migration for electronic assemblies that are soldered in selective soldering applications is higher than for other solder processes when: The selected flux is not inert The surface energy of the solder mask is high (not a dedicated mask for this process) The flux is not activated The flux is drained into areas that are not soldered The selective soldering flux is not compatible with the solder paste in the SMD process. When the right materials are selected and the soldering process is setup properly the risk for dendrite growth is minimal. RELIABLE SOLDER JOINTS Solder joints made in a selective soldering process are mainly leads in supported holes. These connections are very strong approximately 20 s stronger than a SMD connection. In other words once the solder joints are made according to the standards the joint will be very robust and withstand vibration, temperature shocks and other outer noise very easy. THT connections typically are not the bottle neck that defines the reliability of a product. However there are some potential solder defects that may affect the reliability. The process temperatures of a selective soldering process are relatively high, which may influence the quality. Potential defects that should be monitored are: Bridging Through hole filling Blow holes Fillet and pad lifting Solder balls Excess of solder Webbing All these defects may occur during the selective soldering process and should be identified during quality inspection in the factory. This is different from electro-migration. A soldered assembly with no visible flux residues can still have a potential risk for dendrite growth in the field. To avoid this risk a proper flux selection and control during the selective soldering process is required. FLUX REQUIREMENTS SELECTIVE SOLDERING The selection of the flux for a selective soldering process is most critical with respect to reliability. Unfortunately due to the high cost of a flux qualification a number of mainly

2 automotive users run their selective soldering with a flux that has been qualified for their wave soldering applications. This discussion can have a big impact on the life of the product. Four reasons not to use a wave solder flux are: 1. In a wave soldering application all the flux contacts the liquid solder. The high temperature of the solder de-activates all chemicals in the flux and makes the assembly safe. In selective soldering not all flux contacts the solder. 2. The solder temperature in a wave solder process is approximately 260 ºC which is around 30- ºC lower than a selective solder process. These fluxes may not be strong enough to withstand the high selective solder temperatures and result in solder defects like bridging, solder balling or webbing. 3. Wave soldering fluxes have surfactants that make the flux spread very easy to cover the complete solder side of the printed circuit board. In selective soldering a wide spreading of flux is critical and unwanted. For this reason a flux should be selected that does not spread easy, but still is capable to penetrate into the Copper barrels of the PCB. 4. In air wave soldering machine there is no inert (nitrogen) environment. These fluxes must be strong to eliminate oxidation and remove the oxides on the wave. A selective soldering process can only be successful when the oxygen levels in the solder area are below 1000 PPM. These conditions can deal with a much milder flux than a wave solder flux used in air. Whatever flux is selected the goal is to have an inert residue after the soldering. Heat is the method to de-activate flux residues. Some engineers try to bake assemblies after the soldering to make sure all activators are killed. Only the flux supplier is able to give and temperature guidelines to de-activate their fluxes. Baking the assembly is additional process step that is not preferred. For those assemblies that require high reliability cleaning and or conformal coating are alternatives. Figure 1: the impact of the solder mask is visualized. For wave soldering a board with a high surface energy (> mn/m) is preferred. This makes the flux spread to all areas and thus eliminates the risk for open solder joints, bridging, spikes and webbing. Due to the good spreading the residues will be less visible; which is a cosmetic advantage. For selective soldering applications a lower surface energy of the board is wanted. A typical value for a good spreading is ~35 mn/m. One should be aware that heating processes before selective soldering (like reflow soldering) may have an effect on the surface energy of the solder mask. CONTROL FLUX AMOUNT Selective soldering machines use a dropjet device to apply flux on the bottom (solder) side of the PCB. This device is mounted on a x,y controlled robot. The PCB is held during fluxing at a fixed position enabling the robot to apply flux on the areas where the solder is in contact during the process. How much flux is applied depends on three factors that can be programmed: 1. The robot speed during jetting (mm/s) 2. The open of the dropjet (ms) 3. The frequency of the dropjet (Hz) PROCESS OPTIMIZATION Today many engineers are struggling to control the applied flux amounts. For good soldering more flux is somes needed. Apply more flux also results in potential risk that flux penetrates into SMD areas. To avoid this it is important to understand properties of flux and printed circuit board. The solder mask of the printed circuit board is a very important parameter that impacts the spreading of the flux a lot. The flux spread depends on several factors including: Flux surface tension (flux type) Surface energy of the board (solder mask) Temperature of the flux Temperature of the board Figure 2: The number of droplets is defined by the frequency [Hz] (drops per second). The open [ms]

3 defines the amount per droplet. Longer open gives larger droplets. The recommended amount of flux required for soldering is defined by the flux supplier in the flux datasheet. It depends on several properties of the flux and is unique for every flux. These properties include solid content, acid number, density, activation system, etc. The flux supplier defines the amount of dry flux per square inch. How much flux this will be for the assembly is defined by the spreading of the flux on that specific assembly. In a selective soldering process the flux amount that is jetted to the solder side of the assembly can be measured and monitored. A thermal micro sensor technology is used to measure these very small amounts of flux. The short response and high precision allows accurate monitoring of high dynamic fluxing processes PCB Flux Flow [µl/min] Figure 3: Shows the flux flow in µl/min per for an automotive muldia player. The lines are drags for connectors and the spikes show the flux sprayed on single spots. Each product has its own flux graph from which the amount for every single spot on the assembly can be verified. This data can be used for traceability. This enables quality analysis of the product as well as verification of flux amounts in case of solder defects or field returns. DESIGN OF EXPERIMENTS TO MAKE FLUX CALCULATIONS Flux amount and spreading are important factors that have influence on the reliability of the product as well as on the soldering performance. There is a demand to calculate the flux per square inch in order to avoid excess of flux on the assembly. The flux amount per square inch is a result of the flux that is sprayed to the board and the spreading. The more spreading the fewer solids will remain on the spots that needs to be soldered. For good soldering typical µg/in² of solids (the remaining material of the flux after all solvents have evaporated) is required. To determine a formula that defines the flux amount a Box Behnken experiment was done. In this experiment the three parameters (dropjet open, frequency and robot speed) were changed at three levels. In total this experiment requires 15 different runs with predefined settings. The flux used in the experiment was IF2005C; an alcohol based no clean flux with a solid content of 3.3%. The PCB was a Vitronics Soltec test board with a surface energy of approximately mn/m. The formula can only be applied for this flux in combination with this board material since the solder mask affects the spreading of the flux dramatically and each flux has different spreading properties. Table 1: Spreading and flux amount per square inch. Open Frequency Robot speed Flux width Solids per in 2 [ms] [Hz] [in/s] [in] [µg] The system is able to make a 0.20 inch wide line of flux with different solid concentrations from 175 to 12 µg/in². The engineer can decide when making the flux program to flux one or more drags per connector to meet the requirements. From this data the formula for the flux width in this application is defined as: Flux width = * Frequency * Open * Robot speed [inch] Flux amount = 513 * Open * Frequency 3.84 * Robot speed 131 [µg/in²].

4 Frequency [Hz] Flux width after preheat [inch] Robot speed = 3.15 in/s Flux width inch < > 0.3 In this experiment the flux is jetted on four different materials to compare the spreading. The first material is the test board that was used in the previous experiment. The second board is a dedicated selective soldering material with surface tension 35 mn/m. The third material is heat sensitive fax paper. This is often used by engineers to make the spreading visible. The final material is bare Copper over FR4 board material simulating a Cu finished pad Open [ms] Figure 4: Flux width in inches for a robot speed of 3.15 in/s for this combination of flux and solder mask. The next pictures show the visual appearance of the flux on a one row connector with a pitch of inch. The first picture shows a flux amount that is close to the minimum requirements to make soldering possible, where the second has a high solid concentration and therefore may solder better, but has a higher potential risk for dendrites. Figure 5: One row connector having a dry flux concentration of 590 µg/in². (Open 1.5 ms, Frequency 155, Hz and Robot speed 3,15 in/s) Spreading diameter (inch) Selective Soldering Board 35 mn/m Open Time 2 Wait Frequency Figure 7: Straight forward data increase the factors result in more spreading. The design of experiment is a full factorial design with three factors at three levels. Open and frequency of the dropjet are similar to the previous experiment. In this experiment the robot does not move and therefore the wait is introduced. Depending on how long the dropjets sprays on one spot there will be a smaller or larger spread diameter. The most interesting part is to see the flux flow on the Copper material. The effect of all parameters are identical for all board materials only the spreading on Copper is much larger then on FR4 solder mask. 175 Copper finish on FR4 material Surface tension 43 mn/m 0.54 Open Time Frequency Figure 6: Same component with more flux. The amount of dry flux is 1373 µg/in². (Open 2 ms, Frequency 155, Hz and Robot speed 0.40 in/s). Spreading diameter [inch] Wait SPREADING AND BOARD MATERIAL Flux applied on the solder side of the board tends to drift away. How far the flux will spread depends on the material and more the surface energy of that surface. In another experiment a low VOC flux was used. This flux contains water and alcohol as a solvent; which makes it spread less Figure 8: There is wider spreading on Copper material (surface energy = 43 mn/m). For the soldering this is excellent that the flux tends to cover the complete Copper area. One of the major tasks of a flux

5 is to clean the Copper surface from oxides which will improve the wetting. SATELLITES DURING JETTING Dropjet fluxing is an application method to apply very small amounts of flux very on a very accurate way. Like in inkjet technology during the jetting satellites are formed by nature of the liquid and jetting method. These satellites are very small droplets at the start of the spraying (called head satellites) and at the end when the plunger closes (called tail satellites). Many studies are done using high speed cameras to understand the formation and behavior of these flux satellites. Since the size and direction are out of control this flux particles may end in unwanted SMD areas and are a potential reliability risk. Satellite drops, which often follow each fast-moving main drop ejected from the nozzle, are undesirable because they are far more readily misdirected by aerodynamic and electrostatic forces. To eliminate the satellites a new HF (High Frequency) dropjet has been introduced. This device has a sapphire orifice and incorporates an internal pressure compensation bladder. The droplets will retain their integrity longer and travel further. A way to visualize the flux patterns is fluxing on fax (heat sensitive) paper. To illustrate the satellites and the difference between a HF satellite-free and a common dropjet flux was sprayed on fax paper using the same settings. The spray pattern and number of satellites were analyzed. Again a Box Behnken design was used to visualize the performance of the dropjets The experiment showed some interesting data. First the R- Sq = 94 % that indicates that the data from the experiment is reliable (>80%). The wait has a significant impact on the number of satellites. (The longer the dropjet sprays, the more satellites are found on the PCB). There is an interaction between open and frequency of the dropjet. Open [ms] Number of satellites per drop Normal dropjet Sat Wed < > 35 Hold Values Wait Frequency [Hz] Figure 10: Higher frequency gives more satellites. A longer open increases the number of satellites when the frequency is high. At a low frequency there is no significant difference between a long and short open of the dropjet. 2 Figure 9: The Box Behnken experimental layout. Table 2: Number of satellites per dot Open Frequency Wait Typical dropjet HF Dropjet [ms] [Hz] [ms] # #

6 Open [ms] Number of satellites per drop HF Dropjet Frequency [Hz] 2 Sat HF < > 0.8 Hold Values Wait 80 Figure 11: The HF dropjet gets less stable at lower frequencies. Satellites are unwanted because they may end up in areas that are not soldered and heated during the process. In that case they may affect long term reliability since the chemistry is not activated. IMPACT OF SOLDER MASK The next case study is to illustrate the impact of the solder mask on the spreading and flux amount. An automotive company was reviewing their board material and asked to investigate two different solder masks and their effect on the selective soldering. One board was from their wave soldering lines (Mask 2) and the other was a special modified solder mask (Mask 1) for selective soldering. The HF dropjet is designed for higher frequencies (HF). This is the reason why there are more satellites when the frequency is lower than 100 Hz. Figure 14: Mask 2 = wave solder board with a solder mask that has an surface tension > 44 mn/m Figure 12: The flux satellites of the normal dropjet for Open 1.0 ms, Frequency 160 Hz and Wait 10 ms. Figure 13: High Frequency dropjet flux deposit for same settings. Figure 15: Mask 1 = modified solder mask for selective soldering. Surface tension is 36 mn/m. In a Design of Experiment the spreading and flux amounts were verified and compared with the requirements of the flux supplier. Table 3: Flux spreading for different solder masks Open Freq. Robot speed Mask1 Flux width Solids per in 2 Mask2 Flux width Solids per in 2 [ms] [Hz] [in/s] [in] [in] [in] [µg]

7 Due to the high surface energy of mask 2 (wave solder board) the flux spreads very easy and the remaining amount of dry solids per square inch is too low in 80% of the runs. The modified mask has less spreading and therefore the flux amounts are meeting the needs for good soldering. Only for two of the settings the solid content is too low. The dry flux amount can be calculated based on the formula defined with the data from this experiment: Dry amount flux = *Open + 3.5*Frequency-12.7*Robot speed [µg/in²] CONCLUSIONS The flux is a critical material in the selective soldering process. A strong flux is required to clean oxidized metal surfaces and support the wetting. However a strong flux is a potential risk for reliability because of dendrite growth when exposed to temperature, humidity and current. The spreading of the flux may contribute to poor reliability. If flux spreads too far the remaining amount of solids might be too small and flux may not be activated completely after selective soldering. To control the spreading of the flux the solder mask is playing an important role. It is recommended to use solder masks with a lower surface energy for selective soldering applications (preferred surface energy ~35 mn/m). The HF dropjet contributes to a more robust fluxing process. This device generates much less satellites, which are a potential risk for electro-migration.