How to improve SMT PCB assembly quality

MISPRINT CLEANING

INTRODUCTION

Assemblers surveyed report that cleaning misprinted circuit assemblies is a production gap that has not been adequately addressed. Traditionally, the industry has used stencil cleaning agents and equipment to address this rework need. One of the benefits of cleaning misprinted assemblies with the stencil cleaning process is the ability to collect and filter wet solder paste. The major short coming of cleaning misprints within stencil cleaning processes is the inability to remove B-side reflow flux residues from both the surface and under bottom termination components.

 

REWORKING/CLEANING MISPRINTED ASSEMBLIES

Stencil printing is a highly automated process. During machine setup, a small group of boards are misprinted. During production stencil printing, circuit boards are periodically misprinted due to clogged apertures, stencil out of alignment, solder paste rheology shifts and other issues. Stencil misprints are defined as A-Side (Initial print out of alignment with no components previously placed) and B-Side (A-Side was successfully printed and components placed and soldered. The subsequent process of printing the B-Side results in the solder paste being out of alignment resulting in a B-Side misprint).

 

Printed Circuit Board misprints are a costly problem with no easy rework methodology. Production cleaning processes are normally not used to clean misprint assemblies. Potential quality issues such as:

  • Solder balls collecting into the wash tank and being transferred back onto the assembly
  • Solder balls migrating into the rinse streams resulting in hazardous waste from metals in the wash and rinse holding tanks

These complexities potentially compromise repeatability and reliability standards. Due to these complex issues, most assembly houses do not allow misprints to be cleaned within their production cleaning process.

 

Assemblers commonly address the misprint cleaning need by either hand wiping the misprinted side of the circuit card and/or clean the misprint in a stencil cleaning machine. Both methods create the potential for quality issues. First, when wiping solder paste from the misprinted side of the board, solder paste can be trapped in solder mask defined channels, through-hole vias, and other board geometries (Figure 1). Numerous quality problems can result due to lack of control and definition.

 


Figure 1: Solder Balls Wedged into No Solder Mask Defined Channels and Via Holes

 

Second, stencil cleaning machines are designed to remove wet solder paste from stencils. Most stencil cleaning processes do not rinse the stencil with water. For those that use a water rinse, the water is reused since trace levels of metals in water prevent disposal to local treatment works. Cleaning a production board in a machine designed to clean stencils fails to meet ionic cleanliness standards required for a production assembly. Additionally, on B-Side misprints, the stencil cleaning agent is typically not adequate for cleaning reflowed flux residues on the A-Side of the board. In most cases, the stencil cleaning agent partially removes the reflowed no-clean flux residue resulting in white residue and an ionically dirty assembly.

 

FILTRATION OPTIONS

Cleaning the misprinted circuit board within an electronic assembly production cleaning process has the potential to achieve cleaning of wet solder paste and reflowed flux residues as well as meet quality and yield objectives. The problem with cleaning a misprinted circuit board in a production cleaning process is the deposits of solder spheres collected into the wash holding tank. Free solder spheres within the wash holding tank can be picked up by the inlet of the pump and sprayed onto production assemblies. There is also the potential that the solder spheres can be dragged into the rinse sections. Both quality and waste treatment issues result from this practice.

To resolve the quality and water treatment issues, collection and filtration method systems are needed to trap and filter solder spheres. Filtration systems designed tocontain the solder spheres and capture them prevents spraying solder balls through the pump and spray manifolds. The mechanical and filtration systems resolve the issues of redepositing solder balls onto production assemblies and the potential to contaminate rinse streams. The overriding quality advantages in using production cleaning equipment, which is designed for repeatedly removing all solder spheres from the assembly, remove reflowed flux residues and render an ionically clean printed circuit board provide a reproducible and repeatable product.

EQUIPMENT OPTIONS

Inline Cleaning Equipment

Two types of aqueous production machines are used to clean electronic assemblies, inline and batch. For the inline machine, the pre-wash section of the cleaning machine is designed to wet, elevate the circuit board to wash temperature, and soften reflowed flux residues from production circuit assemblies. On option for containing solder balls is to equip the pre-wash sectionwith deflectors that contain the raw solder paste as it is being displaced from the circuit assembly. The deflectors close in the pre-wash spray manifolds using two trays and plates to prevent solder spheres from escaping the housing of the pre-wash section. As the boards enter the pre-wash section, the displaced solder balls and wash fluid drain into the catch trays. By capturing and containing the pre-wash liquid, the majority of the solder balls can be channeled into a series of sluice boxes. This important design feature contains the bulk of the solder balls with a minimal amount escaping to the wash holding tank.


 

 

A series of Sluice Boxes can be designed to capture the heavy raw solder spheres similar to the techniques used in mining precious metals from water streams (Figure 5). Three separate sluice boxes capture the majority of the solder paste. Each sluice box is equipped with a wire mesh. The weight of the solder balls drop through the wire mesh and collect into the sluice box trays. The first sluice box captures the majority of the solder spheres with the remaining two sluice boxes used to collect the residual solder spheres.


Figure 3: Sluice Box Collection Boxes courtesy of Speedline Technologies

Solder balls that are not collected within the sluice boxes will drain into the wash fluid holding tank. To prevent these stray solder balls from being sprayed onto circuit boards, three pump intake strainers prevent large spheres from entering the pump (Figure 6). The smaller solder spheres that pass through the strainers will be captured in a bag filter from wash liquid pumped through the outlet of the pump.


Figure 4: Strainers in Suction Inlet of the Wash Holding Tank courtesy of Speedline Technologies

Following the suction strainers, the wash solution is pumped through a filtration system designed to collect any remaining solder spheres before reaching the spray manifolds. The wash outlet enters the top side of the filtration canister, exits the clean side of the filter and then goes to the spray jets.


Figure 7: Filtration Canister

Within the canister, there are internal bars that prevent the bag filter from getting next to the exit side of the filter housing. This design feature prevents back flow or resistance as the liquid pumps through the filter canister. The 10/5 bag filter cartridge (ten microns on the inside and 5 microns on the outside of the filter cartridge) provides double redundancy to contain any solder balls from escaping the filter (Figure 9). The 10 micron side captures the heavy particles and the fine 5 micron side of the filter assures no solder spheres are sprayed onto circuit cards. The filtration design removes solder balls as small as Type 5 Solder Paste while preventing solder balls going to the manifolds. Pressure drops are minimal due to the solder paste being captured within the bag filter. Should the pressure drop, the machine is equipped with a user defined interface, which sends an alarm to the operator. The design is such that thousands of misprint boards could be cleaned before having an impact on the bath integrity, pressure and cleaning performance.

Batch Cleaning Equipment

One main difference between batch cleaning machines versus an in-line type cleaner is the ability to program the type of wash cycle, the sequence, and cycle times within the cleaning process. It is therefore critical that the ability to effectively trap and collect wet solder paste be integrated into the batch cleaner wash cycles.

 

The design objective is to provide the board assembler the flexibility to deflux their normal production runs (A/ B side), deflux an A-side with B-side misprint, clean A/B side misprint, plus the ability to completely rinse and dry the product within the same batch type cleaner.

Similar to the design for the in-line cleaning system, the same equipment manufacturer used the multi-stage filtration approach to effectively collect solder spheres and to prevent the spheres from being sprayed onto the board assembly. A pre-wash type cycle in the batch cleaning process will wet, elevate the circuit board to wash temperature, and soften the reflowed flux residues from the production circuit assemblies. The flux composition with the raw solder paste is easier to remove than the reflowed paste. An internal bag type filter is used to capture the raw solder paste that is removed during the Flood Wash cycle . The main purpose of the internal bag filter is to minimize the amount of solder paste that would be drained into the wash fluid holding tank.


Figure 8: Bag Filter in Wash Holding Tank

Solder spheres that are not collected in the bag filter will collect in the wash fluid holding tank. To prevent large particles from entering into the wash pumps, two intake strainers are located in the wash holding tank.


Figure 9: Batch Intake Strainers

Following the suction strainers, the wash solution is pumped through a filtration system that is designed to capture the smallest of solder spheres before being sprayed through the wash fluid spray delivery system. The filtration system is designed to capture solder paste as low as type 5 paste (Figure 10).


Figure 10: Batch Filtration Design

SUMMARY

Cleaning both A-Side and B-Side misprints has been a complex problem for assemblers. Using a stencil cleaner to clean misprints has numerous flaws. Two key issues is the inability to remove reflowed flux residues with stencil cleaning agents and poor rinsing. Notwithstanding, most assembly houses do not allow misprints to be cleaned in production cleaning machines due to the risk of contaminating product boards with stray solder balls and due to waste water metal contamination issues.

 

Collection and filtration systems designed into inline and batch production cleaning equipment safely captures and contains solder spheres from being sprayed onto production assemblies. Additionally, the containment and filtration systems prevent raw solder paste from entering the rinse water streams.

 

Using a production cleaning machine provides numerous benefits to the assembler.

  1. Recovery and rework of expensive hardware
  2. Removal of wet solder paste
  3. Containment of solder spheres
  4. Removal of reflowed flux residues
  5. Exceptional rinsing
  6. Ionically clean assemblies
  7. Repeatable
  8. Reproducible

 

Wiping wet solder paste from production assemblies is a bad practice. When wiping wet solder paste, solder spheres can be wedged into no solder mask defined troughs, vias and other offsets. When these solder balls become wedged, high levels of energized sprays may not be sufficient in displacing a wedged solder ball.

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