| |
|
Technical note
Pb-free Specification for Terminations
Zetex, in line with other manufacturers, has elected to replace the tin/lead (SnPb) solder finishes on all its components with 100% matte tin without any change to the leadframe material, the thickness of the plating finish, or the performance of the product.
Zetex will not introduce any additional layers, such as Nickel, under the tin finish unless already present on the existing component.
The grain size of the tin will be a few microns and will be matte in appearance.
There is no impact on the bending and the abrasion caused by the change to matte tin. The behavior is analogous to SnPb finish.
|
 |
| |
|
| Properties |
Plating Composition |
| Sn85Pb15 - Sn95Pb5 |
Pure matte Sn |
| Melting temperature |
215°C (nominal) |
232°C |
| Plating Method |
Electroplating |
Electroplating |
| Thickness |
8-15µm |
8-25µm |
| Hardness, Ductility, Abrasion & Cracking |
Similar to SnPb. |
Evaluation of the various Pb-free plating options
| Pure matte Tin |
Ease of deposition; Compatible with PbSn & SnAgCu solder pastes; No composition control issues; Properties similar to PbSn; Concerns over "whiskering".
|
| NiPd/Au |
Commercially available; Cracking on bending; Not compatible with NiFe42 lead-frames; Expensive; Unsuitable for eutectic die attach.
|
| SnAg |
Comparable fatigue life to SnPb; high cost; Tight control required on alloy; complicated waste treatment
|
SnBi |
Very brittle and prone to fracture; Poor wetting and joint characteristics; Bi is a by-product of Pb mining, avoid any
applications with Pb containing solder materials.
|
| SnCu |
High strength; Brittle alloy; Poor wetting characteristics
|
Whisker Formation
- Whisker formation is a well-documented phenomenon of electroplated tin. It is also present on SnPb plated finshes.
Whiskering is a diffusion controlled mechanism cause by mechanical stress in the layer. Electrical fields, humidity, and ionic contamination can all influence the growth. They can grow from a few microns up to a few millimetres (worst case) in length.
- The Zetex matte tin finish is stress-free plating process that minimizes the risk of whisker growth.
- During the soldering process with Pb-free solder paste (e.g. SnAgCu) the matte tin finish alloys with the solder paste further reducing whisker formation.
Zetex Reliability Test Methods:-
- Standard reliability tests
- Pre-conditioning: Minimum of 4 hours storage 15~30°C 30~80%RH (JESD22A121 level B)
- Temperature cycling:-40°C ~ +85°C 10minute dwell, 500 & 1000 cycles
- Temperature/ Humidity Storage:60±5°C and 87+3/-2% RH, 3000 hours
30±2°C and 60±3% RH, 3000 hours
Evaluation and assessment (SPC, visual inspection by magnification: 25-40x & SEM)
Whisker count, whisker dimensions, preferred appearance on termination, tin thickness.
Acceptance criteria: Zetex has defined an acceptable maximum whisker length of 50µm
Qualification results for Pb-free plating (100% pure matte Sn)
Process Monitoring:
Zetex has implemented the following control measures to minimize the risk of whiskering
- Sn thickness monitoring(SPC)
- Continuous control of the tin electroplating process (process parameters, grain size, surface texture)
- Regular monitoring with temperature cycling at -35°C/ +125°C, 504 cycles
Recommended Reflow Soldering Profile
Figure 1 shows the recommended solder profile for devices that have Pb-free terminal plating, and where a Pb-free solder is used.
Figure 2 shows the recommended solder profile for devices with Pb-free terminal plating used with leaded solder, or for devices with leaded terminal plating used with a leaded solder.
These profiles are also available here
Reflow Soldering Profile
| Profile Feature |
Sn-Pb System |
Pb-free System |
| Average ramp-up rate |
~3°C/second |
~3°C/second |
Preheat
-Temperature range
-Time |
150-170°C
60-180 seconds |
150-200°C
60-180 seconds |
Time maintained above:
-Temperature
-Time |
200°C
30-50 seconds |
217°C
60-150 seconds |
| Peak temperature |
235°C |
260°C |
|
Time at peak temperature |
10 seconds |
40 seconds |
| Ramp-down rate |
3°C/second max. |
6°C/second max. |
Recommended Wave Soldering Profiles
The recommended solder profile for devices with Pb-free terminal plating where a Pb-free solder is used.
Wave soldering profile
The recommended solder profile for devices with Pb-free terminal plating used with leaded solder, or for devices with leaded terminal plating used with a leaded solder.
These profiles are also available here
Wave Soldering Profile
| Profile Feature |
Sn-Pb System |
Pb-free System |
| Average ramp-up rate |
~200°C/second |
~200°C/second |
| Heating rate during preheat |
Typical 1-2, max 4°/sec |
Typical 1-2, max 4°/sec |
| Final preheat temperature |
Within 125°C of solder temp. |
Within 125°C of solder temp. |
| Peak temperature |
235°C |
260°C |
|
Time within +0 -5°C of actual Peak |
10 seconds |
10 seconds |
| Ramp-down rate |
5°C/second max. |
5°C/second max. |
|
|
|
Moisture Sensitivity Level (MSL)
MSL indicates the maximum allowable time period (floor life time) before the soldering process is performed for which a moisture sensitive plastic device, once removed from the dry bag, can be exposed to an environment with a maximum temperature of 30°C and a maximum relative humidity of 60%RH.
Zetex products are qualified to the requirements of JEDEC J-STD-020C, MSL 1.
Please note that MSL 1 classified devices come in standard packaging materials and not in dry bags. |
