Following the lead free process, we must follow up, monitor and analyze the data regularly to keep the process under control.
Lead free soldering has been introduced, so numerous problems have been raised. In spite of this, many questions still have to be answered, including the definition of lead, its implementation costs, and even if all technical issues have been resolved. However, the experiment continues to provide good figures on the reliability of the new lead-free alloys.
This paper discusses the cost and energy effects, and the display process must be continually tested as the technology and process knowledge will be improved in the future. A standard improvement mode, such as Deming cycle (Deming cycle), used to maintain the lead-free welding process control, to make adjustments and improvements, and when possible to achieve cost savings.
material cost
tin solder
As an example, a tin pot of a welding machine contains about 760 kg of tin lead (SnPb) alloy. Using SnPb to fill the tin pot will cost about $$3960. The density of SnPb was 8.4 g/mm3. With tin copper (SnCu) alloy to fill the same tin pot needs 661 kg, its density is 7.31g/mm3:
Quality (7.31 / 8.4) = x 760 = 661
The result is solder costs increased by $28% or $$5063. Other lead-free alternatives, such as tin (SnAg, 135%) and Sn Ag Cu (SnAgCu, 145%) are more affected by solder costs.
Taking into account the welding point and the comparison between SnPb and lead free, we can make the following calculation. If the shape is the same, the quality of lead free alloys will be less, because of the high density. For a SnCu pin connector through hole welding, soldering quality: (P SnCu x P SnPb x massSnPb)
Because the solder joint looks different, wetting may be poor, the solder joint angle is different, we must verify that the quality of the calculation is approximately equal to the actual quality of the welding point increased.
In order to confirm, we weld a piece of a plate with a connector (192 pins per plate), which is called the weight difference before and after welding (Table 1). The weight of the increase in the number of welding solder.
Table 1 Comparison of the welding quality of SnCu and SnPb
SnPb
SnCu
Soldering 192 pins to the board,
Mass increase (g)
One thousand five hundred and eighty-four
One thousand two hundred and ninety-six
Solder cost
100%
128%
Welding 470 through holes of the plate,
Average quality per hole (Hao Ke)
Ten thousand three hundred and eighty-two
Eight thousand eight hundred and eighty
Solder cost
100%
126%
Flux
As in all welding processes in the same flux, play a major role. Weldability and welding defects can be improved and reduced, if you use the correct flux. If we implement "green" welding process, we use no VOC water-based flux, it than alcohol based flux has some advantages.
Several tests have now shown that the flux VOC and lead-free solder flux shows better results than. In particular, they are better for residues and. One reason is that the number of applications to the board is small. In the flux agents and chemicals in the water more than chemical reactivity in alcohol. Although VOC free flux is more expensive, with the total cost of these fluxes is about the same or even less, because for welding to reduce the total number of.
If the weld is improved, the number of rework will be reduced. To reduce the number of flux will also result in reduced maintenance. Cleaning the parts of the machine will be easier and can be done with hot water instead of chemicals and instruments.
However, the number of solder joints increases with the use of VOC free flux. This increase is partly due to the high temperature in the process, so that solder resist (resist solder) to soften. Compared with tin lead solder process, the removal of the more easily.
The new VOC free flux is now being developed. The supplier is trying to dissolve rosin flux and water flux in solder balls on the number of other people to reduce appreciation. These studies will continue, because most of the flux suppliers have not succeeded in finding the right formula.
element
For many components, changing the final surface coating of the pin will not be a major problem. Component suppliers are more likely to switch in the future if they have a large demand for lead free surface coatings. Because the technology is readily available, the prices of these components are not expected to increase substantially.
SnAg and SnAgCu ball seems to be an acceptable alternative to SnPb for BGA. For the surface coating of the surrounding package instead of the pin is under study, reliability and whisker tin (SN) issues must be addressed. High process temperature increases the moisture sensitivity of the components and the requirements for package integrity. Can withstand higher temperatures, such as 280 degrees C 5 seconds, the plastic is now being designed, will push up the price. Therefore, it is necessary to a with reflux of high precision (delta T is small and good heat transfer) welding furnace to run unleaded temperature curve and meet the cheaper the specifications of the components. If you can limit the maximum peak temperature in 245 DEG C, and all of the solder in accordance with the specifications of the lead-free solder paste to above the melting point, then for the user can get the component cost reduction.
Plate material
In addition to ban lead, halogenated flame retardant (halogenated flame retardants) will also be eliminated from the plate material. Therefore, the use of lead-free surface coating of the new sheet must be used with a higher glass transition temperature (Tg) to withstand higher process temperatures. These new sheets, as well as lead-free surface coatings, will affect the price. It is not clear how much these prices will increase, because most of the board suppliers are still in the process of optimizing the selection of the board.