Soldering reflow and rework

Soldering Guide

This is just a quick guide to Soldering, I may add photos at a later time.

I'm not claiming this is a full on guide or anything amazing, there are probably mistakes, so please forgive them.
If you're going to comment, keep it strictly constructive.

Soldering is a very useful skill when working as an engineer and learning how to do it quickly and efficiently can speed up any testing or board re-work you may have to do.

After completing a short half day simple course in a soldering, I just want to write a quick run down on the different tasks that will help you solder in a quick and safe manner which will minimise the damage done to both the PCB and the components that you are working with.

What is solder?

Solder itself is a fusible metal alloy with a melting range from 90°C to 450°C. For the application we’ll be using it for, the range will be typically 180°C to 240°C. Leaded solder is a tin and lead alloy. The two most common alloys are 60/40 Sn/Pb and 63/37 Sn/Pb. Leaded solder is used in older electronic boards and circuits that need stronger solder joints.

Lead Free solder is becoming the norm with the introduction of the RoHS directive (Restrition of Hazardous Substances). The Leaded solder would give off harmful emissions in use, whereas lead free solder does not. Lead free solder does have a higher melting point, from about 5-20°C higher. The most commonly used solder is SnAgCu which is a mix of Tin (96.5%), Silver (3%) and Copper (0.5%).

Flux Pen

A Flux pen is very handy for helping to achieve clean joints and the reduction of dirt around the joint. Flux comes in many forms but the best is in a pen form, where you can simply drag the pen over the area which you will be soldering. This will protect the board from heat damage and help to conduct heat, to help reflow of the solder, helping the solder connect a pin to a pad or to a wire. Flux is corrosive however and needs to be removed after each job.

Electrolube

The best way to remove flux after each application, use electrolube. This stuff comes in a big can and cleans up the excess flux around the board, leaving the pcb clean and shiny and presentable.

Taking Care of your soldering iron

I think to start off, it is probably best to discuss taking care of the soldering iron. The irons I’ve worked with were the MetCal and the Pace models. A variable temperature can be very helpful as there is a difference between the different solder you can use. Also you have to be aware of the damage that applying heat to the board and the components around. Some IC’s can be more sensitive than others. Applying Kapton tape to the surrounding circuits can help prevent damage to heat sensitive components.

The Soldering iron itself is the device that heats up the solder in order that it can flow and attach itself to the joints, connecting pins to pads or wires to pins, or whatever. The iron consists of a metal tip and an insulate handle. Care must be used when using a soldering iron as they are very hot and can cause a very nasty burn.

To take care of your iron, simply turn it off when not in use. When powering the iron down, dip it into some tin replacement gel. Can’t remember the name of it properly. But will change it later. Clean off the end of the iron and melt some solder onto the end of it, then leave the iron on its stand. Now you can power off in the knowledge that the next time you power on your iron, it will be in good condition.

Components Removal

When applying heat to any components, the rule is, you should only apply 2-3 seonds of heat maximum or damage may occur.

THT (Through-Hole)

These are probably the easiest parts to remove. Through-hole resistors and diodes are easy enough. Apply some flux beforehand. Apply some heat to each joint and gently pull from the opposite side of the board to pull the joint through the hole. You can also use a solder sucking gun to suck the solder out of the hole while you apply heat to the solder joint.

For larger parts, like connectors or headers, it is probably best to apply an amount of solder that connects the pins together, apply the heat to this one big joint now and pull the component from the opposite side and you’re done.

SMT (Surface Mount Technology)

These components are connected to pads on the PCB. These pads are easy to rip off and damage if you’re not careful. So take care in removing components off of these. Pads are difficult to re-attach to the PCB and their vias. Often this process is impossible if the via connects to different layers in the PCB.

For resistors, you can use a number of methods. Firstly, you can use two irons, one from each side. Apply some flux, apply heat from both sides and it should pop right off. Another method is to use a specific iron head that sits exactly over the component, applying heat simultaneously on both sides. Apply some sodler to the tip first for this one. The third method is to use just the one iron and apply some more solder so that it connects both terminals at once and removes immediately.

For larger components like SOIC’s and TQFP’s, the above methods can cause damage to the device and are considered destructive methods of component removal. For IC’s it is best to use a heat gun. Using the minimal heat on the gun (around 200-220°C) with a small amount of fan pressure, move the gun in a circular motion around the IC, and apply a little pressure with a probe or knife to move the IC once the solder melts.. You can also use a heat protector with suction nipple to remove the IC while you apply heat to the device. It is recommended to use Kapton tape all around the surrounding PCB for this method as it distributes heat all around the board.

If you don’t care about destroying the IC, then you can simply apply a strip of solder around the IC. Place one end of the strip of solder to one corner of the IC and wrap it around the IC. Apply heat using the soldering iron, using either a single point tip or a specialised tip that covers the IC completely.

It is recommended to clean off the pads after removing components to leave it clean for future use. Apply some flux to a length of solder wick. Place the wick over the pads you wish to clean and then apply some heat. The solder wick should be the same width as the pads you are cleaning. A wide head tip for the iron is recommended for this operation. The wick will soak up the unnecessary solder, leaving the pad nice and clean. It is also recommended to clean the board from the flux (as most flux is corrosive) using Flux cleaner. Electrolube is a recommended brand for ths.

Component Replacement

Component replacement is fairly straight forward enough. Care should be taken with IC’s that are sensitive to static electricity. Make sure you are grounded at all times for this.

THT (Through-Hole)

This is the easiest of component placement or replacement. Simpy line the component up with the holes in the PCB, assuming they match of course. Once component legs have been inserted through the PCB, start applying solder to each leg. If the legs are protuding too far on the other side of the PCB, snip off the ends. The usual amount to leave on the opposite side is amount 1-2 millimetres from the top of the solder.

SMT (Surface Mount Technology)

For resistors, it is an easy procedure. Apply some flux to the pads. Clean off the pads using some solder wick. Do not apply too much pressure on the pads or you may damage or remove them. Apply some more flux on the pads and add a tiny amount of solder to one pad. Place the resistor or diode down using a tweezers and make sure it is straight. Heat the pad with solder on it and place the resistor on top. Once it is straight, you can apply a tiny amount of the opposite side of the resistor.

Placing/Replacing IC's is slightly harder but not by much.Clean the pads as mentioned above. Apply some flux to the area you wish to solder. The trick here is to place the IC down correctly, observing that Pin 1 on the device matches the Pin 1 pad on the PCB. Solder one corner pin to it's relevant pad. Then using this point as pivot point, ensure the IC lines up with the rest of the pads. Then solder the the pin in the opposite corner. Using a technique called drag soldering, apply a tiny amount of solder to a few of the legs and simply drag the iron gently along the row of pins. Do this to each side. If the solder stops flowing so easily, apply more flux and continue on. If some of the legs are soldered together, it is advisable not to use solder wick unless as a last course of action. The solder wick can damage the pins and lift the pads. Simply apply more flux to the joined pins and drag downwards between the pins, each time wiping the iron after each stroke. Apply flux in between strokes as well. This will speed up the process and the legs will be free of the extra solder. If during this process, the legs move, simply use a sharp knife or a probe to gently move the pin back into it's proper position.

Alternatively, if you don't like using the drag solder method, you can solder each pin individually. By applying the tiniest amount of solder to the iron and touching the pin at the base so the solder joins. Then with the iron drag upwards and onutwards at a 45degree angle.

Lifting pins

Sometimes it is necessary to lift pins on an IC to test a signal coming off of it or to stop a signal passing through the IC and into the pad.

This is a simple but very delicate process. Apply the flux, apply some heat with the iron. Then gently using a sharp knife or a very thin probe (ideal), pry the leg outwards and upwards. Ideally the leg should not be lifted any higher than the height of the IC.

Replacing the pin or leg is simply the reversal of the above. Move the leg downwards towards the pad using the probe and apply some solder to the pin and pad to ensure they joined.

Placing a probe/wire

For testing or rework purposes it is sometimes necessary to attach a wire to a pin on an IC. This is not a difficult process, but like the pin lifting above, it is a delicate one.

Get a thin piece of wire, ideally the core should be the same width as the leg or pin. Strip some of the plastic, exposing a small amount of the core. Most packages have whats called a gull wing pin, which is a reference to the shape of the pin. Older packages used to use J-Leads. I won't cover these here. However to make a proper connection, 3/4 of the exposed core must be in contact with the pin. This has to be taken into consideration when exposing the core. It may be necessary to bend the core to fit alongside the pin. Then having applied the flux, apply heat and some solder to the wire and attach to the pin. Ensure there are no holes or gaps between the pin and the attached core, if so, reflow the solder and ensure it sits fully against the pin.

You may be attaching the wire to a lifted pin, in this case, it is recommended to attach the wire to the top of the pin, fitting along the pin. Although it is not unheard of to attach the pin underneath, it just isn't common practice.

You may want to work the wire around the board to connect to another point on the board. In this case, simply hold the wire down and when you bend the wire, hold the tweezers at the point you wish to bend and then bend the wire, thus causing no undue stress at the solder joint. If the wire is a permanent fixture, then an application of glue at each corner is advised.

TombStoning

When you can't get a signal directly from the source and your only option is from a connecting component, then this is a very helpful trick. It's similar to removing the component, except instead of removing the component, you simply stand it up on one end, whilst still connected to the pcb pad. Attach your wire as you would with above methods. Be careful with the wire as this connection is very weak and can rip off your component as well as the pad, leaving you with a mess to fix up.

IPC Standards

For more information on the IPC standards, visit http://www.ipc.org

Handy and necessary Tools

Soldering Iron - Pace, Metcal (variable temperature setting ideal).

Solder - Lead Free for lead free applications if using leaded, 63/37 is ideal.

Heat Mats - place under component so that no heat transfer onto work surface occurs.

Kapton tape - ideal for protecting surrounding components from heat damage.
Solder Wick - varying widths for varying sized pads, good for cleaning solder off pads.
Flux - Helps for wetting action in soldering applications.
Flux cleaner - removes corrosive flux from board after job is done.
Tinner - used for tinning tip of soldering iron, helps prolong life of tip.
Variety of solder tips for different purposes - speeds up soldering and can specific tips can minimise damage

First Blog

I'm not completely sure about this whole blog thing, but I figure I've heard enough about it to finally give in.

However I'm going to try and put more technical stuff up here on things I'm currently learning as I work up the ladder of working as a Hardware Design Engineer.

Here's some background. My name is Michael Peter O'Keeffe, the Peter part being important as I'm named after my dad and everyone finds him first, thinking it's myself. I completed a BEng in European Electronic Systems in 2006, in DCU (Dublin City University). Basically I studied Electronic Engineering with an emphasis on telecommunications with French for technology and a number of business modules for non-business people, like introduction to law, marketing, accounting and new entreprise development.

I've been working here in Ircona (trading as Telemed Technologies) since I finished college with a few breaks in between to go travelling. So far, I've done a lot of schematic entry using Allegro Design Entry. Projects I've worked on have varied from high speed servers to small ticketing systems and indoor mobile solutions.

I'm hoping this blog will be of some help to engineers starting off, out there. If not, it'll help me understand it by trying to explain to this blogger yokey thing.

Laters,
-Mikey