Meaning
Surface flatness tolerances for land grid array packages dictate the maximum allowable height variation across all contact pads. To prevent soldering defects during board assembly, lga coplanarity must remain within the micrometric limits specified by the package manufacturer. This metric determines the risk of open joints or solder bridges beneath the module.
Solder Consistency
Placement of the module on the printed circuit board requires a perfectly uniform solder paste deposit. If the lga coplanarity is poor, some contacts will fail to touch the solder paste when the board is heated in the reflow oven. This results in intermittent electrical connections that are difficult to diagnose during functional testing.
Low coplanarity values ensure that every pad is wetted properly by the molten solder.
Board Attachment
Mechanical forces must be applied evenly across the package during the mounting and reflow processes to prevent warping. A deviation in lga coplanarity can cause the substrate to lift at the corners, creating an open circuit on the outermost pins. This risk is especially high for large modules with over one hundred contacts.
Using a rigid circuit board substrate helps minimize the board-level contribution to this flatness error. Testing these assemblies after reflow involves automated x-ray inspection to verify joint thickness.
Inspection Procedure
Measurement of the pad heights is performed using high-resolution optical profile scanners during the packaging phase. Any batch that exceeds the specified lga coplanarity limit is rejected before being sent to the assembly line. This inspection saves the manufacturer from wasting expensive system-on-chip components on assemblies that cannot be soldered reliably.
