Basic Soldering Guide – How to Solder Electronic Components

Proper soldering technique and quality of solder are the lifeline of any PCB manufacturing and assembly. If you are into electronics, you must be knowing that soldering is basically a technique to join two metals using a third metal or alloy. In electronic PCB Manufacturing, assembly and rework, the metals to be joined are the leads of the electronic components (thru-hole or SMD) with the copper tracks on the PCB. The alloy used to join these two metals is solder which is basically tin-lead (Sn-Pb) or tin-silver-copper (Sn-Ag-Cu). Tin-lead solder is called leaded solder because of the lead present in it while the tin-silver-copper solder is called lead-free solder because no lead is present in it. The solder is melted using either a wave soldering machine or a reflow oven or a normal soldering iron and this molten solder is then used to solder the electronic components on to the PCB. A PCB or Printed Circuit Board after assembly of electronic components is called PCA or Printed Circuit Assembly.

Few other terms such as brazing and welding is often linked with soldering. But

one should remember that soldering, brazing and welding are different from each other. Soldering is done using solder while brazing is done using a lower melting-temperature filler metal. In welding, the base metal also melts while joining two metals whereas this is not the case with soldering and brazing.

Quality of solder and the soldering technique decides the life and performance of any electronic equipment, appliance or gadget.

Flux – Types and Role of Flux in Soldering

Flux plays a vital role in any soldering process and electronics PCB manufacturing and assembly. Flux removes any oxide and prevents oxidation of metals and hence helps in better soldering quality. In Electronics PCB Assembly process, flux removes any oxide from the copper tracks on the PCB and oxides from the leads of the electronic components. These oxides are biggest resistance in good soldering and by removing these oxides, fluxes play a very vital role here.

There are basically three types of Flux used in Electronics:

1. R Type flux – These flux are Non-Activated and are used where there is least oxidation.
2. RMA Type Flux – These are Rosin Mildly Activated Flux. These fluxes are more active than R-Type fluxes and are used at places where there is more oxidation.
3. RA Type Flux – These are Rosin Activated Flux. These are very active flux and are used at places that have too much oxidation.

Some of the fluxes available are water-soluble. They get dissolved in water with no pollution. Also there are No-Clean Flux which require no cleaning after the soldering process.

The type of flux to be used in soldering depends on various factors such as type of PCB to be assembled, type of electronic components used, type of soldering machine and equipments used and the working environment.

Solder – Types and Role of Solder in Soldering

Solder is the life and blood of any PCB. The quality of solder used during soldering and PCB assembly decides the life and performance of any electronic machine, equipment, appliance or gadget.

Different alloys of solder are available but the real ones are those that are eutectic. Eutectic solder is one that melts exactly at the temperature of 183 Degree Celsius. An alloy of tin and lead in the ration 63/37 is eutectic and hence 63/37 tin-lead solder is called eutectic solder. Solders that are non-eutectic will not change from solid to liquid at 183 Degree Celsius. They may remain semi-solid at this temperature. The nearest alloy to eutectic solder is tin-lead in the ration 60/40. The favorite solder for electronic manufacturers have been 63/37 for years. It is stilly widely used across the world.

Because lead is harmful to the environment and human beings, the European Union took the initiative to ban lead from electronics. It has been decided to get rid of lead from solder and electronic components. This has given rise to another form of solder called lead-free solder. This solder is called-free because there is no lead in it. Lead-free solder alloys melt around 250 °C (482 °F), depending on their composition. Most common lead-free alloy is tin / silver / copper in the ratio Sn96.5/Ag3.0/Cu0.5 (SAC). Lead-Free solder is also called “No-Lead” Solder.

Forms of solder:

Solder is available in various forms:

• Wire
• Solder Bar
• Solder Preforms
• Solder Paste
• Solder Balls for BGA

Electronic Components

There are two types of electronic components – Active and Passive.

Active components are those that have gain or directionality. E.g. transistors, integrated circuits or ICs, logic gates.

Passive electronic components are those that do not have gain or directionality. They are also called Electrical elements or electrical components. E.g. resistors, capacitors, diodes, Inductors.

Again, electronic components can be in thru-hole of SMD (Surface Mount Devices or Chips).

Electronic Companies

Since, electronic companies are the ones doing all the soldering and PCB manufacturing, they cannot be ignored here. Some of the top electronic companies are: Apple, Cisco , Texas Instruments, Fujitsu, Mitsubishi Electric, TCL, Bharat Electronics Limited, Siemens, Philips.

Tools and Equipments needed for Soldering

As explained above, soldering can be done the 3 ways:

1. Wave Soldering : Wave soldering is done for mass production. Equipments and raw materials needed for wave soldering are – wave soldering machine, solder bar, flux, reflow checkers, dip tester, spray fluxers, flux controller.
2. Reflow Soldering: Reflow Soldering is done for mass production and is used for soldering of SMD components on to the PCB. Equipments and raw material needed for reflow soldering are – Reflow Oven, Reflow checker, stencil printer, solder paste, flux.
3. Hand Soldering: Hand soldering is done in small scale production and repair and rework of PCB. Equipments and raw materials needed in hand soldering are – Soldering iron, soldering station, solder wire, solder paste, flux, desoldering iron or desoldering station, tweezers, solder pot, hot air system, wrist straps, smoke absorbers, static eliminators, heating gun, pick-up tools, lead formers, cutting tools, microscopes and magnifying lamps, solder balls, flux pen, desoldering braid or wick, desoldering pump or sppon, overcoat pen, esd material.
4. BGA Soldering: Another form of electronic components are BGA or Ball Grid Array. They are special components and need special soldering. They do not have any leads, rather they used solder balls used under the component. Because the solder balls have to be placed under the component and soldered, soldering of BGA becomes a very difficult task. BGA soldering need BGA soldering and rework systems and solder balls.

Wave Soldering

A wave soldering machine can be of different kinds, suitable for leaded wave soldering and lead-free wave soldering but all of them have the same mechanism. There are three zones in any wave soldering machine –

• Preheating zone – This zone preheats the PCB prior to soldering.
• Fluxing zone – This zone sprays flux on to the PCB.
• Soldering zone – The most important zone where there is molten solder.

There can also be a fourth zone zone called for cleaning of flux after the soldering is done.

Process of wave soldering:

A conveyor keeps moving across the plant. Employees inserts electronic components on to the PCB that keeps moving forward on the conveyor. Once all the components are in place, the PCB moves to the wave soldering machine passing through the different zones. Solder waves in the solder bath solders the components and the PCB moves out of the machine where it is tested for any possible defect. If there is any defect, some rework / repair work is done using hand soldering.

Reflow Soldering

Reflow Soldering uses SMT (Surface Mount Technology) to solder SMD (Surface Mount Devices) on to the PCB. In Reflow soldering there are four stages – preheat, thermal soak, reflow and cooling.

In this process solder paste is printed on the track of the circuit board where the component is to be soldered. Printing of the solder paste can be done using a solder paste dispenser or through stencils printers. This board with solder paste and components of the paste is then passed through a reflow oven where the components get soldered to the broad. The board is then tested for any defect and if there is any defect, rework and repair is done using hot air systems.

Hand Soldering

Hand soldering is basically done for small-scale manufacturing or repair and rework.

Hand Soldering for thru-hole components is done using a soldering iron or a soldering station.

Hand soldering of SMD components is done using Hot Air Pencils or Hot Air Rework Systems. Hand soldering of thru-hole components is easier as compared to hand soldering of SMDs.

Key points to remember while soldering:

Soldering is accomplished by quickly heating the metal parts to be joined, and then applying a flux and a solder to the mating surfaces. The finished solder joint metallurgically bonds the parts forming an excellent electrical connection between wires and a strong mechanical joint between the metal parts. Heat is applied with a soldering iron or other means. The flux is a chemical cleaner which prepares the hot surfaces for the molten solder. The solder is a low melting point alloy of non ferrous metals.

1. Always keep the tip coated with a thin layer of solder.
2. Use fluxes that are mild as possible but still provide a strong solder joint.
3. Keep temperature as low as possible while maintaining enough temperature to quickly solder a joint (2 to 3 seconds max for electronic soldering).
4. Match the tips size to the work.
5. Use a tip with shortest reach possible for maximum efficiency.

SMD Hand Soldering Methods:

1. Method 1 – Pin by pin Used for : two pin components (0805 caps & res), pitches >= 0.0315″ in Small Outline Package, (T)QFP and SOT (Mini 3P).
2. Method 2 – Flood and suck Used for : pitches <= 0.0315″ in Small Outline Package and (T)QFP
3. Method 3 – Solder paste Used for BGA, MLF / MLA packages; where the pins are underneath the part and inaccessible.

BGA or Ball Grid Array is one type of packaging for surface-mounted PCBs (where components are actually ‘mounted’ or affixed on the surface of the printed circuit board). A BGA package simply looks like a thin wafer of semi-conducting material that has circuit components on only one face. The Ball Grid Array package is called such because it is basically an array of metal alloy balls arranged in a grid. These BGA Balls are normally Tin/Lead (Sn/Pb 63/37) or Tin/Lead/Silver (Sn/Pb/Ag)

RoHS: Restriction of Hazardous Substances [ lead (Pb), mercury (Hg), cadmium (Cd), hexavalent chromium (CrVI), polybrominated biphenyls (PBB) and polybrominated diphenyl ethers (PBDE).]

WEEE: Waste from Electrical and Electronic Equipments.

Lead-Free Solder: Solder with NO Lead (Pb).

Lead-Free is taking rapid momentum around the world after the EU (European Union) Directives to wipe off lead (Poison) from electronic soldering considering its health and environmental effects.

There will undoubtedly come a time when you need to remove the solder from a joint: possibly to replace a faulty component or fix a dry joint. The usual way is to use a desoldering pump.

Static electricity or ESD is an electrical charge that is at rest. This is mainly created by an imbalance of electrons that stay on a specific surface, or in the environmental air. The imbalance of electrons (in all cases, is caused by absence or surplus of electrons) thus causes an electrical field that is capable of influencing other objects at a distance.

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