Cl, Br and F are halogens that are traditionally present in soldering materials like solder paste, soldering fluxes and solder wire, but for the most part they are found in solder masks, printed circuit boards, cables, connectors and wires. Halogens have the property of staying active on all temperatures. That is why they are very interesting to use as activators to clean the surfaces that need to be soldered.

The chemical reaction:
Halogens have the ability to react with metals. The reaction product is a metal salt that usually stays on the board after soldering. These metal salts have the ability to substract moisture from the air. When this happens, and voltage is present on the board, corrosion is very likely to manifest.

2CuO + 2Cl2 = 2CuCl2 + O2: Cu is consumed, metal salt formation

The difference with lead containing alloys:
When changing over to lead free alloys, leakage currents were measured over 100 times their normal value in some voltage protection devices. When using a halogen free, lead free solder paste, the leakage currents get back to normal. Fact is that the original lead containing solder paste also contained halogens, but this did not influence the leakage currents.

The water solubility of the metal salts:
Water solubility is an indicator for corrosion sensitivity. Looking at the metal salts
formed with Cl, the most commonly used halogen in soldering materials; one sees that the

Metal salt	Chemical designation	Solubility in cold water (g/100cc)
Lead chloride	PbCl2	0,99
Copper chloride	CuCl2	70,6
Tin chloride	SnCl2	83,9
Silver chloride	AgCl	89x10-6

Table 1 : water solubility of the metal salts
tin chloride and the copper chloride are the most water soluble. When changing over to lead free, the increased tin content will have the biggest influence on the increased water solubility. The metal salts formed with SnAgCu for example will be about 50% more water soluble than the metal salts formed with SnPb.

The electrical conductivity:
Another parameter that influences corrosion is the conductivity of residues after soldering. This is also tested by a standard surface insulation resistance (SIR) test. To test the difference between a halogen free and a halogen containing solder paste, the following test was conceived. Two copper plates were printed with respectively 5g of a halogen containing and 5g of a halogen free SnAgCu solder paste. The halogen containing paste was the same paste as the halogen free one but < 0,5% halogens were added. After a reflow process of about 5 minutes and a peak temperature of 240°C, the plates were washed in a 75%IPA - 25%Demi-water mixture. The conductivity in microSiemens of the mixture was measured before and after with a high precision conductivity measuring device. The values for the halogen containing paste were clearly higher.

What are the dangers?
Electro migration or corrosion is very dependent on the atmosphere in which the electronic unit is working. Also the frequency of usage is a very important parameter. When an electronic device is being switched on and off frequently, the solder joints will expand and
contract accordingly, together with the residues that cover and protect the metal salts. After some time, the residue can show some fatigue signs like cracks and the metal salts can come in contact with the atmosphere. When the electronic unit is working in an atmosphere with high humidity, corrosion is very likely to happen. The biggest problem is that the standard surface insulation resistance (SIR) tests and electro migration tests are not always able to predict this phenomenon. These tests are performed with a certain temperature and relative humidity but do not take fatigue into account. The safest solution for this moment is to solder without halogens.

What is absolutely halogen free?
Logically, absolutely halogen free means that the product does not contain any halogens.
Unfortunately it is not so easy to recognize such a product on the market. Most solder pastes,
soldering fluxes and solder wires are classified according to IPC-standards. The IPC Standard,
IPC J-STD-004A (2004) still allows a halogen content of 0.05% halogens for a 'L0'-
classified product. But also the European standard EN 61190-1-2 (2002) allows 0.01%
halogens for a 'L0'- classified product. In most of the datasheets of the producers of soldering
materials, the presence of halogen is only vaguely described or totally missing. Unfortunately
there is no easy way to determine the presence of halogens or to quantify them. The silver
chromate paper test is very easy to mislead with the so-called hidden halogens. Titration, on
the other hand, is very accurate, but the specific laboratory tools and chemicals are needed
and it almost takes an entire day to perform this test. This means that in most cases, one will
have to rely on the information that is provided by the producer.

Conclusion:
If for a certain application, a halogen free soldering product is available, it
certainly is the safest choice.