The SMT soldering process for a bent shielding can is discussed in SMT Soldering Process Analysis for a Conventional Bent Shielding Can. This article focuses on the validation of SMT soldering for a single-sided insulated shielding can.
1. Application Background of Single-Sided Insulated Shielding Cans
The specified distance between SMT components and a shielding can is generally at least 0.15 mm for insulated components and at least 0.2 mm for non-insulated components. As the overall product thickness continues to decrease, shielding cans are becoming thinner, and some projects require a clearance of only 0.1 mm—or even 0.8 mm—between the shielding can and nearby components. To meet the mechanical design requirements while preventing short circuits between components and the shielding can, electrical insulation must be applied to the can.
There are two methods for insulating a shielding can. The first is to apply insulating tape to the inside of the can. The second is to use a shielding can with a single-sided insulating coating. Applying insulating tape to the inner surface is a mature process that is already used in many products. Single-sided insulated shielding cans have also begun to see wider adoption in recent years. The two types are compared below.
Single-sided insulated shielding cans provide the following advantages:
- Lower cost. When the shielding can is small or requires a large number of insulating-tape pieces, the single-sided insulated version can provide a cost advantage. According to supplier information, the labor cost for applying one piece of insulating tape is RMB 0.08. If the material cost of the shielding can is a and the number of insulating-tape pieces is n, the cost of the tape-based solution is a + n × 0.08, while the cost of the single-sided insulated shielding can is 1.8 × a.
- Thinner insulation. The insulating tape is 0.05 mm thick, whereas the insulating coating is 0.03 mm thick.
Insulation-performance testing has already been completed for the insulating coating, and the results met the design requirements.
2. SMT Soldering Validation
2.1 Validation Plan
The only difference between a single-sided insulated shielding can and a conventional shielding can is the insulating coating added to the inner surface; the remaining construction is the same. Because the inner surface is coated, the inner side of the folded edge cannot normally be wetted by solder. This creates two potential effects on SMT production:
- Because soldering may occur on only one side, the mechanical strength of the shielding-can attachment could be reduced.
- Excess solder could cause the shielding can to shift during reflow soldering.
For conventional shielding cans, the stencil aperture width is normally 1.3 mm. For shielding cans with dimensions below 15 cm, the aperture width is 1.0 mm. Because the inner surface of a single-sided insulated shielding can receives little or no solder, the stencil apertures can be reduced appropriately to prevent post-soldering displacement or excessive solder volume.
For shielding cans below 15 cm with good incoming flatness, an aperture width of 0.8 mm was used. Shielding cans larger than 15 cm generally have more complex structures and slightly poorer flatness. Two stencil-aperture options were therefore evaluated for these parts: 1.3 mm and 1.1 mm.
Solder-joint quality and post-soldering displacement can be checked by visual inspection. Whether the soldered shielding can has sufficient mechanical strength can be verified through push-force testing and reliability testing.
2.2 Test Conditions and Results
Three single-sided insulated shielding cans were used in the project. Two were larger parts and one was a smaller part. Their external shapes are shown below. The test quantity was 500 pieces for each type.
2.2.1 Stencil Apertures
A conventional laser-cut stencil with a thickness of 0.10 mm was used for the evaluation.
- For the smaller LCD shielding can, measuring 12 mm × 7.7 mm, the stencil aperture width was 0.8 mm.
- For the larger shielding can measuring 28 mm × 17 mm, the stencil aperture width was 1.3 mm.
- For the larger shielding can measuring 22 mm × 20 mm, the stencil aperture width was 1.1 mm.
2.2.2 Trial Production Process
A trial run was performed on a production project. The line equipment, process parameters, solder paste, and other conditions were kept the same as those used in normal production, with no adjustments. Solder paste was printed using the newly fabricated stencil. The assemblies underwent 100% AOI inspection after reflow, followed by focused visual inspection of the three single-sided insulated shielding cans.
Solder-Joint Quality Inspection
The inspection reference was IPC-A-610, Acceptability of Electronic Assemblies. The soldering requirements were good wetting, a minimum end connection width equal to 75% of the terminal width, no simultaneous opens on two adjacent sides, and no open solder joint at a corner.
The castellated-edge feet of the shielding cans showed good wetting, and the end connection width was close to 100%. Visual inspection also found no open solder joints. The solder height was lower than the recessed locking points of the shielding can and therefore did not interfere with can installation.
However, some solder joints on the larger shielding can using the 1.3 mm stencil aperture showed relatively high solder climbing. Although the solder height met IPC requirements, the excess solder could potentially increase the risk of post-soldering displacement. The amount of solder climbing on the larger shielding can using the 1.1 mm aperture was close to that observed with the 1.3 mm aperture.
Because the inner surface of the single-sided insulated shielding can contains an insulating coating, solder theoretically should not wet the inner side of the castellated edge. One shielding can was pried open to inspect the internal soldering. A small amount of solder climbing was found on the inner side as well. This partial internal solder connection is beneficial to the overall mechanical strength of the shielding-can attachment.
Shielding-Can Displacement Inspection
The inspection reference was IPC-A-610, Acceptability of Electronic Assemblies. Lateral displacement, identified as A, and toe displacement, identified as B, were not permitted.
A review of the AOI inspection records found no shielding-can displacement. Manual visual inspection also showed that the castellated edges were generally aligned with the centers of the solder pads. Neither the terminal sides nor the toe regions extended beyond the pads.
Soldering Strength
IPC does not specify a push-force requirement for SMT components. This test therefore used engineering experience and an internal industry requirement as the reference. The shielding-can push force was measured with a push-pull force gauge, and the required value was at least 8 kgf.
Ten PCBAs were tested. The average push force for the small shielding can was 12 kgf. The two large shielding cans achieved average push forces of 15 kgf and 16 kgf, respectively. All results met the requirement.
The remaining 500 pieces also completed testing, assembly, and reliability testing. No shielding-can soldering defects were found.
3. Conclusions
Based on the validation process and results, the SMT soldering performance of the single-sided insulated shielding cans was normal. No open solder joints, displacement, or other defects were found, and the soldering quality was considered controllable. Where the cost advantage is applicable, this type of shielding can can be considered for broader use.
The recommended stencil-aperture designs are as follows:
- For conventional shielding cans smaller than 15 cm, use a 1.0 mm aperture width.
- For conventional shielding cans larger than 15 cm, use a 1.3 mm aperture width.
- For single-sided insulated shielding cans smaller than 15 cm, use a 0.8 mm aperture width.
- For single-sided insulated shielding cans larger than 15 cm, use a 1.2 mm aperture width.
