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Lean free wave soldering


Lead Free Wave Soldering
Technical Report 212 Frequently Asked Questions
The Wave Soldering assembly process is, potentially, the most complex area for Pb-Free implementation. This

document is intended to provide an introduction to the wave soldering process for PCB engineers. It also provides some information which may be used by assembly operators to optimize their results when making the transition to Pb-Free. The document contains a considerable amount of information relevant to both Pb-Free and VOC Free Wave Soldering. The first section gives brief answers to some of the most common questions on wave soldering. The second section provides some additional text describing ways to improve particular sections of the wave solder process.

. Source:Nihon Lead Free (Pb-Free) Wave Soldering: 25 Most Frequently Asked Questions: 1) What Is The Wave Soldering Process? During wave soldering, many soldered connections of high reliability can be made with one pass of a printed wiring assembly over a wave of molten solder. A continuous flow of solder is formed in a dedicated machine, with the surfaces to be joined passed over the solder. Surface of solder can be protected by Nitrogen to prevent oxidation and reduce dross formation. 2) What Are Main Areas In Wave Soldering? Flux Application (Foam or Spray Fluxing) PCB Support Conveyor System Pre- Heat Wave solder (Wave Turbulent & Laminar or Laminar only) – Thermal Profile Cooling Cleaning
MacDermid Electronics Solutions 245 Freight Street, Waterbury, Connecticut 06702-9984 USA Telephone: +1 (203) 575-5700 www.macdermid.com/electronics Revision: January 16 2006

Source:EPS

Lead Free Wave Soldering

Frequently Asked Questions

Page 2

2.1 PCB Support As preheat and solder temperature increase with Pb-Free soldering, the PCB will be exposed to longer duration at higher temperatures. The PCB support system includes the conveyor system, pallets, and stiffeners (strain guards). 2.2 Conveyor System Ensure that the conveyor system offers the most efficient PCB support possible as increases in the Preheat Temperatures shall increase PCB Warpage. Main Types of Conveyor; Finger Conveyor (V Groove, L Shaped Rigid Fingers) Belt Conveyor

If using a Finger Conveyor system, trial a variation of the fingers by mixing L Shaped Fingers with the V Groove - usually 1 in 5, to improve PCB support while traversing through Wave Solder Process. Always allow room for the PCB to expand during preheat to avoid central PCB warpage resulting in inconsistent wave solder contact as the center bows deeper into the solder wave than the extremities. Another method for ensuring PCB support during wave soldering on larger PCBAs is the use of center support. 2.3 Pallets Pallet design is critical to achieving a very efficient form of PCB support. Manufacturability with following considerations: PCB thickness Highest and lowest component height Components requiring hold downs Location of edge rails Direction of pallet Width of edge rail Distance from bottom of edge rail to bottom of pallet Wave requirements Process requirements

2.4 Stiffeners/ Strain Guards: When using stiffeners, ensure that they are located in center. Check that stiffeners are firmly positioned on front and back of PCB. Ensure that the stiffeners are cleaned and maintained in good condition. Replace titanium stiffeners when warped or damaged. 3) What Is Flux? Flux is a chemically and physically active compound that, when heated, promotes the wetting of a base metal surface by molten solder by removing minor surface oxidation and other surface films. It also protects the surfaces from reoxidation during a soldering operation.

MacDermid Electronics Solutions 245 Freight Street, Waterbury, Connecticut 06702-9984 USA Telephone: +1 (203) 575-5700 www.macdermid.com/electronics Revision: January 16 2006

Lead Free Wave Soldering 4) What Flux Types Available For Pb-Free?

Frequently Asked Questions

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There are two types of fluxes available, alcohol based and VOC free fluxes Alcohol based fluxes: are less acidic and have varying solids. VOC Free Fluxes: Free of VOC’s (Volatile Organic Compounds). These solvents evaporate under normal usage. They are released in the atmosphere and can contribute to global warming. Most hydrocarbons such as alcohols, ethers and alkanes are considered VOCs. Water is not a VOC. Water has a higher surface tension than alcohol. Fluxes with a base carrier of water are harder to atomize into fine drops, and they do not spread out as well across the metal surfaces to be joined.

5) What Flux Options Available For Each Flux Type? There are two flux options available, No Clean and Aqueous, for each type of flux mentioned above No Clean flux: is made of benign constituents, residues are meant to be left on the assemblies. Aqueous: (Water Wash) flux contains halogens and conductive ionic residues, which must be washed off after assembly

6) What Are Flux Application Methods? The flux application method depends on the flux type but generally there are three methods for depositing the required flux onto the PCB. Foam Fluxing Reciprocating Spray Nozzle Fixed Spray Nozzle

7) During Spray Flux How Is Flux Coverage Checked? Flux coverage can be checked using one or both of following methods: Unpopulated PCB with paper attached on component side or / and Cardboard (Same Dimensions as PCB)

MacDermid Electronics Solutions 245 Freight Street, Waterbury, Connecticut 06702-9984 USA Telephone: +1 (203) 575-5700 www.macdermid.com/electronics Revision: January 16 2006

Lead Free Wave Soldering

Frequently Asked Questions

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8) What Is The Method For Determining Deposition Rate Of Spray Fluxers? Method for Determining the Deposition Rate of Spray Fluxers (Source - Kester) 1. Take a sheet of transparency film for copy machines. 2. Roll up the film and put a rubber band around it. 3. Weigh the paper on an analytical balance. Weigh to the nearest 0.0001 grams. 4. Unroll the film and affix it to the bottom of a board using paper clips. 5. Run the board through the fluxer and remove to dry. 6. After drying (15-30 min.) look at the paper. Look for anomalies in the spray pattern. 7. Reweigh the paper and rubber band (1,000,000 micrograms per gram). 8. Calculate the weight of flux in micrograms per in2 (remember to subtract the weight of the paper and rubber band). 9. Minimum value is 700 micrograms/in2 10. Recommended value is 1000 micrograms/in2

9) What Are Pb-Free Solder Alloys Used In Pb-Free Wave Soldering? Lead-free solder alloys for wave soldering include SnAg3.8Cu0.7, SnAg3.5, SnCu0.7 and related alloys. Melting points for these alloys are 217 °C, 221 °C and 227 °C, respectively. Tin-silver-copper is recommended as a multipurpose alloy for wave soldering, but both tin-silver and tin-copper could be good alternatives. The tin-copper alloy is cheaper and may be considered for low-cost products.

10) What Are The Component Considerations For Wave Soldering? There are three considerations when using approved and acceptable components for wave soldering. Correct component sizes, coupled with the right operating pre-heat range to avoid thermal shock, and minimizing the temperature delta between the end of pre-heat and wave solder temperature.

Approved 0805 , 1206

Acceptable 0603, 1210,1812, 1825 50 mil pitch SOIC< 16 Leads

Not Acceptable < 0603, 1210, 2512 PLCC

Comments - All surface mount devices glued with adhesive & cured before wave solder SMT capacitors or resistors. Figures are length and width in mils for each component type. SMT Active devices SOIC= Small outline Integrated circuit PLCC= Plastic Lead Chip Carrier

TSOP Electrolytic SMD Tantalum SMD Through Hole Components

TSOP =Thin Small outline Package SMT Devices

MacDermid Electronics Solutions 245 Freight Street, Waterbury, Connecticut 06702-9984 USA Telephone: +1 (203) 575-5700 www.macdermid.com/electronics Revision: January 16 2006

Lead Free Wave Soldering

Frequently Asked Questions

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Preheat Requirements: For most Pb-Free flow soldering applications, the topside PCB temperature will increase by as much as 125% to approximately 110- 130° C to limit the thermal shock as the PCB contacts the higher temperature of the lead-free molten solder. The ideal method is to heat up the PCB as quickly as possible to 100°C, and continue to heat the PCB with forced hot-air convection preheating for optimal evaporation of the water medium from the water-based flux. Avoiding Thermal Shock: Working pre-heat range is mentioned above, as the PCB and components contact the higher temperatures of Pb-Free Solder.



11) What PCB Considerations Should Be Considered For Pb-Free Wave Solder? PCB Laminate Selection: New Pb-free substrates will tolerate the increasing assembly temperatures at different rates. Selection of new substrates will use glass transition temperature (Tg,) Temperature of thermal decomposition (Td), and several other considerations. Test new substrates with your design prototypes to ensure the ability to withstand higher Pb-Free temperatures without excessive discoloration and/or warpage. PCB Surface Finish: Pb-Free surface finishes are all different in their ability to withstand the increased temperatures of Pb-Free assembly. OSP will suffer from heat, and the duration of thermal exposure(s). Immersion Tin forms a copper-tin intermetallic with normal storage and each thermal exposure, so check supplier documentation for guidelines. Electroless nickel immersion gold (ENIG) is generally tolerant of increased temperature, but may require more heat than other finishes to form an acceptable solderjoint intermetallic thickness. Immersion silver is not sensitive to temperature or storage time, but may discolor with exposure to low levels of sulfur and/or chloride. Tarnish should not affect solderability until the tarnish reaches a dark blue/black appearance. All finishes will benefit from the use of inert blankets, such as nitrogen, especially those finishes more sensitive to temperature exposure.



12) Pb-Free Wave Solder –DFM Requirements? Critical Design for Manufacture Requirements (e.g. components, hole size, orientation, etc.) must be followed to achieve higher yield. Check the documentation with each component to ensure compatibility with Pb-Free soldering conditions. In particular, check for any compatibility in mixed Pb/Pb-Free systems. The presence of bismuth in lead frames, pins, and discrete terminations may be a cause of concern.

13) What Is A Pre Heating Guideline? The temperature of the board (component side) during preheating should rise as quickly as possible to 45°C (with maximum slope of 2°C/Sec), and just before soldering at a temperature of >105 °C. For better results, 110C-120C is preferred. Lower preheat is known to cause poor PTH fill. 14) What Is The Wetting Time? Wetting time is defined as the time between the moment of first contact (between the parts to be soldered and the solder,) and the moment that the solder in the joints starts solidifying. Typical time is less than 2 seconds. 15) What Is The Dwell Time? Dwell time is defined as the time between the moment of first contact (between the parts to be soldered and the solder,) and the moment of last contact with the solder. For Pb-Free dwell time should be at least half a second more than the Pbcontaining process. Typical time is 3-5 seconds (Pb-Free Solder is 3.5 – 5.5 seconds). 16) What Is The Soldering Time? Soldering time is defined as the time between actual contact of the PCB with the solder and the onset of solidification. (Dwell time + approximately. 5-10 Sec.) 17) What Is The Solidification Time? Solidification time is defined as the time required for the solder to become solid on the bottom of the board. This time is dependant on cooling rate

MacDermid Electronics Solutions 245 Freight Street, Waterbury, Connecticut 06702-9984 USA Telephone: +1 (203) 575-5700 www.macdermid.com/electronics Revision: January 16 2006

Lead Free Wave Soldering

Frequently Asked Questions

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18) Where Is Cooling Performed After Wave Solder? Forced cooling after soldering reduces the maximum temperature reached in the areas on the component side of the board. However, forced cooling directly after solder bath, may cause unequal contractions, which will initiate cracks and therefore is not advised. If cooling is needed because the temperature from the carrier is rising too high, the best place will be before or in the lift itself. 19) What Is Wave Solder Process Optimization? In wave soldering the interaction between the PCB, flux, solder alloy and processing equipment must be considered and optimized for each product. Specification of typical wave solder parameters (solder bath temperature, flux system…) will vary. Some values will give good performance for some products and equipment, and will cause problems for others. It is recommended to use a solder bath temperature between 250°C and 260°C, with 250°C as an absolute minimum. Tests performed in the Norwegian lead-free project shows acceptable or good results at 250°C, 255°C and 260°C, both for tin-copper alloy and tin-silver-copper. The preheating of Pb-Free wave soldering is more critical than for conventional wave soldering, and it is recommended to optimize the temperature profile with the use of top and bottom convection heating, especially for complex boards with heavy components. Wave height settings will be higher for Pb-Free than leaded wave solder process due to lower capillary action and wetting spread. 20) What About Wave Height In Wave Solder? This is very critical; solder must reach the top of the hole prior to exiting the wave. If the solder does not reach the top of the hole then insufficient amount of solder will remain the hole. 21) Will There Be A Reduction Of Capillary Action In Pb-Free Solder Alloys? Capillary action is a physical effect caused by the interactions of a liquid with the walls of a thin tube. The capillary effect is a function of the ability of the liquid to wet a particular material. Pb-Free solder alloys have decreased capillary action compared to tin lead solder alloys and therefore care must be taken to ensure top side penetration of solder alloy. If the thermal load is too high, and the Pb-Free solder does not achieve 100% solder fill, it is important to ensure that Workmanship Standard Acceptable Conditions are allowed and remove the focus from the Target Condition. (IPC STD 610D)

Target

Acceptable

MacDermid Electronics Solutions 245 Freight Street, Waterbury, Connecticut 06702-9984 USA Telephone: +1 (203) 575-5700 www.macdermid.com/electronics Revision: January 16 2006

Lead Free Wave Soldering

Frequently Asked Questions

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Pb-Free solder joints in reliability studies have shown increased Pull Test Strength even when 100% fill not achieved. (Courtesy – Dr. M. Warwick, Multicore Solders Limited) 22) How Does Surface Tension In Pb-Free Solder Affect Spreading? The cohesive forces between liquid molecules are responsible for the phenomenon known as surface tension. The molecules at the surface do not have other like molecules on all sides of them and consequently they cohere more strongly to those directly associated with them on the surface. This forms a surface "film" which makes it more difficult to move an object through the surface than to move it when it is completely submersed. Source (Indium www.indium.com) Pb-Free Solder Alloys have a higher surface tension than tin lead solder alloys, therefore reducing Pb-Free solder alloys’ wetting spread capability. 23) What Is The Affect Of Wetting Behavior On The Wetting Angle In Pb-Free SAC Alloy? The surface tension of the Pb-Free alloys is significantly higher than SnPb40. This can be inferred from wetting balance measurements where it has generally been recorded that the wetting angle of conventional Tin Lead solder alloy is approximately 5° on a HASL Finish and Pb-Free SAC 387 is approximately 20° on Pb-Free Surface Finishes* *Reference Source - Impact of Intermetallic Growth on the Mechanical Strength of Lead-Free BGA Assemblies, P.Roubaud, etal, Alpha Metals/ Sanmina) 24) How About Dross or Oxidation In Pb-Free? One effect that is present in tin-lead wave soldering, and becomes a problem in Pb-free wave soldering, is the growth of an oxide layer on top of the wave. The use of nitrogen atmosphere will prevent the growth of an oxide layer and the dross formation in the process. A dramatic difference in wave oxide is shown, and an 80% improvement in dross formation was observed for N2 compared to air. This could lead to the conclusion that an inert atmosphere gives the best overall economy for the process. Alloy economy aside, the stability of the process increases dramatically when using nitrogen.

MacDermid Electronics Solutions 245 Freight Street, Waterbury, Connecticut 06702-9984 USA Telephone: +1 (203) 575-5700 www.macdermid.com/electronics Revision: January 16 2006

Lead Free Wave Soldering

Frequently Asked Questions

Page 8

25) What Is The Temperature Summary For Tin/Lead vs. Pb-Free (SAC Alloy) & Tin /Copper/Nickel? Alloy Melting Point Wetting Range Reflow Process Wave Soldering Process Hand Soldering Tip 63Sn 37Pb 183°C 210 – 220°C 280°C Peak 240 – 250°C 315°C SAC 305/307 217°C 230 – 250°C 300°C Peak 250 – 270°C 350°C Sn Cu Ni 227°C N/A N/A 260 – 270°C 350°C

ADDITIONAL ADVICE: Factors That Affect PTH Assembly Results If you’ve read this far into this document, it’s likely that you have an assembly problem at the wave solder process. Most commonly, the affected PCB will have marginal PTH wetting, which means that some through holes are wetting well, while some are wetting less than required. In general, marginal wetting occurs because the wave soldering process window is too narrow. With a narrow process window, slight PCB variations will show up as reduced wetting. As it is impossible to eliminate all minor PCB variations, the only solution is to open the wave soldering process window. This is especially important during the transition to Pb-Free, since Pb-Free solders are generally much more sensitive. The wave soldered through hole solder joint is over-designed. It has a larger surface area to wet than a SMT solderjoint, and if a lead pull experiment is conducted, the component pin normally breaks before solder joint fails. Recognizing this, industry documents and OEM specifications are beginning to allow PTH wetting of Pb-Free assemblies with solder rise less than 100% Effect of Pre-Heat: For Pb-Free wave soldering the pre-heat temperature is critical. It is important to determine your thermal profile. As with the higher temperature of Pb-Free, you need to eliminate the guess work. Higher temperatures can burn off all the active flux ingredients prior to hitting the wave, or result in insufficient preheat, yielding a poorly wetted through hole. To complement the use of thermal profiling, use an IR pyrometer to check top board temperature prior to soldering assembly. The IR pyrometer is often used for verification purposes. The temperature of the board (component side) during preheating should rise as fast as possible to 45 °C. Just before soldering, the temperature should exceed 105°C. For better results, 110-120C is preferred. Lower preheat is known to cause poor PTH fill. In one case study associated with preheat, a 5-8C increase in preheat temperature resulted in a yield improvement of more than 40%. The major difference between tin/lead and lead-free flow soldering is the higher melting point required by the lead-free alloy. Because lead-free alloys require higher preheat temperatures, thermal shocking of components as they enter the molten liquid wave should not exceed 100°C.

MacDermid Electronics Solutions 245 Freight Street, Waterbury, Connecticut 06702-9984 USA Telephone: +1 (203) 575-5700 www.macdermid.com/electronics Revision: January 16 2006

Lead Free Wave Soldering

Frequently Asked Questions

Page 9

Example of Wave Solder Profile Effect Of Flux Since the wetting characteristics of lead-free alloys are less than those typically exhibited by SnPb alloys at lower process temperatures, the selection of a good flux is mandatory to assure good solderability. The flux selected must be able to withstand exposure to the higher process temperatures required for lead-free alloys. In general, fluxes used for lead-free through hole soldering must be able to withstand topside PCB preheat temperatures as high as 130°C and solder temperatures as high as 280°C for three seconds of contact time or longer. A good flux will open up the wave soldering process window. It can create worry-free, consistent results. If the flux is marginal, any slight change will cause you to tweak the process. In a case study, Pb-Free flux opened the process window from unacceptable to consistently acceptable all the time. It is not worth penny pinching when selecting flux. Make sure to use the flux that gives a wider process window and yields. You can realize higher cost improvement on yield than saving on flux pricing. Effect of Wave Height: It is very critical to select correct wave height. Wave height should be such that solder must reach the top of the hole prior to exiting the solder wave, which involves a combination of wave force and capillary action. If the solder does not reach the top of the hole then insufficient amount of solder will remain the hole. In this case the solder that is in the hole will continue to rise due to capillary action after leaving the wave and leaving gap in the hole. In a case study Wave height adjustment yielded 5% improvement.

MacDermid Electronics Solutions 245 Freight Street, Waterbury, Connecticut 06702-9984 USA Telephone: +1 (203) 575-5700 www.macdermid.com/electronics Revision: January 16 2006

Lead Free Wave Soldering

Frequently Asked Questions

Page 10

Example of unpopulated board Through Hole Soldering
50-60% wave height

Example of populated board Through Hole Soldering

50-60% wave height

Immediately after wave

Immediately after wave

75-140 mm after wave hole at reflow temperature

75-140 mm after wave hole at reflow temperature
Capillary Action

5% Condition
Capillary Action

l

Effect of Dwell Time: For Pb-Free due to sluggish wetting of Pb-Free alloys compared to tin/lead, dwell time will be greater than tin/lead alloy wave usually at least half a second more. During the wave solder dwell you are taking board barrel temperature higher than the tin/lead alloy soldering. Boards with higher concentration of holes per unit area, with inner layer hole connections makes holes cooler, which results in slower heat up and cool down. Effect of Wave Temperature: Higher temperature will assist wetting faster, as it will heat the substrate faster for soldering. Any thick boards (above 1.5 mm) and/or boards with an internal thermal plane (power/ground at layer 3,4,5) will rob heat from the PTH. Simply raising wave pot temperature will be inadequate. A more intense preheat and wave-height adjustment is needed.

In conclusion Pb-Free wave soldering is the potentially the most complex area for Pb-Free implementation. The topics listed above are only a few of the interacting factors. Other factors to consider include through hole to pin size ratio, alloy and impurities control etc. The key is process optimization, which allows for the widest possible process window so that you have more latitude. Contact: Bihari Patel, OEM & Assembly Applications Manager, MacDermid Inc. 245 Freight St. Waterbury, CT 06702 Tel: 203 808 1378 bpatel@macdermid.com

MacDermid Electronics Solutions 245 Freight Street, Waterbury, Connecticut 06702-9984 USA Telephone: +1 (203) 575-5700 www.macdermid.com/electronics Revision: January 16 2006


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