When a connector sits lifted after SMT, or a few pins haven’t taken solder, the common causes fall into four groups: the pins themselves are not in one plane (poor coplanarity), the plastic housing or the PCB deforms from heat during reflow, the paste volume is wrong, and placement is offset or uses the wrong force. A connector maker’s note says that if even one pin sits noticeably higher or lower than the others, an open joint may result, and most specifications require coplanarity within 0.10 mm to 0.20 mm; the background section of a connector patent says the housing material’s coefficient of thermal expansion often differs from the PCB’s, so the housing tends to warp during reflow; taking a BGA-type connector as an example, coplanarity tends to worsen and joints may be open after reflow. The order of checks: look at where the symptom is, measure coplanarity, then check deformation during reflow, paste and placement, and finally verify.
This article covers how to find the cause when surface-mount connectors (board-to-board, USB and others) sit lifted, have open joints, or have shorts and opens at the same time after reflow; connector mating force, electrical specifications and press-fit connectors are outside its scope. It is organized in five stages:
- Stage ① Look at the symptom: whole connector lifted or only a few pins without solder, and where.
- Stage ② Measure coplanarity: what it is, the usual specification, how to control it.
- Stage ③ Check deformation during reflow: housing warpage, temperature changing too sharply, PCB warpage.
- Stage ④ Check paste and placement: too little or too much paste, clogged stencil apertures, placement offset, shorts and opens together.
- Stage ⑤ Verify and improve: inspect, measure, set limits, change the process.
Each stage covers what to do and what public sources say, plus how JIAHON can help where relevant; sources and limits are summarized at the end. This describes the general case; individual cases still need their own assessment.
Figure 1. Five stages (illustration): ① symptom (whole connector lifted → check deformation first; a few pins → check coplanarity first; position: ends or middle → know where to look) → ② coplanarity (definition: highest minus lowest pin; specification per data sheet; control by sampling incoming parts → pins in one plane) → ③ deformation (housing warps from heat; too steep a ramp may warp; the PCB warps too → understand the heat effects) → ④ paste and placement (too little or too much paste; stencil clogged or contaminated; placement offset or too much force → right solder volume and position) → ⑤ verify (visual and optical inspection; set acceptance values; improve profile and printing → change with evidence).
Stage ①: Look at the symptom
What to do:
- What a lifted lead is: the part’s leads or terminals are not in one plane, so some leads don’t solder to their pads during reflow (source 4).
- One high pin can cause an open: ideally all pins lie in one plane; if even one pin is noticeably higher or lower than the rest, an open joint may result (source 1).
- Whole connector or a few pins: if the whole connector is raised, check housing and PCB deformation first; if only a few pins lack solder, check those pins’ coplanarity and paste first (this article’s own summary).
- Note the position: whether opens cluster at the ends, the middle or always the same pins; record it and compare with the next batch (this article’s own summary).
- Consequences of lifted leads: joints may be incomplete, with poorer electrical and mechanical connection and a greater chance of cracking later; depending on how far the lead is offset, an open circuit may result (source 4).
Figure 2. Two symptoms (side-view illustration; based on source 1): whole connector lifted — every pin sits some distance above the paste, so check housing and PCB deformation first; only a few pins without solder — two pins sit higher and don’t touch the paste, so check those pins’ coplanarity and paste first.
What public sources say: an SMT process knowledge site’s page on lifted leads and coplanarity says the problem is leads or terminals of surface-mount parts that are not level and don’t solder to their pads during reflow, causing poor joints, electrical problems and mechanical stress; such joints may crack and fail over time, and depending on the offset may be open. A connector maker’s surface-mount process note says coplanarity is critical to the joint; ideally all pins lie in one plane, and one pin noticeably above or below that plane may cause an open.
Stage ②: Measure coplanarity
What to do:
- Definition: with the connector on a perfectly flat surface, the distance between the highest and lowest pins (source 1).
- Typical specification: most specifications require at most 0.10 mm to 0.20 mm deviation from the seating plane, meaning the highest and lowest pins can differ by no more than this (source 1).
- Control by sampling: coplanarity is hard and time-consuming to measure, but controlling it with statistical sampling and quality procedures is essential; laser measuring equipment is available (source 1).
- Placement machines check it too: to ensure lead coplanarity, placement machines perform a vision coplanarity check (source 4).
- Handling can bend pins: a research institute’s FAQ says a pin with a large vertical offset (for example more than 0.1 mm) may result from poor handling, raising the chance of an open (source 3).
- Check incoming parts: for a new lot or a new supplier, sample and measure coplanarity before the line (this article’s own summary, based on source 1).
What public sources say: the connector maker’s note gives most specifications as at most 0.004 in (0.10 mm) to 0.008 in (0.20 mm) from the seating plane, and says that because coplanarity is critical and hard to maintain, statistical sampling and quality procedures matter a great deal, with high-precision laser measuring equipment available. A U.K. welding research institute’s FAQ says a common cause of opens after reflow is leads not in one plane, especially on multi-lead ICs, and a lead offset vertically by more than about 0.1 mm may come from poor handling.
Stage ③: Check deformation during reflow
What to do:
- Housing warps from heat: the housing material’s coefficient of thermal expansion often differs from the PCB’s, so the housing tends to warp at reflow temperatures (source 2).
- Warpage worsens coplanarity: the patent, taking a BGA-type connector as an example, says housing warpage along the bottom tends to worsen coplanarity and may leave joints open after reflow; warpage of the housing walls may also misalign the mating connector (source 2).
- Temperature changing too sharply: if temperature changes too sharply during reflow, the part and PCB expand and contract at different rates, which may also lift leads (source 4).
- The PCB warps too: an expert answering a question on BGA corner opens says the BGA itself can warp and lift the part’s edges off the board, and PCB warpage and twist during soldering can do the same; check the temperature profile together with the board’s and part’s warpage characteristics (source 5).
- The part’s own design: body size and lead design also affect how prone a part is to coplanarity problems (source 4).
Figure 3. Two kinds of deformation during reflow (illustration, based on sources 2 and 5): housing warpage — both ends rise and those pins can’t reach the paste (housing and PCB thermal expansion often differ, source 2); PCB warpage — the board sags in the middle and the part’s ends leave the board surface (a warped or twisted board can lift part edges, source 5). Actual warpage direction may be reversed.
What public sources say: a connector patent’s background section (published 2009) says one problem in PCB manufacturing is the effect of soldering heat on parts; housing materials often have a different coefficient of thermal expansion from the PCB, so housings tend to warp at reflow temperatures; for BGA-type connectors, warpage along the bottom tends to worsen coplanarity, possibly misaligning contacts and pads or leaving opens after reflow, and warpage along the walls may misalign mating connectors. The SMT process site says sharp temperature changes lift leads and that body size and lead design matter. An expert on an electronics assembly Q&A site answering on BGA corner opens recommends checking the profile and the warpage characteristics of board and part.
Stage ④: Check paste and placement
What to do:
- Too little or too much paste: insufficient or excess paste on the pads may make joints uneven (source 4).
- Clogged stencil apertures: if an aperture is clogged, the pad gets no paste and won’t solder in reflow (source 3).
- Placement offset: a misplaced part may leave leads off their pads; incorrect placement force may also shift or lift leads (sources 3 and 4).
- Poor wetting: if pad or lead solderability is poor, solder may not bond to the surface (source 3).
- Solder flowing into vias: if a pad connects to an adjacent via, solder flowing into it may leave too little between lead and pad (source 3).
- Shorts and opens together: common causes of bridging are stencil aperture size, shape and spacing, an unclean stencil underside, poor board support causing excess paste, and placement pressure squeezing paste off the pads (source 4); when paste is pushed sideways, one pin may short while another lacks solder (this article’s own summary).
What public sources say: the U.K. welding research institute’s FAQ lists clogged stencils, misplaced parts, poor pad or lead solderability and solder flowing into connected vias as common causes of opens after reflow. The SMT process site says poor handling or excess force during placement or reflow may shift or lift leads, too little or too much paste may make joints uneven, and its page on solder paste bridging lists aperture size, shape and spacing, paste on the stencil underside, poor board support and high placement pressure.
How JIAHON can help:
- Missing parts or open joints may come from paste printing, stencil design, the reflow oven, PCB design or placement position; JIAHON’s engineering department can help find the cause across these areas. For the reflow oven, the temperature the connector body actually sees can be measured with K-type reflow thermocouple wire attached to the connector.
- Paste printing: JIAHON’s published article “How to troubleshoot solder paste printing defects: work back from the print to the consumable to replace” says: “A worn or loose blade clamp lets the PCB shift during the squeegee stroke, directly causing print offset, open joints and other defects.”
- Stencil cleaning: JIAHON’s published article “Is your stencil cleaning done right? 70% of SMT defects start here” says: “Many people take cleaning to mean ‘it looks clean,’ but that standard is not enough for fine-pitch and ultra-fine-pitch products.”
- Placement position: JIAHON’s published article “Notes from learning SMT mounter programming” says: “The machine uses the Mark camera on its head to look at the Fiducial Marks on the board, compares their actual positions with the coordinates in the program, then corrects all placement positions together.”
Stage ⑤: Verify and improve
What to do:
- Look first: check parts and boards for lead offset visually or with automated optical inspection (AOI) (source 4).
- Then measure: measure the offset with measuring tools, vision systems or 3D scanning (source 4).
- Set limits: set coplanarity acceptance limits from industry standards or internal quality standards; parts beyond them may need correction (source 4).
- Ways to correct: reflow the joints again, replace with parts that meet coplanarity, or adjust assembly process parameters (source 4).
- Change the process: optimize the reflow profile, improve placement accuracy and improve paste printing; monitor with solder paste inspection (SPI) after printing to catch anomalies at once (source 4).
- Work both directions: if the connector still sits lifted after paste and placement are corrected and coplanarity is within limits, go back to housing and PCB deformation (this article’s own summary).
What public sources say: the SMT process site says the first step is usually visual inspection by technicians or AOI, then measurement with calipers, vision systems or 3D scanning; makers set acceptable coplanarity limits from industry or internal standards, and parts beyond them may need correction by reflowing, replacing parts or adjusting parameters; prevention includes optimizing the profile, placement accuracy and paste printing; its bridging page adds that SPI right after printing finds problems at once.
Sources and limits
Sources: a connector maker’s surface-mount process note (maker content, footer copyright 1997, no publication date; Stages ① and ②); the background section of a connector patent (published 2009; Stage ③); a U.K. welding research institute’s FAQ (research institute, page undated, page data field December 2015; Stages ② and ④); an SMT process knowledge site’s pages on lifted leads and on solder paste bridging (process site, undated; Stages ① to ⑤); an expert answer on an electronics assembly Q&A site (industry expert, April 2011, updated August 2018; Stage ③); and JIAHON’s engineering support scope and three published JIAHON articles (paste printing defects, stencil cleaning, mounter programming).
Limits: this article draws on 5 public sources; the full text of connector coplanarity standards was not read. Source 1’s footer copyright is 1997 with no publication date; sources 2 and 5 discuss BGA-type connectors and BGAs, and this article takes only the reasoning shared with ordinary surface-mount connectors; source 3 is undated (page data field December 2015) and source 4 is undated. The sources cover the causes of bridging and of opens separately; “paste pushed sideways, one pin shorts while another lacks solder” is this article’s own summary, as are “whole connector or a few pins,” “note the position,” “check incoming parts,” “work both directions” and Figures 2 and 3. “How JIAHON can help” gives only JIAHON’s engineering support scope and sentences from published pages. Follow the connector maker’s data sheet, your product specifications and your own verification. This article is a general summary, not a restatement of standard text, nor an endorsement of any connector maker or research body, nor a guarantee for your products and processes.