BGA Assembly and SPI Inspection on an Edge Computing Board

An edge computing board packs a controller, memory, power management, high speed interfaces and a communication module onto one board, and the pads vary widely in size and spacing between one region and the next. For work like this the accuracy of the placement machine is only one of the conditions. Whether the volume of paste is stable, whether the stencil opening suits the pad, and whether the board is properly supported decide the result earlier than the machine does.

Why the Problem Starts Before Placement

If the paste over a ball grid array is short, the joints can fail to form during reflow. If it is excessive, the risk of a bridge or of an abnormal solder ball rises. Around a memory device the parts are packed tightly, so a single print offset can affect several pads at once. The check therefore happens at the printing stage, where the board can still be cleaned, rather than after the placement where the only remedy is rework.

The support under the board matters just as much as the aperture, and it is the condition most often overlooked on a dense order. On a large, thin high density board the squeegee can deflect the panel, and a small deflection over a small pad changes the deposited volume. A support tool designed for the panel rather than for the product is the usual cause.

What the Order Should Contain

Beyond the Gerber data, a complete bill of materials, the placement coordinates, the assembly drawing, the panel information and the revision notes are wanted. Where programming, interface testing or functional verification is included, the program file, the programming method, the test steps, the acceptance conditions, the quantity and the expected date come with them.

The review is not a check that the files open. The reference designators, the packages, the orientations and the part numbers are compared across the bill of materials, the coordinates and the assembly drawing. Where one designator is listed as a particular memory device but the package in the coordinate file does not match the part, the difference is raised before production rather than after. Because the joints under a ball grid array cannot be seen, a discrepancy in the data is far more expensive to correct once the board has been through the oven. This is the reason BGA assembly places so much weight on the review that happens before the first board is printed rather than on the inspection that happens after the last one.

Stencil Design and SPI Inspection

The stencil cannot be designed by scaling every opening by the same factor. The controller, the memory, the bottom terminated devices and the small passives all need different volumes of paste, and the opening is evaluated from the pad size, the spacing between parts, the layout and the thickness of the board. A large thermal pad is normally divided into a set of openings so that the total volume is controlled and the device does not float during reflow.

After printing, the paste height, area, volume and offset are measured in three dimensions. The purpose is not only to reject a defective board but to establish whether the printing is in a state where placement can begin. If the volume is consistently low in one region, the cause is a blocked aperture, the squeegee pressure, the support under the board or the condition of the paste, and each of those is checked in turn. Stopping to correct a printing drift is normally cheaper than collecting the defects after the placement has been completed.

The stencil aperture design and the inspection plan are set from the actual board, the customer drawing and the acceptance requirement, rather than by applying one set of parameters to every order.

Material, Loading and the First Article

Before placement the material is checked and the loading is confirmed against the programme. The controller, the memory, the power devices and the interface parts on an edge computing board often share similar packages, so the physical marking, the reel label, the bill of materials part number, the reference designator and the orientation are all confirmed together.

Where a substitute or a different batch of material is proposed, an identical package is not sufficient evidence that it can be used. The ratings, the temperature grade, the suffix and the packaging may all differ. Where substitution is permitted, the range and the method of confirmation are stated in the data; a change that has not been confirmed is discussed before it is made.

On the first article, the offset of the small parts, the orientation of the polarised devices and the condition around the ball grid array are all observed. A coordinate origin error, an angle error or an incorrect footprint library entry found at that point costs far less to correct than the same error found after the batch has been built.

Reflow and the Hidden Joints

The reflow profile is evaluated from the size of the board, the area of copper, the thermal mass of the components and the paste. A board that carries both a large ball grid array and small passives has to melt the solder on the large device properly while not overheating the small ones, and a satisfactory appearance at one joint does not prove that the thermal process was right across the board.

After reflow, optical inspection covers the offset, the missing parts, the tombstones, the bridges and the visible joints. Where the customer requires verification of the joints beneath a ball grid array, the extent of the X-ray inspection is agreed from the quality requirement. Once the order moves into volume, and the BGA assembly process has been proven on the pilot batch, the material lot, the production revision and the inspection results are all kept so that the scope of an anomaly can be established rather than guessed.

Programming and Testing

Some boards need the boot program written after assembly and the supply rails, the communication interfaces or the basic start-up state checked. The programming file, the interface definition, the supply conditions, the fixture description and the acceptance criteria belong with the order. Without a defined test sequence the factory cannot distinguish a soldering problem from a program revision problem or from a peripheral condition that was not met.

Testing, programming and assembly can be connected in one order, so that the scope is agreed before production rather than added after it. Our SMT assembly lines place and inspect the board, component procurement controls the material, and the testing is covered by PCBA testing.

FAQ

Why does the factory want more than the Gerber data? Because the bill of materials, the coordinates and the assembly drawing have to agree with each other, and a disagreement found before production is much cheaper than one found afterwards.

What does paste inspection actually prevent? It prevents a print that is out of specification from becoming a soldering defect, which is a correction made at the press rather than at the iron.

Is X-ray used on every board? The extent is agreed with the customer, from a sample to a full inspection, according to the design and the quality requirement.