Repair- and reballing-Stencils

Tailor-made stencils for demanding SMD repairs

With the introduction of SMD technology, despite the low error rates in the new technology, procedures and repair processes had to be developed to enable material-friendly and reproducible repairs, as a first pass yield of 100% is rarely achieved in production.

In addition, with the introduction of SMD technology, the variety of component shapes and connections has steadily increased, especially in area array components, rework systems, and repair stencils in particular.

The ever-shrinking pitch spacing of component connections poses a further challenge for repair procedures and stencils.

Due to high demands on quality and reliability, the repair of components has become much more complex and costly. LaserJob reliably manufactures laser-cut printing templates for a wide variety of rework stations and repair areas.

Advantages

Precise aperture geometry
Aperture size accuracy ± 3 μm
Aperture positioning accuracy ± 10 μm within a 200 x 200 mm area
Smooth walls
Absolute flatness
Positioning aid for the component

Repairing assemblies

Since a first pass yield of 100% is rarely achieved in SMD technology, procedures and repair processes must be developed to enable material-friendly and reproducible repairs, despite its low error rate.

The SMD components can be replaced either manually or using a rework station.

First, the faulty component is desoldered, then the new component is soldered back in.

Depending on the component shape and LP layout, there are various options for creating a solder paste deposit for the solder connection:

Printing the PCB connections
Printing the component connections
Reballing the BGA (Ball Grid Array) land areas
Special solutions, e.g., assembly aid

Printing the PCB connections

Repair stencil for printing on the circuit board:

Only possible with sufficient space on the circuit board.
Only way to create a paste deposit for designs with lead frames or connection pins
Can be done with rework stations as well as simple hand squeegee stencils
Hand squeegee stencils are the more cost-effective option as no rework station is required
The material thickness and size of the apertures of the rework stencil correspond to those of the SMD stencil

Printing the component connections

Repair stencil for printing on components:

Only possible for components with connection surfaces located underneath the component (QFN).
Can be carried out using both rework stations and simple hand squeegee stencils.
Advantage: By soldering the component, repairs are possible even with very dense assembly.
The material thickness and size of the apertures of the rework stencil correspond to those of the SMD stencil.

Reballing of BGA land areas

Reballing stencils for BGA components

Repairing or re-soldering BGAs is a common practice in electronics manufacturing. However, the connections of desoldered BGAs or CSPs are often so damaged which means they cannot be soldered directly back onto the circuit board. After removing the residual solder, the BGA connections are wetted using flux in the form of a flux pen, and the BGA balls are distributed over the mask using a precisely fitting pattern in the corresponding connection grid. The excess solder balls not placed in the openings of the mask are returned to the container via a drain channel. In the subsequent reflow process, the solder balls for the BGA connections are remelted.

Special solutions: e.g., assembly aid

General problem faced by the customer: THT connectors had to be assembled for an application. Due to the high requirements (fill level and clearance) for the solder joint, it was not possible to apply sufficient solder using PIP paste printing. Additional solder in the form of preforms (circular rings) had to be threaded onto the connection pins. This laborious manual work needed to be replaced by a faster and error-free solution. A case for the experts at LaserJob!

Within five working days, LaserJob found the solution: an assembly aid with movable insert plates that allows the preforms to be threaded securely, reliably, and quickly in two passes. Thanks to its modular design, the preform holder plates can be exchanged quickly. This makes the assembly aid easy to convert for different layouts.

Device for placing preforms with a sliding mechanism for releasing the preforms:

Production

The printing stencils are manufactured in air-conditioned production rooms using fiber lasers. Fiber lasers are characterized by their very high beam quality. The smaller cutting gap with the same depth of field ensures reduced heat input into the material. The precision of the slightly conical openings expands the process window of the placement process and facilitates the release of the solder paste from the stencil.

By default, repair templates are available in sizes ranging from 10 x 10 mm to 100 x 100 mm. However, any other size or shape can also be requested. For this purpose, the apertures for the connection pads are cut into a stainless steel sheet in a tensioned state and welded into position with a frame.

This procedure ensures that the sensitive printing mask is protected against external damage and is absolutely flat when stretched.

The stiffening frame can also be used as a positioning aid for the component.