Chemical reliability. lead-free. soldering. Steven Teliszewski Interflux electronics NV October 2005

Transcription

1 Chemical reliability in lead-free soldering

2 Chemical Reliability What is chemical reliability? The residues and reaction products of the soldering process that are left on the board, cannot interfere with the functionality of the electronic circuit. What can happen? Reduction of Surface Insulation Resistance. (SIR) Leakage Currents Electromigration Corrosion/Pickling (removal of surface metal) Electromigration (deposit of surface metal somewhere else)

3 Chemical Reliability How does it happen? 1) Spontaneous Corrosion without electrical Voltage SnO + 2Cl - SnCl 2 + O - At soldering temperatures the Cl atom gets free and reacts with the Sn SnCl 2 + H 2 O + CO 2 SnCO HCl The Tinchloride (metal salt) remains on the board. Being unstable, it reacts with moisture and CO 2 from the air and forms Hydrochloric Acid. The SnCO 3 is porous and allows HCl to penetrate to react again. SnO + 2HCl SnCl 2 + H 2 O The Hydrochloric Acid reacts again with the SnO

4 Chemical Reliability How does it happen? 2. Corrosion under electrical Voltage: Standard electrolytic process or electromigration When SnCl 2 (or another metal salt) attracts moisture an electrolytic solution is formed. Positive Sn ions and Negative Cl ions are formed. Sn-ion is deposited at the negative contacts/component leads. Cl-ion is attracted by the positive contacts where it dissolves another Sn-ion. This phenomenon, in worst case scenario, can lead to dysfunctional electronics within a few days or even hours!!!

5 Chemical Reliability What are the parameters? Atmospheric conditions: Temperature, humidity Water solubility of the residues Protection capacity of the body of the residue Working condition of the electronic circuit: ON/OFF ratio (fatigue of the body of the residue)

6 Practice What did already happen? In the past Corrosion was mostly related to a high presence of halogens either coming from the PCB, components or soldering materials. With the change over to lead free soldering alloys more cases of corrosion seemed to be popping up on the market. In these cases only a small presence of halogens was measured on the board. Removing the halogens,resolved the corrosion problem.

7 Practice Practice example Manufacturer fort he automotive industry Automotive Rank 1 ( IPC Class 3) application Cu selectively soldered with SnCuNi Application very sensitive to Cu-corrosion 5 VDC 2000 cycles C (1 hour/ cycle) Examining Cu for corrosion under electron microscope.

8 Practice Steven Teliszewski Interflux electronics NV October 2005

9 Practice Absolutely halogen free soldering chemistry!

10 What is the difference between SnPb and the lead free alloys? Solubility in Metal salt Chemical designation cold water (g/100cc) Lead chloride PbCl 2 0,99 Copper chloride CuCl 2 70,6 Silver chloride AgCl 89x10-6 Tin chloride SnCl 2 83,9 water solubility of the metal salts Water solubility is an indicator for corrosion sensitivity Increased Sn content will have the biggest influence SnAgCu-salts are theoretically about 50% more water soluble SnPb-salts

11 How to predict corrosion? Due to the increased corrosion sensitivity, standard corrosion tests might not be suitable to predict corrosion. Corrosion tests under static conditions (e.g. 85 C,85% r.h.) do not test fatigue cracking of the body of the residue that protects the reaction products from the atmosphere. Standard corrosion tests do not test unconsumed soldering chemistry (e.g. selective soldering, selective solder carriers) Automotive, Siemens and others already use special corrosion tests.

12 Special corrosion test: example Test performed by a german institute on request of an automotive manufacturer due to corrosion problems in the field. Test done on soldering chemistry that is most established on the market and passes standard corrosion tests. Test parameters Pattern of naked Cu sprayed with flux 85 C 20% R.H. 85 C 85% R.H. 5 VDC permanent Measuring SIR continuously Visually controlling the boards for corrosion

13 Test results Flux A SIR dropping bad result

14 Test results Flux B Immediate drop verybad result

15 Test results Flux C Late recovery LowSIR Bad result

16 Test results Absolutely halogen free chemistry IF2005C Good result

17 Test results Flux E SIR Dropping Bad result

18 Test results Flux E Corrosion??? No Migration

19 Test results Flux C Corrosion Migration

20 Test results Absolutely halogen free chemistry IF2005C No Corrosion? No Migration

21 Conclusions In all cases where an increased corrosion sensitivity was witnessed when changing over to lead free soldering alloys, the problems were eliminated by removing the halogens from the soldering chemistry. Soldering with absolutely halogen free soldering chemistry is the safest way. What is absolutely halogen free soldering chemistry??? 0 ppm = absolutely halogen free L0 allows 500ppm of halogens (IPC J-STD-004A (2004)) L0 is no guarantee for corrosion safety! No fast and easy way to measure the exact ppm-level Steven Teliszewski Interflux electronics NV October 2005