PCB Assembly Process

PCB Assembly Process

At Yingstar Electronics, we pride ourselves on our efficient and high-quality turnkey PCB assembly service. Yingstar employs a number of strategies in Quality Management and process control to ensure that every PCBA order is done right the first time. In order to achieve the fastest possible turnaround of the highest quality product, we continuously strive to improve our services and to make each step as efficient as possible.

Below we briefly introduce the main procedures of making PCB assembly in Yingstar Electronics.

DFM / DFA Check

Once a turnkey order is released to Yingstar’s production team, their very first task is the validate the design through a thorough DFM / DFA Check process. These checks include verification of consistency across different design documents (i.e. BoM, Gerbers, Centroid, etc.), part spacing, footprint accuracy, and clear orientation markings. The main objective of this procedure is to minimize as much as possible the potential for design errors to affect the finished product, which acts to protect clients against the additional time and cost involved with board-level rework.
Clients should watch for emails from Yingstar’s production team over course of the first one or two days after placing an order. If any issues or discrepancies are detected during this initial check, the production coordinator for the order in question will reach out to the client directly for resolution. Generally, an itemized report will be sent for response, and the order will be placed on hold pending full confirmation, so it is important to reply to these questions as soon as possible in order to avoid delays.

Incoming Material Inspection

As parts are received at Yingstar’s production facility, our Incoming Quality Control (IQC) team conducts a thorough inspection before warehousing any particular material or component. Inspections include sample operational testing as well as date code verification and entry into a software material management system. Our sophisticated software management system ensures that rules of first-in-first-out are strictly followed, and that parts used in PCB Assembly are always in good working order.
After DFM/DFA Check and incoming materials inspection, the assembly process will be started, please see below chart for all following steps:

SMT Solder Paste Screening

The first step in the actual PCB Assembly process is the application of solder paste to the bare PCBs. Here the stainless-steel stencil that was created during PCB Fabrication is fit over the bare board, leaving only the pads for assembly of surface-mount components uncovered. The stencil is held in place by our automatic solder paste machine, and an applicator moves over the surface of the board to meticulously distribute solder paste over those uncovered spaces. Following the automatic solder paste machine, there is a 3D SPI which can performs a thorough inspection to ensure that the solder has only been applied to the necessary areas, and that all pads are covered with a sufficient amount of paste with proper thickness.  
Yingstar’s solder of choice is SAC305, which is a Lead-Free alloy containing 96.5% tin, 3% silver, and 0.5% copper, and is compliant with the RoHS, REACH, and JEIDA directives. We use the paste version of this material for reflow soldering, and solid versions for manual and wave soldering.

SMT Mounter Placement

Once solder paste is applied to the bare PCBs, they are moved to automated Pick & Place machines for the actual mounting of components on their associated pads. Part placement is 100% machine automated for maximum accuracy and efficiency, and uses the project’s Centroid file for component coordinates and rotation data. The boards are again inspected after components have been mounted to ensure all placements are accurate before the soldering process begins.
This stage might need to be performed multiple times, depending upon the specifics of a given project. Double-sided SMT boards require one round of placement for the top and one for the bottom, and projects requiring wave soldering due to a high number of through-hole parts will normally have their components machine-placed as well.

Reflow Soldering

With parts mounted securely in place with solder paste underneath their pads, it is time for the PCBs to enter the reflow soldering phase. This is the most common method for PCB assembly in the industry today since it is much more flexible in terms of PCB layout requirements compared with wave or manual soldering.  
For double-sided SMT projects, the boards will need to be reflowed once for each side. A special adhesive is applied underneath the components that were soldered in the first run to prevent them from detaching and falling off the board when their solder is re-heated.
The main concern in reflow soldering is that components must withstand high levels of heat for a more prolonged period of time than what would be required for either wave or manual soldering. The vast majority of modern SMT components are designed with these heat profiles in mind, but many through-hole components are not suited to reflow soldering for this reason.  

X-Ray Inspection

After a reflow cycle, any boards including BGA, QFN, or other lead-less package types are sent for X-Ray Inspection; this service is included by default on all Yingstar quotations which include such parts.
X-Rays penetrate the silicon of an IC package and reflect from the metal connections underneath, forming an image of the solder joints themselves that can be analyzed by advanced image processing software similar to Automated Optical Inspection (AOI). Higher-density features in the captured area create a darker resulting image, allowing for quantitative analysis to determine quality of the solder joints and compare against industry standards.
Not only does X-Ray inspection detect issues in PCB assembly, but the analysis of an X-Ray image can help to determine the root cause of a given defect, such as insufficient solder paste, skewed part placement, or improper reflow profile.

Wave Soldering

Wave soldering is a method of PCB Assembly that involves sending boards on a conveyor through a “wave” of molten solder. This technology is for through hole parts, it will be much faster and more cost-saving than manual solder. Our engineer will design a wave soldering fixture for your boards, it will hold the boards and protect the SMD parts, but only let the pins of the through hole parts exposed to the molten solder. It’s low cost, high speed and fewer mistakes, it’s the best choice for soldering through hole parts.

Final Inspection

These final inspections always include visual inspection by our highly experienced quality assurance team, and 100% Automated Optical Inspection (AOI) for complex or high-volume projects, and X-Ray inspection for BGA, QFN, or other lead-less package parts . We can also provide additional services such as Functional Circuit Testing (FCT) and In-Circuit Testing (ICT) upon client request. The more involved testing methods of ICT and FCT will require some additional lead time and labour cost depending upon the specific requirements for a given project, but the manufacturing quality can be highly guaranteed.