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--- ee:soldering [2025/03/06 13:41]
FKR@staff.hsrw
+++ ee:soldering [2025/03/14 14:50] (current)
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@@ Line 69: / Line 69: @@
 ==== Flux ====
-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 hen repairing or when the soldering process is taking too long. 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.
+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.
@@ Line 89: / Line 89: @@
   * 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 diffrence is now smaller, soldering with thick ground placnes should be easier.
+  * 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.
@@ Line 114: / Line 114: @@
 ===== SMT soldering =====
 Many of the rules and hints for THT soldering also apply for soldering **S**urface **M**ount **D**evices (SMD).
+==== Contact soldering ====
+Contact soldering (i.e. with soldering iron) of SMD components is most commonly done in repair-work. Usually only a few components 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.
+You can see the flow soldering with a "Hohlkehle" tip in [[https://hs-rw.cloud.panopto.eu/Panopto/Pages/Viewer.aspx?id=c5301793-a3d4-4684-913c-b2a000d7e954|this video]].
+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 the tin liquid using your soldering iron. The component is now fixed to the PCB and the other pin(s) can be soldered next without the risk of moving (see [[https://hs-rw.cloud.panopto.eu/Panopto/Pages/Viewer.aspx?id=9b16628b-bd9a-4cbb-bb18-b2a000dfabfe|this video]]. 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).
 ==== Microscope soldering ====
+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.
+==== Reflow soldering ====
+=== Hot air ===
+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.
+=== Infrared ===
+Currently the lab does not own an infrared or hybrid soldering station.
+=== Reflow oven ===
+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.
+==== Most common problems in SMT hand soldering ====
+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.
 ==== Dispensing solder paste ====
@@ Line 122: / Line 169: @@
 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).
 ===== ESD protection =====

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