In the implementation of lead-free process, we need understand the principles, characteristics and requirements of lead-free soldering, strictly manage lead-free materials, printed boards, components, testing, and standards, especially material management.

1. Solder paste screen printing process requirements

1.1. Thaw and stir: first remove the solder paste from the refrigerator for at least 4 hours, and then stir. The stirring time is 2 minutes for mechanical and 3 minutes for manual. Stirring is to physically separate the solder paste stored in the library, or to reduce it due to the high metal content caused by recycling. At present, lead-free solder paste Sn / Ag3.0 / Cu0.5 replaces the alloy with a specific gravity of 7.3, and the Sn63 / Pb37 alloy has a specific gravity of 8.5. Therefore, lead-free solder paste can be stirred for a shorter separation time than lead-containing solder paste.

1.2. Stencil template: stainless steel laser opening, thickness 80-150 mesh (0.1-0.25mm), copper and electroformed Ni mold analysis can be used.
1.3. Scraper: Hard rubber (polyurethane scraper) and stainless steel metal scraper.
1.4. Squeegee speed angle: 2cm-12cm per second (depending on the size and density of PCB components). Angle: 35-65 ° C.
1.5. Squeegee pressure: 1.0-2Kg / cm2
1.6. Reflow mode: suitable for various reflow equipment such as compressed air, infrared and gas phase reflow.

1.7. Process requirements: The solder paste screen printing process includes 4 main processes, namely registration, filling, leveling and release. To do the whole job well, there are certain requirements on the substrate. The substrate needs to be flat, and the dimensions between the pads are accurate and stable. The design of the pads should match the screen printing stencil, and there should be a good reference point design to assist in automatic positioning and alignment. In addition, the label imprint on the substrate must not affect the screen printing part. The design must be convenient for the automatic loading and unloading of the screen printer. The shape and thickness must not affect the flatness required during screen printing.

1.8. Reflow soldering process: The reflow soldering process is currently the most commonly used soldering technique. The key to the reflow soldering process is to adjust the set temperature curve. The temperature curve must match the solder paste product requirements of different manufacturers.

2. Reflow soldering temperature curve

2.1. Four key points on the lead-free reflow optimization process curve:
a. The heating rate in the preheating zone should be as slow as possible (select a value of 2-3 ° C / s) in order to control the solder bridges, solder balls, etc. caused by the collapse of the solder paste.
b. The active area must be in the range of (45-90sec, 120-160 ° C), in order to control the temperature difference of the PCB substrate and the change in flux performance, etc., to cause defects during reflow soldering.
c. The maximum welding temperature is maintained above 230 ℃ for 20-30sec to ensure the wettability of the welding.
d. The cooling rate is selected at -4 ° C / s.

2.2. Explanation of Humidity Change of Reflux Curve:
a. The flux of the solder paste begins to melt when the humidity rises to 100 ° C (begins to enter the active period). The main role of the solder paste in the active area is to remove the oxide layer on the surface of the solder. If the active area is too long, the flux will play too fast, but also cause the solder joint surface is not smooth and grainy. The time for solder paste to completely melt above the melting point and humidity (into the reflow zone) is about 30-45 seconds, depending on the thickness of the PCB, the size of the component, and the density to determine whether to extend the time.
b. The temperature of the active area can also help PCB components to absorb and reduce the temperature difference between small and large components, reducing the occurrence of malfunction.
c. The temperature difference between the large and small components entering the reflow furnace is about 11.4 ° C. Therefore, to reduce their temperature difference is also controlled from the active area. The maximum temperature difference can be reduced to 5-8 ° C.
d. Because lead-free solder paste is composed of multiple alloys, the cooling shrinkage time of the metal is different. In order to brighten the solder joint, in addition to other methods, rapid temperature reduction is the most effective method.

3. Defects in reflow soldering and their solutions

3.1. Welding defects are divided into major defects, minor defects and surface defects:
a. The main defect causes the SMA function of the product to fail.
b. Minor defect means that the wetting between the solder joints is good and the SMA function is normal, but it will affect the life of the product.
c. Surface defects do not affect the life and function of the product (usually based on the production process, appearance, etc.).

3.2.SMT Lead-free Requirements Problem formation and solution:

A. Tin beads
the reason:
a. In the component placement process, the solder paste is placed between the chip components and the pads. If the pads and component pins have poor wetting (poor solderability), the liquid solder will shrink and Make the weld insufficient, and all the solder particles cannot be aggregated into a solder joint. Part of the liquid solder will flow out of the weld and form tin beads.
b. The pressure of the Z axis during the placement process is too short to squeeze the solder paste out of the pad.
c. If the heating speed is too fast and the time is too short, the moisture and solvent inside the solder paste will not completely evaporate, which will cause the solvent and water to boil when it reaches the reflow soldering zone, and splash the tin beads.
d. The opening size and outline of the template are not clear.

Solution:
a. Follow up the pads, component pins and solder paste for oxidation.
b. Adjust the precise alignment of the template opening and the pad.
c. Precisely adjust the Z-axis pressure.
d. Adjust the temperature rising speed of the preheating zone activation zone.
e. Check whether the opening and outline of the template are clear. If necessary, replace the template.

B. Tombstone
(Manhattan phenomenon) One end of the component is soldered to the other end of the pad and it stands up.
the reason:
a. The two ends of the component are unevenly heated or the width and length of the pads are too wide and the gap is too large.
b. The position of the placement component is shifted.
c. The flux in the solder paste causes the component to float.
d. Poor solderability of components.
e. The thickness of printed solder paste is not enough.

Solution:
a. Uniform components and reasonable design the dimensions of the pads are symmetrical at both ends.
b. Adjust printing parameters and placement.
c. Use a moderate amount of flux (lead-free solder paste flux at 10.5 ± 0.5%).
d. Use lead-free solder paste or solder paste containing silver and bismuth.
e. Increase printing thickness.

C. Bridging
(Disconnected solder joints are connected together), one of the most common defects in SMT production, it can cause short circuits between components.
the reason:
a. The quality of the solder paste, the metal content in the solder paste is too high, and the printing time is too long.
b. There is too much solder paste, low viscosity, and poor slump. After preheating, it flows out of the pad and leads to the solder bridge of the denser gap.
c. Printing misalignment or printing pressure is too large, it is easy to cause fine-pitch QFP bridging.
d. The pressure of the placed components is too high and the solder paste overflows after being pressed.
e. The chain speed and heating speed are too fast. The solvent in the solder paste is too late to volatilize.

Solution:
a. Replace or add new solder paste (new solder paste can be replenished regularly during the printing process to maintain its metal content and viscosity).
b. Reduce the scraper pressure and use a solder paste with a viscosity of 190 ± 30Pa · S.
c. Adjust the precise alignment of the template.
d. Adjust the Z-axis pressure.
e. Adjust the reflux temperature curve and adjust the chain speed and furnace temperature according to the actual situation.

D. Less solder joints and insufficient solder volume
the reason:
a. Insufficient solder paste, printing after machine stop, stencil opening clogged, solder paste quality deteriorated.
b. Pads and components have poor solderability.
c. Less reflow time.

Solution:
a. Increase the thickness of the stencil, increase the printing pressure, and check whether the stencil is blocked after restarting after stopping. The template opening used for lead solder is ≥100% larger than the pad when the design allows it.
b. Use pads and components with better solderability.
c. Increase the reflow time.

E. False welding
the reason:
a. Poor solderability of components and pads.
b. Improper reflow temperature and heating speed.
c. Printing parameters are incorrect.
d. The stagnation time after printing is too long, and the solder paste activity becomes worse.

Solution:
a. Strengthen the screening of PCBs and components to ensure good soldering performance.
b. Adjust the reflow temperature curve.
c. Change the scraper pressure and speed to ensure good printing results.
d. Paste the solder paste as soon as possible after reflow soldering.

F. Cold welding
(The solder joint surface is dark and rough, and it does not melt with the object to be welded.)
the reason:
a. The heating temperature is not suitable.
b. Solder deterioration.
c. Preheating time is too long or the temperature is too high.

Solution:
a. Adjust the reflow temperature curve, according to the curve reference provided by the supplier, and then adjust according to the actual situation of the produced product.
b. Replace with new solder paste.
c. Check whether the equipment is normal and correct the preheating conditions.

G. Wicking
There are not many problems with Sn / Pb solder paste, and this problem often occurs when using lead-free solder paste.
Reason: Lead-free solder paste has less wetting and expansion than lead-containing solder paste. The wicking phenomenon is generally considered to be due to the large thermal conductivity of the component pins, the rapid temperature rise so that the solder preferentially wets the pin, and the wetting force between the solder and the pin is much greater than the wetting force between the solder and the pad. The upturn of the pins will further exacerbate the wicking phenomenon. In infrared reflow soldering, the PCB substrate and the organic flux in the solder are excellent absorption media for infrared rays, but the pins can partially reflect infrared rays. In contrast, the solder preferentially melts, and its wetting force with the pads greater than the wetting force between the solder and the pin, so the solder will not climb up the pin, instead the solder will climb up the pin.

Solution: When reflow soldering, the SMA should be fully preheated before being placed in the reflow furnace. Carefully check and ensure the solderability of the PCB pads. The device should not be used in production.

H. IC pin open / virtual soldering
After the IC pins are soldered, some pins are soldered, which is a common soldering defect.
the reason:
a. Poor coplanarity of components, especially QFP devices, due to improper storage, lead deformation and sometimes difficult to find (some chip mounters do not have coplanarity inspection function).
b. The solderability of the pins is not good, the pins are yellow, and the storage time is long.
c. The solder paste is not sufficiently active and the metal content is low. Usually, the solder paste metal content for soldering of QFP devices is not less than 90%.
d. The preheating temperature is too high, which will cause the pieces to oxidize and the solderability will deteriorate.
e. The opening size of the template is small and the amount of tin is not enough.
Make corresponding solutions to the above problems.

I. Solder beading
Solder beading is a special phenomenon of solder ball formation when using solder paste and SMT processing technology. Simply put, solder beading refers to those very large solder balls with tiny solder balls attached (or not). They are formed around components with extremely low standoffs, such as chip capacitors. Solder beading is caused by the flux exhaust. This exhaust effect exceeds the cohesive force of the flux during the preheating phase. The exhaust promotes the formation of vertical clusters of the solder paste under the low-gap components and melts the solder paste during reflow Rising out from under the component again and agglomerating.
the reason:
a. The thickness of the printed circuit is too high, and the solder joints and components overlap too much.
b. Too much solder paste is applied under the component, placing too much pressure on the component.
c. The preheating temperature is too fast, and the preheating temperature is too high.
d. Components and solder paste are wet, the activity of the flux is too high, the solder powder is too fine or too many oxides.
e. The solder paste slumped too much.

Solution: Change the pore shape of the stencil so that less solder paste is sandwiched between the low-footed component and the solder joint.

[Summary]

1. Assess, manage and confirm the ROHS compliance of materials, PCB, component manufacturers to distributors. This is the key to ensuring that the toxic and hazardous substances restricted in ROHS are controlled from the source.
2. Select various ROHS-compliant materials according to specific products and conduct a systematic evaluation.
Solder Paste: Alloys and Fluxes
Device: Solder end and body material
Substrate: substrate, pad surface and solder mask
3. Evaluate whether the current furnace meets lead-free requirements, establish a ROHS-compliant production line, and strictly manage pollution according to the system Bb.
4. Strict material management. Including procurement, packaging protection, identification, inventory conditions, management, use.
5. Check and modify PCB pad design (heat dissipation, layout) and stencil design.
6. Correctly set, carefully optimize the humidity curve, and keep the process stable.
7. Adopt lead-free quality inspection standards

In short, strict management, mastering the correct process methods, and making the process more detailed are the keys to the correct implementation of lead-free processes.