Professional hand soldering

This guideline aussumes you have basic knowledge of soldering and have soldered THT components before. If that is not the case contact the laboratory staff for an introduction to soldering.

• Make sure your workplace is clean and tidy.
• It should be well-lit.
• Check your tools and supplies before working.
• Inform the laboratory staff if something is missing or broken.
• Leave your workplace as neatly prepared as you would like to have found it.

• A good heat transfer between the tip of the soldering iron and the component and pad on the PCB is important. Liquid solder will enable a good heat transfer.
• Keep the tip at the soldering joint until the solder has moved where it is supposed to be. In a good (THT) soldering joint the solder will form a cone (not a ball) on both sides of the PCB. The solder should at least be visible from the other side of the PCB.
• The components should not
• Cut off the protruding wires of the components before you solder. Cutting them off after soldering puts too much stress on the joint. he protruding part should be less than 1.5mm. There are specials pliers for that available in the lab.
• Do not use too much solder. The joint should have a cone shape (as mentioned before) and the wire of the components should still be visible.
• Use a bending help for small axial components (like resistors).
• Componts may not touch the PCB. If needed use beading pliers (“Sickenzange”) to form your components legs accordingly.
• Usually it is a good strategy to start with the smallest components because clamping will be easier in this order.
• Place components in a consistant manor, e.g. all resistors read the color bands left to right.
• In order to clamp your components you can use reverse tweezers in the lab as well as third hands.
• The size and shape of the soldering tip should match the size of the joints. Large tips can provide more heat. Generally chisel-shaped tips are preferred over conical tips.
• Change the temperature of the soldering station frequently when soldering. A good starting temperature might be 330°C. For small components you can go down to 280°C. If you need more than 350°C consider changing to a bigger tip or to using a preheater.

• Use lower temperature whenever possible.
• Clean the tip right before you want to make a new solder joint using the spiral wool in the stand.
• DO NOT clean the tip before putting it back to the stand. The leftover tin will help preserve the tip longer. Even better would be to put on new solder.
• In the lab the soldering station will go to a standby temperature of 150°C after 15 minutes. If you do not need your sodlering station for some time go to standby manually or turn the station off completely.
• Inspect your tip before using it. It should be silverish and it should be easily to wet it with new solder.
• In case the tip is not easily wetted with new solder, clean it and wet it several times.
• As a last resort you can use a tip reactivator on your tip.
• If it is still not okay replace the tip with a new one. There are repalcement tips in the lab.

In case of excess solder or if a components needs to be desoldered there are some options:

• Desoldering pump
  • We have several hand operated desodlering pumps.
  • Make sure the joint is completely liquid before trying to remove the solder.
  • Sometimes it is helpful to add more solder (and/or flux) before desoldering due to the better heat transfer.
  • Sometimes it is helpful to find a second person for this operation.
  • The silicone tip can be replaced if used up.
• Continuous vacuum desoldering iron
  • Currently we do not have a desoldering iron in operation in the lab.
• Desoldering wick
  • Apply flux to the desoldering wick before use
  • Only use the first part of the wick.
  • Cut off already used pieces.

Solder wire is an alloy which mostly consists of tin (Sn) and caontains some smaller quantities of silver (Ag) and/or copper (Cu). Other metals are possible but less common. In the lab we only use lead-free solder (Pb free). Lead-free soler wire usually has a melting temperature of 217-227°C. Solder wire usually also contains flux inside. For most soldering operations extra flux will not be necessary. Some hints:

• Do not tear off solder wire from the spool as you will not have flux at the points of separation. Use a cutting tool or use a soldering iron to cut of pieces of solder wire.
• Do not curl up solderwire in your hands. Skin fat and oil can hinder soldering.
• Choose the right solder wire diameter for your task. In the lab you may find 0.35mm, 0.7mm and 1mm wire.
• Solder wire likes to be stored dry an cool. It may have an expiration date (important for industry), it will also work years after that time.

The purpose of flux is to remove the oxidised layer on the PCB and the component directly before the soldering process. Usually there is a sufficient amount of flux in the solder wire (or paste). In some occasions it is needed to add extra flux to improve soldering. For example when repairing or when the soldering process is taking too long. If there is a “nose” in the soldering there was not enough flux. The flux pens you will find in the lab are of the “No-Clean”-type, which means it can stay on the PCB after soldering and does not need to be cleaned. Using flux gel is sometimes help as it will stay in place after dispensing it. Most soldering fluxes come in liquid form.

• Use only as much flux as needed.
• Use REL0 or ROL0 types preferrably as they are least aggressive and are mostly free of halogen.

As stated above the flux available in the lab can stay on the PCB. In case you want or need to clean your PCB from flux residues complete the following steps:

• Wet the affected areas with isopropyl alcohol.
• Scrub the area carefully with a suitable (ESD) brush.
• Rinse the PCB by tilting it. Remove residues with fuzz-free (ESD) rag (pink). Residues should not go under neighboring components on the PCB.
• Dry Let it dry by air. Do not use compressed air as it may cause ESD issues. Slowly drying flux may be removed with the fuzz-free rag (pink: ESD). Test: a clean surface will not feel sticky.

In case of high-mass components and or thick ground plane connections it might be helpful to first preaheat the PCB.

• Preheat the PCB to 100-120°C using the IR preheater.
• Wait and check the temperature fo the PCB using the IR thermometer.
• Use a standard soldering iron to solder. As the temperature difference is now smaller, soldering with thick ground placnes should be easier.
• Alternatively you can use a hot air gun to preheat the PCB locally.

In order to tin cable ends you can use a solder bath.

• Wear sfatey glasses.
• Turn it on and wait until the solder melts.
• Remove the oxidised first layer with the spatula.
• Use a cable stripper to prepare your cables.
• Dip the cables in flux.
• Dip the cables in the solder bath. Stop 1 mm before the insulation of the cable.
• Let the bath cool down and clean up the workspace.

• Temperature too high
• Soldering too long
• Damages due to pressing or scraping with the tip
• No wetting due to dirty tips
• Passive tips
• Corroded tips

Many of the rules and hints for THT soldering also apply for soldering Surface Mount Devices (SMD).

Contact soldering (i.e. with soldering iron) of SMD components is most commonly done in repair-work. Usually only a few compoents will need to be (de)soldered by hand. Attention: not all components can handle the shock immediate heat from contact soldering (e.g. be careful with ceramic capacitors). The choice of tip for your soldering iron becomes more important. Chisel-shaped tips are still preferred over conical tips, however you might want to use smaller tips or conical tips for fine pitched but multi-pin components. In addition to the normal tips there are tips for flow soldering (german “Schwalllöten”). You can use a tip with a “Hohlkehle” which has a small deposit for tin to solder multi-pin components with gullwing connectors (like QFP or SOIC) in a single operation. Similarily there are blade-shaped tips for J-type connectors (PLCC or SOJ). In both cases you will need to use extra flux. The “Hohlkehle” tip is availabe in the lab. Generally it is a good idea to apply some soldering wire to a pad first and then use tweezers to move the component in position while you keep to tin liquid using your soldering iron. The compoent is now fixed to the PCB and the other pin(s) can be soldered next without the risk of moving. For bigger ICs it might be a good stretegy to fix two or three corners first. Specifically for desoldering two pin SMD components we own a set of desoldering tweezers. Basically a toll with two soldering tips.

The soldering tin wire be the same as in THT soldering, however there are smaller sizes available. In the lab we stock 0.35mm solder wire. The soldering temperature will often be smaller than in THT soldering, however there are SMD components with big ground connectors and pads with big connections to the ground plane(s) which require more heat and/or time.

In the best case the solder tin fills the pad completely and it forms a cone shape on the side of the component of more than 25% of the height of the component. Solder may not touch the component (exeption SOT packages). A component offset is acceptable (up to 25% in class 3).

The lab owns a microscope for soldering and inspection purposes. This device is specifically handy for smaller SMD components. There are example PCBs available to learn how to solder small SMDs, ask the laboratory staff about it in case you are interested. The microscope is also connected to a monitor and is able to make screenshots and videos. Clean the binoculars with the provided tissues before you use the microscope.

Hand operated reflow soldering using hot air is less stress for the components if executed correctly. It requires some soldering tin or soldering paste (preferred) already on the pads. For dispensing soldering paste check the corresponding chapter below. This process is often combined with preheating the PCB (see THT). Components need to be placed by tweezers (or the Pick-and-Place maschine).

• Apply solder tin or paste
• Place components using tweezers (or the Pick-and-Place maschine)
• Small components might need to be held until the tin is liquid.
• Preheat from a greater distacne of 50-100mm, if you use sodler apste and do not use a preheater.
• Solder the component using hot air from a distance of 5-20mm (mostly vertical). Use an appropiate nozzle.
• Stop when wetting is of good quality
• Do not overheat components.
• Components might be blown away when the air flow is to high.

In case some parts of the PCB should not be heated use aluminium foil to protect it from the heat. The heat transfer is defined by the temperature of the gas (i.e. air) and the amount of airflow.

Currently the lab does not own an infrared or hybrid soldering station.

This guide is mostly about hand soldering. In case you want to assemble a full PCB the reflow oven might be a good ressource to use.

The most common problems from THT still apply. There are two more problems:

• Padlifting: Too much heat can raise the pad from the PCB.
• Tombstoning: A compoent might tilt to one side. This happens of the solder on one side melts faster as on the other side. Typically this happens in case one side is connected to a big ground plane.

There are two options available to dispense solder paste on a PCB at the university:

• Stencil printing (only possible if you have a stencil for your PCB)
  • Solder paste is applied all at once using a squeegee held at an angle of 45-60°.
  • high viscosity paste
• Individual dispensing using a hand dispenser which employs compressed air
  • very low viscosity paste

Please note that there are different pastes for each of the two options due to the different viscosities needed.

Solder paste consists of powder which comes in different particle sizes. In general the the inner diameter of the dispensing needle shall be 7 times the biggest particle size. For class 4 solder paste (20-38 μm) use an orange needle. Metal needles are for dispensing sodler paste, plastic needles can also be used for flux. Solder paste needs to be stored refrigerated. Take it out of the fridge before you use it and give it some time to heat to room temperature (approx. 1 hour).

The lab is currently not a proper ESD environment. It is possible to destroy components due to electrostatic discharges. However you can take some precautions to reduce the risk of ESD.

• The grey mats on the workplace are ESD protected
• Leave the components in the bag when moving in the lab.
• Use an ESD wristband whenever working with ESD sensitive components.